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1 ON THE EXPLORATION OF NEXT GENERATION INTERCONNECT DESIGN FOR CHIP MULTI PROCESSORS By ZHONGQI LI A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2012
2 2012 Zhongqi Li
3 To my father and mother
4 ACKNOWLEDGMENTS First and foremost, I offer sincere gratitude to my advisor, Dr. Tao Li, who has guided me throughout my PhD pursuit with his great knowledge and patience. It has been an exceptional experience to work with Dr. Li in the past years. H is mentoring is inspiring and her dedication to work is contagious. The dissertation would have been next to impossible without h is vision and research support. I acknowledge my committee members at University of Florida: Dr. Renato Figueiredo Dr. Ann Gordon Ross, and Dr. Peng Jiang I am truly thankful for the time and effo rts that they spent on reviewing and commenting my research proposal and dissertation defense. appreciate all the warm help and encouragement that I received from the lab me mbers during my personal and professional time. My r esearch would have been less colorful without the witty remarks in the lab now and then. Finally, I am deeply indebted to my parents. They have provided me with immense understanding and moral support all these years. I have enjoyed every moment we spent together with care and love.
5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 8 ABSTRACT ................................ ................................ ................................ ................... 13 CHAPTER 1 THE INTRODUCTION TO ON CHIP PHOTONIC COMMUNICATION .................. 15 Introduction to Interconnection Network ................................ ................................ .. 15 A Sample NoC Architecture ................................ ................................ .................... 16 Network Topology ................................ ................................ ............................ 17 Router Architecture ................................ ................................ .......................... 19 Background of Photonic Communication ................................ ................................ 21 The structure of ring resonators ................................ ................................ ....... 21 Application of ring resonators ................................ ................................ ........... 23 2 THE THERMALLY RESILIENT PHOTONIC NETWORK ON CHIP ARCHITECTURE ................................ ................................ ................................ .... 27 A Characterization of Thermal Impact on Photonic NoCs ................................ ....... 27 Motivation ................................ ................................ ................................ ............... 28 Structure of Thermal Resilient Photonic NoC System ................................ ............. 29 Impact of Temperature on Ring Resonators ................................ ..................... 31 Photonic Network Architecture ................................ ................................ ......... 38 Thermally Resilient Photonic NoC Architecture ................................ ...................... 41 Circuit level Technique ................................ ................................ ..................... 42 Architecture level Technique ................................ ................................ ............ 43 Operating System level Technique ................................ ................................ .. 46 Experimental Setup ................................ ................................ ................................ 48 Evaluation Results ................................ ................................ ................................ .. 50 NoC Latency ................................ ................................ ................................ ..... 52 BER and MER ................................ ................................ ................................ .. 53 Power Consumption ................................ ................................ ......................... 55 3 THE ARCHITECTURE OF HIERACHICAL PHOTONIC NOC ................................ 60 Motivation ................................ ................................ ................................ ............... 60 The Proposed Hierarchical Photonic NoC Architecture ................................ .......... 61 An Overview of Hierarchical Photonic NoC Architecture ................................ .. 61 Dynamic Resource Allocation in Photonic Network ................................ .......... 63
6 RapidEngy Optical Switch ................................ ................................ ................ 68 All Optical Adaptive Routing ................................ ................................ ............. 70 Experimental Methodology ................................ ................................ ..................... 78 Machine Configuration and Workloads ................................ ............................. 78 Power Estimation Methodology ................................ ................................ ........ 80 Evaluation ................................ ................................ ................................ ............... 80 The Optimal Network Power Latency Product (PLP) ................................ ....... 80 Network Performance ................................ ................................ ....................... 82 Power and Energy Efficiency ................................ ................................ ............ 85 4 EXPLORING PHOTONIC INTERFACE FOR OFF CHIP PHASE CHANGE MEMORY SYSTEMS ................................ ................................ .............................. 89 Motivation ................................ ................................ ................................ ............... 89 Background ................................ ................................ ................................ ............. 91 Phase Change Random Access Memory ................................ ......................... 91 The Memory Devices Organization and LPDDR2 Protocol .............................. 93 OptiPCM System Organization ................................ ................................ ......... 95 Sub channel Division Technology ................................ ................................ ........... 97 Fixed Channel Division ................................ ................................ ..................... 97 Dynamic Channel Division ................................ ................................ .............. 100 The Structure of PIs ................................ ................................ ....................... 100 The Design of Memory Controller ................................ ................................ ... 103 Experimental Setup ................................ ................................ ............................... 107 Simulation Methodology ................................ ................................ ................. 107 Power Model of the Communication Bus ................................ ........................ 109 Performance Evaluation ................................ ................................ ........................ 113 Power Consumption Breakdown ................................ ................................ .... 113 Latency Evaluation under Different Memory Configurations ........................... 115 System Throughput under Different Number of Ranks ................................ ... 116 Channel Width Impact on OptiPCM ................................ ................................ 117 5 RELATED WORKS ................................ ................................ ............................... 120 6 CONCLUSION ................................ ................................ ................................ ...... 125 LIST OF REFERE NCES ................................ ................................ ............................. 128 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 138
7 LIST OF TABLES Table page 2 1 Chip parameters ................................ ................................ ................................ 50 2 2 Ba seline machine parameters ................................ ................................ ............ 50 2 3 Thermal scenarios ................................ ................................ .............................. 52 2 4 The evaluated techniques ................................ ................................ .................. 52 3 1 The evaluated NoC design ................................ ................................ ................. 79 4 1 Simulation benchmarks ................................ ................................ .................... 108 4 2 Machine configuration ................................ ................................ ...................... 108 4 3 Simulation scenarios ................................ ................................ ........................ 109 4 4 Optical loss in various components ................................ ................................ .. 109
8 LIST OF FIGURES Figure page 1 1 The structure of a 2D Mesh network ................................ ................................ ... 18 1 2 The architecture of a typical router in Mesh or Torus architecture ...................... 20 1 3 The structure of a ring resonator ................................ ................................ ........ 22 1 4 A typical optical communication system ................................ ............................. 25 2 1 Representative schematics of ring resonator building blocks ............................. 31 2 2 Simplified layout of a ring modulator ................................ ................................ ... 32 2 3 Transmission spectra affected by DC bias voltages and temperature ................ 33 2 4 Impact of temperature shift ................................ ................................ ................. 36 2 5 Placement of temperature detecting resonators ................................ ................. 38 2 6 Photonic network layout ................................ ................................ ..................... 39 2 7 folded torus network augmented with access points ................................ 41 2 8 Schematic diagram of the bias circuit used for compensating small range temperature variations ................................ ................................ ........................ 43 2 9 Paths selected by the proposed routing algorithms under various thermal scenarios ................................ ................................ ................................ ............ 46 2 10 Opera ting system level workload relocation ................................ ....................... 48 2 11 Thermal maps of the generated scenarios ................................ ......................... 51 2 1 2 NoC Latency ................................ ................................ ................................ ....... 54 2 1 3 Average BER of the network ................................ ................................ .............. 58 2 1 4 Average MER of the network ................................ ................................ .............. 59 2 1 5 Comparison of network power consumption ................................ ....................... 59 3 1 An overview of ESPN architecture ................................ ................................ ...... 62 3 2 The VCSEL sources ................................ ................................ ........................... 63 3 3 The network components ................................ ................................ ................... 67
9 3 4 The design of optical switches ................................ ................................ ............ 69 3 5 The request signal in routing examination and forwarding ................................ 74 3 6 An example of blocked link in adaptive routing ................................ ................... 77 3 7 The power latency product (PLP) of different networks ................................ ...... 81 3 8 The number of path establishment attempts ................................ ....................... 83 3 9 Network latency under 128 state MMP synthetic traffic ................................ ...... 84 3 10 Power breakdown on synthetic traffic ................................ ................................ 86 3 11 The normalized power consumption on SPLASH 2 and PARSEC Benchmarks ................................ ................................ ................................ ........ 88 3 12 The normalized energy consumption on SPLASH 2 and PARSEC Benchmarks ................................ ................................ ................................ ........ 88 4 1 A single PCM cell ................................ ................................ ............................... 92 4 2 Th e organization of a rank of memory device ................................ ..................... 93 4 3 The example of 16 mini rank prototype design of the OptiPCM system ............. 95 4 4 The timing penalty caused by rank to rank switch ................................ .............. 99 4 5 The structures of important photonic components ................................ ............ 101 4 6 The structure of a memory controller ................................ ................................ 104 4 7 Finite state machine in the Enhanced Wavelength Assigner ............................ 10 5 4 8 The power modeling of the LPDDR2 NVM ................................ ....................... 109 4 9 The power consumption under different memory states per memory chip ....... 111 4 10 The breakdown of power consumption in OptiPCM ................................ .......... 114 4 11 The latency behavior in different test scenarios ................................ ................ 115 4 12 Normalized memory throughput under different rank number .......................... 117 4 13 The latency under different data bus widths ................................ ..................... 118
10 LIST OF ABBREVIATION S 3DI T hree D imensional I ntegration BER Bit Error Rate BW B ufferwrite CMOS Complementary metal oxide semiconductor CMP C hip multiprocessor CPU Central Processing U nit DDR Double Data Rate DLL Delay Lock Loop DIMM D ual in line memory module DSP Digital Signal Processor DBR D istributed Bragg R ef lector DCD Dynamical channel division DRAM Dynamic random access memory DWDM Dense W avelength D ivision M ultiplexing DQS D ata strobe signals ECC E rror C orrecting C ode ER E xtinction R atio ESPN Energy Star Photonic NoC FCD Fixed channel division FIFO First In, First Out FR FCFS First Ready First Come First Serve FSR F ree S pectral R ange IP Intellectual property IPC I nstructions P er C ycle
11 JEDEC Joint Electron Devices Engineering Council LPDDR Low Power Double Data Rate LPDDR NVM Low Power Double Data Rate Non Volatile Memory LVCMOS Low Voltage Complementary Metal Oxide Semiconductor MER Message Error Rate MMI M ultimode I nterference MMP Markov Modulated Process NRZ Non return to zero N O C Network on Chip OS Operating system PCB P rinted C ircuit B oard PCM Phase Change Memory PHOP Path Hop PSE Ph otonic Switching Elements RC R outing computation RHOP Request Hops RMS Root Mean Square RTD Resista nce Temperature Detector SA S witch allocation ST S witch traversal S O C System on Chip SPF Shortest Distance First SSTL Stub Series Terminated Logic TDM Time Division Multiplexing TF Temperature First
12 TSV Through silicon via ToC Thermo optic coefficient VA V irtual channel allocation VC Virtual Channel VCSEL Vertical Cavity Surface Emittin g Laser WDM Wavelength Divi sion Multiplexing
13 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy ON THE EXPLORATION OF NEXT GENERATION INTERCONNECT DESIGN FOR CHIP MULTI PROCESSORS By Zhongqi Li Dec ember 2012 Chair: Tao Li Major: E lectrical and Computer Engineer ing With the emergence of multi and many core processors, the required bandwidth to support effective on chip communication is expected to gro w rapidly. According to ITRS conventional electrical interconnect will become a power and performance bottleneck for future on chip communication. As a result, photonic Network on Chips ( NoCs ) are drawing increased attention as an alternative to ach ieve low power and high bandwidth interconnects in the multi /many core era Nevertheless, the design of energy efficient photonic NoCs faces many new challenges. Our work exploits several aspects of next generation photonic NoC design. For example, phot onic NoCs are sensitive to ambient temperature variations because their basic constituents, ring resonators, are themselves sensitive to those variations. We propose a thermally resilient photonic NoC architecture design that supports reliable and low bit error rate (BER) on chip communications in the presence of large temperature variations. Also, we advocate the hierarchical photonic NoC architecture to optimize energy utilizati on via a three pronged approach: ( 1 ) b y enabling dynamic resource provisioning,
14 it adapts photonic network resources based on runtime traffic characteristics ; ( 2 ) b y leveraging power efficient routers, it minimizes power used for compensating optical signal loss ; and ( 3 ) b y utilizing all optic al adaptive routing, it improves energy efficiency by intelligently exploiting existing network resources without introducing high latency and power hungry auxiliary routing mechanisms. We also exploit the utilization of photonic channels in the Phase Chan ge Memories (PCMs) t o build a high performance and energy proportional system, the memory devices nee d to be reorganized so that ( 1 ) s maller rank preserves unnecessary power waste in contemporary computer systems with small sized cache lines ; and (2) c oncu rrent operations of phase change memory devices hide the long write access latency.
15 CHAPTER 1 THE INTRODUCTION TO ON CHIP PHOTONIC COMMUNICATI ON Introduction to Interconnect ion Network A n interconnection network is a programmable system that transports data between terminals [ 1 ] An interconnection network is usually programmable in the sense that it makes different connections at different points in time. The network may deliver a message from a terminal to another in one cycle and then use the same resources to deliver a message for other terminals in the next cycle. The network is a system because it is composed of many components: buffers, channels, switches, and controls that work together to deliver d ata. The interconnection network may work at many scales. The o n chip networks deliver data between processor cores caches and arithmetic units within a single processor. The system level networks may tie processors to off chip memories or input ports to output ports. Finally, the local area and wide area networks connect disparate systems together within an enterprise or across the globe. The interco nnection network between proces sor and memory largely determines the memory latency and memory bandwidth, which are two key performance factors, in a computer system. The interconnection network between processors and caches is related to I nstructions p er C ycle ( IPC ) directly. The performance of an interconnection network in a communication switch largely det ermines the capacity (data rate and number of ports) of the switch [ 1 ] Since the demand for interconnection especially on chip communication, has grown more rapidly than the capability of the underlying wires, interconnection is now attracting more attentions as a critical bottleneck in most systems.
16 A Sample NoC Architecture In order to meet the growing computation intensive applications and the needs of low power, high performance systems, the number of computing resources in single chip i s enormou sly increas ing and current VLSI technology can provide support to such an extensive integration of transistors in a single chip Especially, when adding many computing resources such as Central P rocessing U nit s ( CPU s) Digital S ignal P rocessor s ( DSP s) spe cific Intellectual Properties ( IPs ) etc to build a System on Chip (SoC) the interconnection between each other becomes an important bottleneck In most existing SoC applications, a shared bus interconnection which implements an arbitration logic is used to serialize several bus access requests This type of bus solution is usually adopted to communicate with each integrated processing unit due to its low cost and simple control characteristics. However, such a shared bus interconnection has some natural limitation from the perspective of scalability since only one master at a time can utilize the bus This requires serialized communication of all bus accesses controlled by the arbitrator. Therefore, more advanced interconnection schemes should be taken in environment s where the number of bus requesters is large and their required bandwidth for interconnection is beyond current bus solutions The NoC architecture is proposed to address s uch scalable bandwidth requirement issues. The NoC generally uses the on chip packet switched micro network of interconnects. Its basic idea is derived from the traditional large scale distributed computing networks. The scalable and modular nature of NoCs and their support for efficient on chip communication lead to the No C based system implementations. Even though the current large scale network technologies are well developed and their
17 supporting features are excellent, their complicated configurations and implementation complexity make it hard to be adopted as an on chip interconnection methodology. Network Topology In order to meet typical SoCs or multi core processing environment, the basic module of network interconnection like switching elements the routing algorithm and its packet definition should be light weighted to result in the implemental solutions on single chips T he NoC approach has clear advantage s over the traditional busses and most notably system throughput. And hierarchies of crossbars or multilayered busses have characteristics somewhere in between traditional busses and NoC, however they still fall far short of the NoC with respect to performance and complexity. We will use an example to explain the components in a typical NoC system.
18 Figure 1 1 The structure of a 2D Mesh network Figure 1 1 presents a sample NoC structured as a 4 by 4 mesh which provides global chip level communication. Instead of busses and dedicated point to point links, a more general 2D Mesh network is adapted, employing a grid of routing nodes spread out across the chip, connected by communication links. In this dissertation, w e will
19 adapt a simp which the NoC contains the following fundamental components. 1. N etwork adapters implement the interface by which cores (IP blocks) connect to the NoC. Their function is to decouple computation (the cores) from communication (the netwo rk). 2. Switching nodes route the data according to chosen protocols. They implement the routing strategy. 3. L inks connect the nodes, providing the raw bandwidth. They may consist of one or more logical or physical channels. Figure 1 1 covers only the to pological aspects of the NoC. The NoC employ s packet or circuit switching or something entirely different and be implemented using asynchronous, synchronous, or other logic. Router A rchitecture The switching node usually contains a router. The architectur e of th e router is depicted in Figure 1 2 The data transmitted between the processors are usually encapsulated into packets. A typical packet encloses a cache line, an invalidation packet, or part of DMA block data. A packet usually contains the data section and the header. In each router, the incoming packets are first received and stored in an input buffer. Then the control logic circuits in the router makes a routing decision and channel arbitration. Finally, the granted packet will traverse throug h a crossbar to the next router, and this process repeats until the packet arrives at its destination. Each head flit of a packet must proceed through the steps of buffer write (BW), routing computation (RC), virtual channel allocation (VA), switch allocat ion (SA), and switch traversal (ST). A head flit, on arriving at an input port, is first decoded and buffered according to its input virtual channel (VC) in the BW pipeline stage.
20 The VC is a n important aspect of NOC. In the case that a VC splits a singl e physical channel into two channels, it is virtually providing two paths for the packets to be routed. There can be two to eight virtual channels in each physical channel The use of VCs can reduce the network latency at the expense of area, power consump tion, and production cost of the NOC implementation [ 2 ] Figure 1 2 The architecture of a typical router in Mesh or Torus architecture In the next stage, the routing logic performs RC to determine the output port for the packet. The header then arbitrates for a VC corresponding to its output port in the VA stage. Upon successful allocation of a VC, the header flit proceeds to the SA stage where it arbitrates for the switch input and output ports. On winning the output port, the
21 flit then proceeds to the ST stage, where it traverses the crossbar. Finally, the flit is passed to the next node through external links in the link traversal (LT) stage. Body and tail flits follow a similar pipeline except that they simply inherit the VC allocated by the head flit. Thus, the time between the header flit of a packet to be received by the router and the downstream node starts to receive the packet wit hout considering the contention could be computed as: Background of Photonic Communication The structure of r ing resonator s In recent years, the i ntegrated ring resonators have emerged in the last few years in integrated optics and have been applied into many applications. The i ntegrated ring resonators require no facets or gratings for optical feedback and are thus particularly suited for monolithic integration with other components [ 3 ] In this way, t he respon se from coupled ring resonators can be custom designed by the use of different c oupling configurations. Thus the response s from the ring resonator filters can be designed to have both a flat top and steep roll of. A typical layout of the channel dropping filter is shown in Fig ure 1 3 [ 4 ] This can be regarded as the standard configuration for an integrated ring resonator channel dropping filter. In this example, t wo straight waveguides also known as the bus or the port waveguides are coupled either by dir ectional couplers through the evanescent field or by multimode interference (MMI) couplers to the ring resonator. A simpler configuration is obtained, when the second bus or port waveguide is removed. Then the filter is typically referred to as notch fil ter because of the unique filter characteristic.
22 Figure 1 3 The structure of a ring resonator Figure 1 3 shows a Prototype of ring resonator channel drop filter (Ring resonate at frequency and Figure is redrawn from [ 3 ]). The r ing resonator filters can be described by certain characteristics which are also generally used to describe optical filters. One important characteristic is the dis tance between resonance peaks, which is called the free spectral ran ge (FSR) [ 3 ] A simple approximation can be obtained fo r the FSR by using the propaga tion constant where is the propagation constant. The vacuum wavenumber is related to the wavelength through: Using the vacuum wavenumber, the effective refractive index can be introduced easily int o the ring coupling relations By neglecting the wavelength dependency of the effective refractive index
23 This equation leads to th e FSR which is the difference between the vacuum wavelengths corresponding to two peak resonant conditions. This equation is also for the resonant condition next to a resonance found for the propagation constant. In the above equations, is the wavelength, and is the circumference of the ring which is given by where is the radius of the ring measured from its center to the center of the waveguides. Thus the phase Application of ring resonators The communication fabric emerges as the critical performance factor when tens or hundreds of cores are integrated into a single chip. Therefore, a high performance network is essent ial for efficient inter core communication. By sharing channels and paths, packets can be routed to their destinations with optimum bandwidth, latency, and power. However, electrical NoCs do not scale well because of large latencies associated with convent ional RC wires and stringent power requirements [ 5 6 ]. Recently, photonic NoCs have been attracting plenty of attention [ 7, 8, 9, 10, 11, 12 13, 14, 15, 16 ]. Compared to electrical NoCs, photonic NoCs offer higher band width density, lower latency [ 17 ], a nd power consumption that is independent of path length. These characteristics seem to be an answer to the shortcomings of electrical NoCs. Moreover, Wavelength and Time Division Multiplexing (WDM and TDM) allow several channels to share an optical wavegui de for transmitting information, thus
24 increasing bandwidth density. An optical waveguide is a structure constructed from two materials having different refractive indices. This allows the waveguide to confine and guide light waves via total internal reflec tion. Unlike electrical wires, energy is only expended at the end points, which reduces power consumption significantly. Since optical signals travel at a speed close to that of light, latencies are also improved. Recent advances in integrating photonic de vices with microelectronics using current Complementary metal oxide semiconductor ( CMOS ) technology have made possible the realization of high speed, low power modulators, switches, and detectors that are essential to the design of photonic NoCs [ 18, 19 ]. The basic building block for these devices is a ring resonator. Ring resonators are waveguides shaped as rings. Resonance occurs when a ring selectively couples one wavelength from a close by waveguide and ignores the rest. The significance of this abil ity is that ring resonators can act as filters, switches, modulators, and detectors. Unfortunately this ability can be compromised due to the effect of temperature variations on refractive index [ 20, 21 ], causing the resonance frequency to shift. Integra ted silicon photonic technology could be used as an ideal candidate for the large scale connection of the multi core processors due to its low latency and high scalability. Silicon nanophotonics have made complete photonic on stack communication systems a promising alternative to electrical communication systems. Nanophotonics significantly improve the interconnect bandwidth density by approximately two orders of magnitude and yields to over 1 0 power reduction [ 22 ]. Figure 1 4 shows the basic optical commu nication components including the laser source(s), the optical waveguides, the modulators, and the photodetectors. The laser
25 source(s) multiplex a number of different wavelengths of laser lights into a single waveguide, using the dense wavelength division multiplexing (DWDM). One feasible laser source, vertical cavity surface emitting laser (VCSEL), is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface. The modulators then modulate laser lights to carry the optic al bits using the Mach Zehnder interferometer [ 3 ]. The SiGe photodetectors couple and absorb the laser lights at their resonant wavelengths and then convert into current flows to be amplified for the final electrical bits. Fig ure 1 4 A typical optical communication system Apart from the basic photonic components, the turn resonators which are properly tuned couple the traversing optical signals and drop them to the intersecting waveguides. A turn resonator works in a set of resonant frequencies which is derived from its material and structural properties. When the resonant frequency of the turn resonator is different from the traversing wavelength(s) of the optical light(s), the light(s) pass through the waveguide intersectio n uninterrupted (the red light in Figure 1 1 ); otherwise they are coupled into the resonator and dropped to the intersecting waveguide (the green light). The material and structural properties of the passive
26 resonators are predetermined when manufacturing and kept constant during run time, as (e) in Figure 1 4 The frequency of the active resonator could be tuned during run time to support different waveguide connections. The frequency tuning is achieved by adjusting the effective index of the resonator and is generally achieved in one of the following ways. The heat tuning applies or cancels the heat on resonator to change the effective index, which usually requires several microseconds. The electrical tuning applies a voltage on the p n contact and injects electrical current into the resonator to tune the effective index of the ring waveguide as (c), which requires ~100 ps. Another way is to apply optical pump pulses to inject free carriers through two photon absorption inside the ring resonator and hence t une the effective index of the ring resonator as (d) [ 23 ]. The optical pulse tuning has the lowest latency among the three (~40 ps) [ 24 ], and is suitable to control the distant resonators which otherwise suffers extra delay from electrical control wire. In our design we apply the electrical tuning at the memory controller while the optical tuning at the memory device.
27 CHAPTER 2 THE THERMALLY RESILI ENT PHOTONIC NETWORK ON CHIP ARCHITECTURE A Characterization of Thermal Impact on Photonic NoCs While proposed architectures [ 13, 14, 15, 16 ] employ a photonic network layer placed on the top of a silicon chip, we use the design proposed in [ 16 ] as a representative 3D chip to characterize the impact of thermal variations on the reliability of photoni c NoCs. In this section we first describe the photonic network architecture; we then discuss BER as an indicator of thermal effects, quantify the BER due to temperature variations, and address temperature sensing issues. Our simulated architecture is based on 3D integration, where a photonic NoC is implemented as a layer of optical devices on the top of the silicon chip. Such an arrangement reduces fabrication complexity, chip dimensions, and total cost. A 2D folded torus hybrid NoC topology is used in our study since it is compatible with the tiled chip multiprocessor ( CMP ) chip, allows the use of low radix switches, and allows light waves to intersect without significant cross talk The hybrid NoC architecture [ 16 ] combines a photonic circuit switched netw ork with an electrical packet switched control network to reduce power consumption while achieving high bandwidth and low latency. In this study we assume a 2D 30 core processor tiled in a arrangement. To transmit a message, a path setup packet is firs t sent on the electrical control network. As the packet is routed through the network, it reserves the corresponding photonic switches along its path. Once the optical path is established, the message is transmitted through the photonic network.
28 M otivation In the ring resonator based photonic NoC, r esonance occurs when a ring selectively couples one wavelength from a close by waveguide and ignores the rest. The significance of this ability is that ring resonators can act as filters, switches, modulators, an d detectors. However, this ability can be compromised due to the effect of temperature variat ions on refractive index [ 20, 21 ], causing the resonance frequency to shift. Because a variation in temperature causes a change in the refractive index, it can pot entially disrupt the proper operation of photonic devices. For instance, ring resonators can be brought in or out of resonance due to a small variation in temperature. A resonance shift of 0.11 nm/K has been reported in ring resonators [ 21 ]. In addition, prior work [ 25 ] has reported high BER when a thermal shift as small as several degrees K caused a significant shift from the base resonant wavelength. Thus, small temperature variations can introduce large BER, or even cause faulty operatio n in photonic NoCs. Conventionally, metal strip heaters em bedded around ring resonators [ 26 ] or overlaid on top of the silicon oxide cladding [ 27 ] are used to control the temperature of the resonators. However, these heaters require substantial electrical tuning power, exacerbate on chip thermal effects, and are not suitable for use in large scale photonic NoCs due their bulkiness and extensive wiring requirement. Other methods resort to overlaying a polymer coating with negative tempera ture coefficient [ 28 29 ]. Unfortunately, polymer is not compatible with CMOS processes yet. The ITRS Roadmap [ 30 ] projects that three dimensional chip stacking for three dimensional integration (3DI) is a viable solution for latency and power dissipation limitations. Hybrid photonic/electrical NoCs [ 13, 14, 15, 16 ] have been proposed to be built on a separate layer on top of the core layer with through silicon vias (TSVs) connecting the two layers.
29 Although latency and power dissipation are improved, thermal effects are compo unded due to heat generated by other layers. Since heat is not easily removed from multi layered integration, techniques to counter thermal effects on photonic NoCs at the architectural and operating system levels become imperative. To mitigate temperatur e effects on photonic NoCs, we propose Aurora, a thermally resilient photonic NoC architecture design that can tolerate a wide range of temperature variations. Our proposed cross layer solution targets the device, architecture, and operating system layers where each can significantly improve the reliability of the photonic NoC. More attractively, combining our proposed techniques provides significant reliability improvements and, as a side benefit, better power efficiency. Our first proposed technique deals with temperature variations within a small range. To achieve this at the device level, we adopt the method proposed in [ 25 ], which varies the bias current through a ring resonator to compensate for small local temperature variations. At the architectural level and for thermal variations across a large range, we propose to reroute the messages through cooler regions of the chip to their destinations. At the Operating system ( OS ) level, we use thermal/congestion aware co scheduling to reorganize the thermal profile of the chip to further lower BER. To the best of our knowledge, w e present the first effort on improving thermal reliability of photonic NoCs at the architecture and operating system levels. Structure of Thermal Resilient Photonic NoC System Ring r esonators are not only applied in the optical networks, the resonators have recently been demonstrated to be used as sensors and biosensors as well Extensive research strived to create optical devices that can modulate, guide, and detect light signals eff iciently while leveraging current CMOS processes. Of those devices, ring
30 resonators are finding wide acceptance in the photonic and architecture communities for serving as a basic building block for various photonic circuits ranging from modulators to swit ches and multiplexers. Their compact size, low power consumption, low insertion loss, and high extinction ratio (ER) per unit length, make them ideal for use in on chip optical networks [ 25 ]. In this section, we give an overview of the structure and operat ion of ring resonators, the role of the refractive index, and the effects of temperature variations on their operation. In our simulated architecture, we assume that 64 wavelengths are used for modulation, resulting in 64 modulators and 64 photodetectors for a total of 128 ring resonators per core. A total of 4680 ring resonators are used to build the photonic network. In order to increase bandwidth density, path multiplicity can be used, where additional parallel waveguides are added to the network. Thes e new paths will need additional modulators, multiplexers, demultiplexers, photodetectors, and switches. This will dramatically increase the number of ring reso nators used in the photonic NoC A typical example of photonic NoC is shown in Figure 2 1.
31 F igu re 2 1 Representative schematics of ring resonator building blocks: (a) Switch : resonator fails to divert a light signal, (b) Multiplexer : resonator fails to add a light signal to a waveguide bus, (c) Demultiplexer : resonator succeeds in removing a li ght signal from a waveguide bus, and (d) Modulator : modulator encodes erroneous data on a light stream (green) Impact of Temperature on Ring Resonators A ring resonator is built by placing a ring next to a straight waveguide, as shown in Figure 2 2 wavelength traveling through the straight waveguide. The index of refraction of the materials that form the ring waveguide plays an important role in determining the resonance frequency Resonance occurs when coupled light circulates inside the ring and is reinforced by interference while light traveling in the waveguide is suppressed. Changing the refractive index changes the resonance frequency. To control the refractive index, the met hod of free carrier injection [ 31 ] is used due to its high speed. In this method, two highly doped regions that form a PIN junction surrounding the ring are built to form a modulator [ 32 ], as shown in Figure 2 2 By applying a voltage Vm to the P
32 and N regions, free charge carriers are injected into the ring, causing its effective refractive index to change. By injecting more free carriers into the ring, the refractive index decreases. On the contrary, extracting free charge carriers increases the refrac tive index. Thus, PIN carrier injection and extraction effectively modulates the refractive index of the ring resonator. A ring resonator can be in one of two states. In the ON state, there are no free carriers in the ring since the PIN junction is reverse biased. By design, the resonance wavelength of the ring is same as the wavelength of the light, hence resonance is ON, and light is coupled into the ring. This coupling causes the optical signal to circulate inside the ring, and prevents the signal from p assing through the waveguide. In the OFF state, the PIN junction is forward biased, and thus free carriers are injected into the ring. The injection of free carriers changes the refractive index and in turn shifts the resonance wavelength. Since the resona nt wavelength is now different from the wavelength of the light signal, resonance is OFF and the light continues its path unobstructed through the straight waveguide. Figure 2 2 Simplified layout of a ring modulator
33 As described above, resonance occu rs only at some specific frequencies where light is coupled into the ring. The wavelen g th at which resonance occurs [ 20 ] is governed by: (2 1) wh ere is the effective refractive index of the optical mode, is equal to where is the radius of the ring, is an integer number, and is the resonant wavelength. A shift of the effective refractive index results in a shift of the resonan t wavelength [ 21 ]: (2 2) where is a c hange in the resonant wavelength, is the resonant wavelength, is a change in the effective refractive index, and is the effective refractive index. Figure 2 3 Transmission spectra affected by DC bias voltages and temperature (a) Transmission spectra of a modulator under two different DC bias voltages, (b) Transmission spectra shifts due to changes in temperature
34 Figure 2 3 (a) shows the transmission spectra of a modulator at a nominal operating temperature The figure shows that for a small positive increase in bias current, the spectrum shifts to the left due to the decrease of the refractive index of the silicon caused by the injection of free carriers in the ring. Consequently, resonance occurs at a shorter wavelength than the original one. This shift means that a light wave at the original wavelength of will be allowed to pass since it has a high transmission value. Before the shift, its transmiss ion value at was small, and the wave was suppressed. Let where is a change in the refractive index, and is the new resonant wavelength. Substituting in Eq (2 2) gives where is a change in the resonant wavelength. In addition to the carrier injection method described above, the refractive index can optic effect [ 3 3 ], ring resonators are sensitive to temperature variations. The thermo optic coefficient (TOC) is given by As in the carrier injection case, temperature variations also affect the refractive index and result in shifting the resonance wavelength. A resonance shift of 0.11 nm/K from the original resonance wav elength has been reported in [ 21 ]. Such resonance shifts are undesirable and increase the BER in systems that use resonant electro optic modulators and switches. Figure 2 3 (b) shows transmission spectra shifts due to 2 K and 4 K temperature shifts. The original spectrum is at a nominal operating temperature and constant bias current. It is interesting to point out that temperature variation and free carrier injection have opposite effects on the resonance frequency. For example, an increase in
35 temperature causes an increase in the refractive index, and a corresponding shift of the spectrum towards the right. Thus, it is possible that electro optic and thermo optic effe cts can compensate each other. Undesirable thermal shifts will cause large BER and even faulty operation of a photonic NoC. With a rise in temperature, rings will not resonate at the intended frequency. Modulators, switches, multiplexers, and demultiplexer s will produce erroneous outputs if thermal shifts are not addressed. Figure 2 3 illustrates several scenarios showing the intended and the actual outputs when a ring fails to resonate at the intended frequency due to a rise in temperature. In 3D packaging the photonic network is usually implemented on top of the core layer. It experiences larger non uniform temperature variations, depending on the temperature of the cores below. Since the photonic layer consists of thousands of ring resonators, the operat ion of the photonic network will be drastically compromised by the variations in temperature. As described in previous section these variations affect the refractive index of the ring resonators, causing the transmission spectra of the resonators to shift unpredictably. For example, a few degrees rise in temperature can cause a photonic switching element to malfunction by diverting light when it should not. To obtain eye diagrams and BER of ring resonators, we simulated optical links with OptiSystem, an op tical communication system simulation software [ 34 ]. The simulated optical channel consists of a VCSEL source, signal generator that generates 10 Gbps pseudorandom Non return to zero ( NRZ ) code, a modulator to modulate the NRZ code to optical signals, int ermediate resonator, and demodulator. The resonance frequency was varied to simulate the effect of temperature variation.
36 Figure 2 4 Impact of temperature shift (a) BER versus temperature shift, (b) Eye diagrams for various temperature shifts As seen in Figure 2 4 (a), BER increases with variation in temperature and reaches 10 12 at a temperature variation of ~ 3.5 degrees K. This value is sufficient for reliable on chip communication [ 35 ]. Figure 2 4 (b) shows the eye diagrams for different temperature variations. Eye diagrams are used to qualitatively examine signal integrity and signal to noise ratio in a communication system. As the temperature varies, the quality of the eye diagrams deteriorates indicating reduced s ignal integrity. To obtain runtime chip temperature, we ran multi core oriented workloads on a cycle accurate, multiprocessor simulator and the generated power traces are then fed into HotSpot [ 36 ]. We modified Garnet [ 37 ] to simulate the photonic NoC. We used average BER as an indicator to provide a measure of how temperature variations affect the operation of our simulated photonic network. We obtained BER along the optical path by evaluating the temperature of the involved photonic devices. We observed t hat if temperature variations were left unaddressed, the average BER across the network would be unacceptably high (greater than 10 1 ) and all messages would be corrupted
37 during transmission, implying the need for a thermally resilient photonic NoC archite cture. Temperature detecting Resonators The temperature information of resonators is necessary for maintaining their initial operating conditions. Integrated temperature sensors like thermistors and Resistance Temperature Detectors (RTDs) are usually used to measure the temperature within a chip. However, these conventional integrated sensors require large areas, making them unsuitable for large scale photonic networks that contain thousands [ 16 ] or even millions [ 15 ] of ring resonators. In Aurora, we empl oy resonators to measure temperature [ 38 ] because of their small area overhead and compatibility with CMOS technology. In these resonators, the amplitude of the output is related to temperature variation. Resonators used for temperature detection are coupl ed to waveguides through splitters to minimize signal loading. In the implementation, the output signal of a resonator is amplified and converted by a Root Mean Square ( RMS ) detector into DC current whose level indicates the amount of frequency shift (Figu re 2 5. (c)). A temperature detecting resonator along with its detection and control circuitry are deployed in each switch and modulator set. Due to the small size of modulator sets and switches (around 640 and 70 m in diameter), we assume that the temper ature measured is the temperature of the whole set or switch. The placement of these resonators within the modulator sets and the switches is shown in Figures 2 5 (a) and (b).
38 (a) (b) (c) Figure 2 5 Placement of temperature detecting resonators (a) Modulator/demodulator sets, (b) Switches (detection and control circuits not shown) (c) Temperature detecting circuit Photonic Network Architecture To char acterize the impact of thermal variations on the reliability of photonic NoCs, we assume an optical network similar to the one proposed in [ 16 ]. In this section we first describe the photonic network architecture, we then discuss BER as an indicator of the rmal effects and quantify the BER due to temperature variations. Our simulated architecture is based on 3D integration, where a photonic NoC is implemented as a layer of optical devices on top of the silicon chip. Such an arrangement reduces fabrication c omplexity, chip dimensions, and total cost. A 2D folded torus hybrid NoC topology is used in our study since it is compatible with the tiled CMP chip, allows the use of low radix switches, and allows light waves to intersect without significant cross talk. The hybrid NoC architecture combines a photonic circuit
39 switched network with an electronic packet switched control network to reduce power consumption while achieving high bandwidth and low latency. Figure 2 6 Photonic network layout (a) folded torus network, (b) Access point (c) Modulators and detectors, (d) switch In this chapter we assume a 2D 30 core processor tiled in a arrangement. The detailed processor, memory and NoC configuration can be found in Section 5. Figure 2 6 (a) shows the layout of the 2D grid of optical waveguides with switches at the intersection points. An electronic sub network of similar layout (not shown) is overlaid on the photonic network. This network is used for control and short messages. Each core connects to the photonic network through an access point. Access points enable the injection and ejection of messages without interference with through traffic, and avoid blocking between injected and ejected traffic. Figure 2 6 (b) provides a magnified in view of an access point excluding a torus switch. As can be seen, an access point consists of a gateway and 3 switches. A gateway, shown as Figure 2 6 (c), acts as a
40 photonic network interface which connects each core to the folded torus network. A gatewa y converts electronic signals to optical and optical signals to electronic. It contains optical modulators and detectors whose structure is based on ring resonators. A gateway is connected to a switch through its West port, while the other 2 switches are f or injection and ejection of messages. As shown in Figure 2 6 (d), injection, ejection, and torus switches are switches controlled by an electronic router. Each switch is made of four Photonic Switching Elements (PSE). A PSE is a switching elem ent capable of switching the direction of a light signal. It is based on a ring resonator structure where two rings are placed at the intersection of two waveguides. Figure 2 7 illustrates the topology of the simulated folded torus network augmented with the access points. Access points comprise injection and ejection switches that lie on additional waveguides to facilitate injection and ejection. A gateway and switch unit is connected to two injection switches and one ejection switch. Torus switches are used to route messages between the cores. To transmit a message, a path setup packet is first sent on the electronic control network once the destination address is known. As the packet is routed through the network, it reserves photonic switches along the path to be followed by the photonic message. A next hop decision is made at every router along the path, depending on the routing algorithm. The process of reserving the photonic path is completed when the packet reaches its destination. To indicate t hat a path is now open, a short light pulse is transmitted through the waveguide back to the source. The source realizes that the optical path is established and sends out the message through the photonic network. At the end of the message, a path teardown packet is sent to release all resources and free the path. An acknowledgement packet may be sent on the electronic control network if guaranteed delivery is requested.
41 Figure 2 7 folded torus network augmented with access points In our simulated architecture, a single core contains four switches and one gateway. We assume that 16 wavelengths are used for modulation, resulting in 16 modulators and 16 receivers for a total of 32 ring resonators. A total of 1920 ring resonators a re used to build the photonic network. In order to increase bandwidth density, path multiplicity can be used, where additional parallel waveguides are added to the network. These new paths will need additional modulators, multiplexers, demultiplexers, phot odetectors, and switches. This will dramatically increase the number of ring resonators used in the photonic NoC. Thermally Resilient Photonic NoC Architecture We propose a holistic approach to mitigate the effect of temperature variations on the operati on of photonic NoCs. Our techniques target circuit, architecture, and OS levels respectively. For small temperature variations, we adopt a circuit level technique
42 [ 25 ] that adjusts the bias current flowing through ring resonators to locally compensate for thermal effects. At the architecture level and for larger temperature variations, we reroute messages away from higher temperature regions through cooler regions to t heir destinations. At the OS level, we employ a thermal/congestion aware co scheduling technique to further reduce BER. More attractively, our solutions at the circuit, architecture, and OS levels can be further integrated with each other to reduce BER. Circuit level Technique We use the circuit level technique proposed in [ 25 ] to combat temperature variations within a small range (e.g. 15 K). The heat generated by the flow of an appropriate DC bias current through a ring resonator is used to maintain th e original operating conditions. The amount of Joule heat generated in the device is proportional to the value of the bias current. As the temperature varies, the bias current is varied to compensate for changes in local temperature in order to maintain th e resonant frequency at its original value. Figure 2 8 shows the schematic diagram used to control the bias current through a PIN resonator based modulator. Only sectors of the ring and the N region are shown for clarity. A bias tee network combines a modu lating signal with the DC bias to modulate the refractive index of the resonator via free carrier injection and extraction. The inductor and the capacitor provide isolation between the DC bias and the RF bit generator inputs. In [ 25 ], the modulation was ma intained for a temperature rise of 15 K by changing the base operating condition from 1.36 mA at 0.2 V to 345 A at 2.2 V bias. In nominal operation, reducing the bias current does not have an effect on the modulation process since the high speed RF signal injects the required amount of carriers to perform switching. The use of this technique is limited to small variations in temperature since the amount of wavelength shift using the free carrier
43 injection and extraction method is limited to about 2 nm. In contrast, the amount of wavelength shift due to temperature variations can be up to 20 nm [ 39 ]. Figure 2 8 Schematic diagram of the bias circuit used for compensating small range temperature variations Architecture level Technique The circuit level solution could mitigate the impact of small variations in temperature. However, due to the variance of running workloads, some regions of the chip area may experience temperature variations beyond the compensation range of the circuit lev el solution. We propose re routing messages away from resonators within these regions, and through cool regions to their destinations. We propose two techniques based on the shortest distance algorithm: shortest path first (SPF) and temperature first (TF). SPF selects the path with the lowest MER among all shortest paths available. On a tie, the algorithm selects the path with the lowest utilization. TF selects the path with the lowest temperature (i.e. the lowest MER path between source
44 and destination) wh en the circuit level technique is unable to compensate. On a tie, the algorithm considers route length and route utilization in order to mitigate link delay and avoid congestion. Figure 2 9 illustrates the routes generated by the proposed algorithms under various thermal scenarios. The regions where the DC bias current was able to compensate for the resonance frequency shifts are indicated in white. The regions that are beyond the compensation range of the DC bias current are indicated in orange. Source an d destination nodes are indicated in blue. The paths selected by SPF distance path to the destinat ion by avoiding hot regions and hence incur low MER. Messages that fail to find a cool path towards the destination incur a higher MER than messages that succeed. Messages that fail to be delivered are retransmitted after a timeout period. The routing pat h is calculated by the source node. In order to compute a routing path, the source node gathers temperature information of the resonators, which is distributed to all nodes through the electrical network. Before sending a packet, the source node first calc ulates the MER at each resonator along the routing path according to where is the BER of one resonator and is the number of bits in one message Then, the MER is obtained by multiplying all MER s for each reson ator on that path, i.e. where is the number of resonators in that path. After that, the source node performs either SPF or TF algorithm utilizing as the weight of a path. Then the source node selects the path with the min imum weight among the shortest distance paths (SPF) or the path with the minimal
45 weight among all paths (TF). Aurora employs an electrical/optical hybrid network structure, and path establishment is performed via the electrical network, so it is reasonable to assume that no error occurs when establishing the path. Deadlock in the electrical network can be avoided by using virtual channel flow control. On the other hand, the photonic network is inherently deadlock free due to circuit switching and predetermi ned routing path. Livelock is also avoided due to the predetermined routing path. Note that as the number of cores increases, the number of paths available for transmission also increases. Therefore, it is expected that the proposed routing algorithms sca le well in large scale multi /many core systems. However, if the source and/or destination cores are located in hot regions themselves, a high MER is inevitable regardless of the selected path. In these situations, thermal management solutions such as dyn amic clock disabling and dynamic frequency scaling can be invoked to halt or power off hot cores for a period of time [ 40 ] to guarantee reliable communication.
46 Figure 2 9 Paths selected by the proposed routing algorithms und er various thermal scenarios Operating System level Technique To further mitigate the effect of temperature variations on the photonic network and reduce the MER, we propose a thermal/congestion aware co scheduling scheme at the operating system level. T he operating system distributes workloads across the multi core substrate in order to reorganize the temperature profile of the chip. The OS prioritizes the outer cores of the chip rather than the inner cores when mapping the workloads to the cores. Usual ly, a set of related workloads occupies adjacent cores and the communications demand within that set is high. We treat related workloads as one set when performing thermal/congestion aware co scheduling. Figure 2 10 (a)
47 shows a scenario in which this co T4). Workload sets can be rotated when necessary as in the case of T3. If the outer cores are already occupied by other workloads, reschedulin g will only be performed when a workload set can be mapped as a block in order to maintain efficient communication among the set. Figure 2 10 (b) shows the pseudo code of the co scheduling algorithm. This thermal/congestion aware co scheduling algorithm pr ovides two benefits: First, relocating workloads to the edges of the chip helps reduce both peak and average chip temperatures since the edges of a chip are more efficient in transferring the heat to the ambience than the center of the chip. Second, chip u tilization and performance are increased. Due to fragmentation, a new workload may be prevented from being allocated to contiguous cores, resulting in increased communication latency. Maintaining the shape of the workload sets and preferentially mapping wo rkloads to the outer cores alleviates the impact of fragmentation. Third, the utilization of links located on the edge of the chip is increased. Using traditional adaptive routing algorithms, messages tend to be routed through the center of the chip, resul ting in significant congestion in that area [ 1 ]. With co scheduling, workloads at outer cores may take advantage of side links within a chip. However, the average packet travelling distance will be increased after applying this co scheduling technique. For tunately, as we will show in next section this drawback could be largely compensated by photonic networks due to the inherent high speed and low power nature of light. This makes our thermal/congestion aware co scheduling highly suitable for photonic netw orks.
48 Figure 2 10 Operating sy stem level workload relocation (a) Relocation of workloads by applying co scheduling (b) Pseudo code for co scheduling algorithm Experimental Setup In our study, we used Simics and GEMS simul ation frameworks. Simics [ 41 ] provides a full system, functional simulation framework whereas GEMS [ 42 ] provides a cycle accurate timing simulator which models timing of multiprocessor memory systems. We used GARNET [ 37 ], which is a detailed cycle accurate on chip network model incorporated inside the GEMS framework, and extended it to support the proposed Aurora architecture. All simulations are performed on the 56 network. Table 2 1 summarizes the parameters of the simulated chip. We evaluated our techniques using a set of representative synthetic traffic patterns (i.e. uniform random, transpose, bit complement and tornado [ 1 ]). Garnet generates traffic during a period of 1 million cycles (including 1K warm up cycles). We assume that the E/O and O/E conversions are carried out at 640 Gbps (64 wavelengths, 10 Gbps each). Since the time needed to
49 establish an optical path is quite costly, especially under heavily loaded situations, the size of messages in photonic networks should be larger than those in traditional electrical networks to increase network performance. Nevertheless, extraordinarily large messages may block the network due to the lack of virtual channels and buffers in the photonic network. Thus, in our simulations, we set the maximum message size to 13312 bits, which is a trade off between link efficiency and blocking probability. Consequently, maximum message transmission time on the photonic network is 208 cycles. We simulated a 30 core processor with a shared 2 M Byte cache to generate the temperature profiles. We assume 3GHz frequency and a 45 nm technology with a supply voltage of 1.2 V. Each core is 4 mm 4 mm for a total chip area of 20 mm 24 mm. The baseline p rocessor and memory architecture are summarized in Table 2 2 To evaluate our proposed techniques, we modeled all of the above components. To evaluate the efficiency of our proposed schemes under a wide range of temperature profiles, we constructed various thermal scenarios using the method. Table 2 3 summarizes the characteristics of thermal scenarios used to evaluate our techniques. Figure 2 11 presents the thermal map of each generated scenario.
50 Table 2 1. Chip parameters Number of cores 30 arran ged as 56 in a folded torus Convection resistance 0.07 K/W Convection capacitance 240.4 J/K Area of demodulator/photodetector set 660 m 40 m Area of switch 70 m 70 m Number of resonators in demodulator/photodetector set 3840 Number of resonators in switches 720 Number of resonators in temperature detecting units 120 Total number of ring resonators 4680 Table 2 2 Baseline machine parameters Parameter Configuration Width 4 wide fetch/issue/commit IQ, ROB, LSQ 64 Issue Queue, 96 ROB entries, 48 LSQ entries TLB 128 entries(ITLB), 256 entries(DTLB), 4 way, 200 cycle Branch Pred. 2 K entries Gshare, 10 bit global history, 32 entries RAS I/D L1 Cache 64 KB, 4 way, 64 Byte/line, 2 ports, 3 cycle Integer ALU 4 I ALU, 2 I MUL/DIV, 2 Load/Store FP ALU 2 FP ALU, 2 FP MUL/DIV/SQRT L2 Cache Private 512K 4 way, 128 Byte/line, 12 cycle Evaluation Results In this study, we evaluate the reliability and performance characteristics of the proposed Aurora architecture using different architecture and OS level thermal management schemes. Table 2 4 summarizes the evaluated techniques. We assume that the circuit level technique is always activated to achieve thermal stability on small range temperature variations.
51 (a) Scenario 1 (b) Scenario 2 ( c ) Scenario 3 ( d ) Scenario 4 ( e ) Scenario 5 ( f ) Scenario 6 Figure 2 11 Thermal maps of the generated scenarios
52 Table 2 3 Thermal scenarios Scenario Synopsis S1 Center b lock A block of hot cores in the center force traffic to use the edges as paths F i gure 2 9 (a) S2 Corner b lock More than half of the hot cores are located at the corner, Figure 2 9(b) S3 Winding p ath Hot regions force traffic to follow a winding path to destination, Figure 2 9( c ) S4 Narrow s trait Hot regions on both side s, dividing the processor into two sections Figure 2 9( d ) S5 Random 1 Randomly generated hot regions F i gure 2 9 ( e ) S6 Random 2 Randomly generated hot regions F i gure 2 9 ( f ) Table 2 4 The evaluated techniques Scheme Routing Algorithm OS level Technique SD (Baseline) Shortest distance No SD +OS Shortest distance Yes SPF Shortest Path First No TF Temperature First No SPF+OS Shortest Path First Yes TF+OS Temperature First Yes NoC Latency Figure 2 1 2 shows the average latency of the simulated photonic NoCs under four traffic patterns (uniform random, transpose, bit complement and tornado) and various thermal management techniques. As described in previous section the architecture level techniques dec rease the average BER but can introduce additional congestion since messages tend to traverse through cool regions. In general, we observed that the average network latency increases by 5 50% compared to the baseline cases. In addition, we found that the w orst performance occurs when a hot region occupies a significant fraction of the chip area and leaves only narrow straits for message routing, as shown in scenarios 1 and 4 in Figure 2 11 The network latency in these cases increases by 1 ~ 4 times. The ne twork latency of SPF falls in between those of the TF
53 and the baseline, since the SPF takes both the path length and BER into consideration. Figure 2 10 further shows that in most cases, network latency can be reduced by combing the OS level technique with architecture level technique. Compared with the SPF and TF cases, the average latency reductions of SPF+OS and TF+OS are 6% and 27% respectively. This is because our proposed OS level technique diminishes the high temperature regions within the chip and h ence provides additional routing alternatives. Note that if we retransmit the messages which are ruined by errors, it may incur additional latency overhead. In this case, the latency of the baseline case would increase significantly more than our proposed techniques. This is because our proposed architecture and OS level techniques dramatically reduce the MER (as will be shown in the next subsection), thus reducing the message retransmission probability. BER and MER Figure 2 1 3 shows the average BER for ou r simulated photonic NoC using various thermal management techniques. The first three bars in each group represent the BER after applying architecture level techniques (i.e. SD, SPF, and TF). The next three bars show the BER after applying both architectur e and OS level techniques, (i.e. SD+OS, SPF+OS, and TF+OS). As indicated, BER is reduced by 10% and 49% after applying the architecture level technique (SPF and TF) alone. On average, combining the architecture and OS level techniques can further reduce B ER by 93% and 92% for SPF+OS and TF+OS respectively.
54 (a) Scenario 1 (b) Scenario 2 ( c ) Scenario 3 ( d ) Scenario 4 ( e ) Scenario 5 ( f ) Scenario 6 Figure 2 1 2 NoC Latency We observed that in Figure 2 1 3 the average BER of the SD (baseline) case in scenario 1 is about 110 3 while it is 210 4 for scenario 2. This indicates that the average BER depends on the thermal map of the chip. The high BER in scenario 1 is attributed to the routes traversing the high temperature region in t he center of the chip. After applying the architecture level technique to scenario 1, BER is significantly
55 reduced compared to scenario 2 since more messages are rerouted through the cooler paths. Furthermore, applying the OS level technique provides more cool paths through the center than scenario 2 by relocating high temperature regions to the outer cores. Among the SD, TF and SPF cases, TF achieves the best BER performance followed by SPF. This is because TF depends upon the heat distribution in the netw ork, and thus tends to route messages through the regions with least MER; whereas SPF uses temperature information as well as the number of hops from source to destination. There is a tradeoff between delay and error rate improvement shown by SPF and TF al gorithms. For cases with high congestion, TF shows more improvement in BER 60% 80% at the expense of increasing network delay. The above observations are also valid for TF+OS and SPF+OS cases. We also recorded the average MER which indicates the ratio of m essages that fail in delivery to total messages as shown in Figure 2 1 4 SPF and TF show 6% and 30% improvement compared to the baseline case, whereas SPF+OS and TF+OS can achieve 76% and 84% improvement on average in our simulation scenarios. Power Consu mption Total power consumption in Aurora is mainly attributed to: 1. Heat generated by the DC bias current (direct localized heating) for each ring resonator 2. Energy consumed by the network f or the transmission of messages The static energy of the netwo rk is also converted to a per bit scale and integrated into part 2 as in [ 43 ]. Compared to conventional metal strip heaters, maintaining the operating temperature by varying the DC bias current co nsumes about 50% less energy [ 25 ] due
56 to direct localized he ating. The metal strip heaters are implemented in a metal layer atop the photonic layer. Due to the top cladding oxide between the metal layer and the waveguides, the metal strips cannot directly heat the resonators and thus are power inefficient. In contr ast, the DC bias current provides localized heating in the PIN junction surrounding the resonator and thus is more efficient. In our simulation, we employ one metal strip heater for each ring resonator. We assume that the size of the heater is 2 m 2 m 5 m and its surface heat release rate is 1 mW/m 3 The thickness of the top cladding oxide is assumed to be 1 m. We also modeled the power consumption for both the electrical and photonic networks. For the electrical network, the dynamic power consume d due to data transmission is obtained through ORION [ 44 ]. The total power consumed on our 2D 56 mesh electrical network is calculated as in [ 43 ]. For the photonic network, the resonators consume energy when free carriers are injecting into the rings. The in plane Poly Si energy consumed is 100 fJ/bit [ 45 ]. Assuming advanced driver circuits with poly Si carrier lifetimes of 0.1 1 ns and modulation speed of 10 Gbps, the power consumed by each modulator is approximately 200 fJ/bit [ 45 ]. The energy consumptio n is also related to link MER since retransmission of messages which fail in delivery will cost additional energy. For six thermal scenarios, we compare the power consumption of a network using conventional metal strip heaters to a network using the DC bia s control method, as shown in Figure 2 1 5 The DC bias current driven heater is about twice as power efficient as the conventional metal strip heaters. Since applying our architecture and OS level techniques reduce MER, message retransmission ratio decreas es which
57 further reduces the power consumption of Aurora. On average, the DC bias method consumes 33% less total power than the metal strip heater. Moreover, by leveraging the architecture level and the OS level co scheduling techniques, Aurora could furt her save another 4% power (TF+OS sch s eme) because of decreasing message retransmissions.
58 Figure 2 1 3 Average BER of the network
59 Figure 2 1 4 Average MER of the network Figure 2 1 5 Comparison of network power consumption. conventional metal strip heaters to compensate for temperature variations in resonators.
60 CHAPTER 3 THE ARCHITECTURE OF HIERACHICAL PHOTONIC NOC Motivation Unlike electrical NoCs, stati c power dominates the overall photonic NoC power budget (e.g. 75% reported in [ 14 ]). Worse, the energy conversion efficiency of the laser sources is low (e.g. 50% reported in [ 46 ]), which further aggravates the total power loss. While the static power of photonic NoCs is fixed owing to the predetermined network design and the constant laser source injection, the network traffic manifests sub stantial runtime variation [ 47, 48 ]. When the traffic is below the provisioned network bandwidth, the NoCs will manif est a significant static power overhead. Furthermore, a large portion of the laser power is lost when traversing through the ring resonators along the traffic path. The optical switches in photonic NoCs contain modulators, photo detectors, and turn resonat ors; all of which are made from ring resonators. These ring resonators act as band pass filters, causing pass band attenuation and power loss on the traversing optical signals. For instance, the intermediate ring reson ators within optical switches [ 13 43 ] can cause 40% optical power loss in an 88 mesh network. In addition, these ring resonators have to be thermally tuned to function, which incurs significant heating power. The above static power overheads make deploying on chip optical components (e.g. ri ng resonators and waveguides) power expensive, and demand a good utilization of the provisioned network resources. Moreover, due to the lack of optical logic gates and storage, existing photonic NoC routing approaches are either static or relying on additi onal components (such as duplicated optical networ k s [ 49 ] and electrical buffers [ 13 ]) to achieve adaptivity. These methodologies, however, fail to exploit existing
61 photonic network resource effectively and increase the overall NoC latency and power due to the inclusion of auxiliary components. In summary, the emergence of photonic NoCs calls for a new set of techniques to optimize their energy efficiency. To this end, we propose ESPN, an energy star photonic NoC architecture. Specifically, we make the fol lowing contributions: 1. We propose a dynamic photonic NoC design that allows network resources to adapt with run time traffic characteristics. In our design, the network resources are partitioned and supplied with separate laser sources to enable dynam ic network resource management strategies via traffic aware bandwidth provisioning. 2. We propose a power efficient router design (e.g. RapidEngy), which alleviates the impact of power loss on the traversing signals due to the intermediate modulator/phot o detector arrays. 3. We propose all optical adaptive routing to accelerate data communication. Our adaptive routing leverages low latency optical links to establish data paths and thus avoids introducing high latency and power hungry auxiliary routing components. The Proposed Hierarchical Photonic NoC Architecture An Overview of Hierarchical Photonic NoC Architecture Figure 3 1 provides an overview of the proposed Hierarchical Photonic NoC architecture design. Hierarchical Photonic NoC is an architecture targeting future high throughput systems, so our exploration and evaluation targets 22nm technology [ 50 ]. Hierarchical Photonic NoC consists of one multi processor chip and two laser source chips connected by off chip optical fibers and electrical wires on the P rinted C ircuit B oard ( PCB ) The multi processor chip consists of three vertically stacked dies using 3D packaging technology [ 17 ]. The processor & caches die contains processor cores, private L1/L2 caches and electrical routers. The control die, which operates as the interface between the processor & caches die and the optical die, integrates driving circuits, sense amplifiers, and control circuits for the optical components (e.g. the
62 ON/OFF switch of turn resonators and modulators/photo detectors). The optical die, which integrates the waveguides and ring resonators, is connected to the control die usin g Through Silicon Vias (TS Vs) [ 51 ]. These optical components are built using CMOS compatible monolith ic integration to reduce cost [ 52 ]. Figure 3 1 An overview of ESPN architecture ESPN employs a 2D mesh dynamic optical network which consists of se veral sub networks to support traffic aware dynamic network resource allocation through real time tuning of the laser sources; each of the sub network is supplied with individual external laser source. The external laser lights are provided by two VCSEL so urce chips [ 53 ] and coupled into the multi processor chip via off chip fibers. The laser lights are separately conducted to the waveguides in horizontal and vertical directions through on chip splitters. A basic switch element of ESPN consists of an electrical router an d an optical switch. The electrical router surrounds a processor core and locates on the processor &
63 cache die; the optical switch is located on the optical die. In addition, ESPN uses the RapidEngy router and all optical adaptive routing to further improv e the energy efficiency of photonic NoCs. Dynamic Resource Allocation in Photonic Network ESPN achieves dynamic resource allocation by partitioning the interconnection network into multiple sub networks. Each sub network provides a fraction of the aggrega ted bandwidth and is driven by separate lasers. The bit widths (i.e. wavelengths) of the data channels are divided among the sub networks. The sub networks can be dynamically activated/deactivated based on the run time bandwidth estimation. To further mini mize the power consumption of an inactive sub network, the driving circuitries, and heaters (we assume each heater is dedicated to one turn resonator or shared by one mo dulator/photo detector array [ 15 ]), along with the photonic components are turned off. Figure 3 2 The VCSEL sources (a). The organization of a VCSEL source (b). The organization of a VCSEL array Unlike hierarchical electrical NoCs [ 54 ], ESPN leverages the characteristics of photon ic components such as the controllable laser source to achieve low overhead
64 sub network switching. ESPN employs two types of optical channels to connect each tile in the 2D mesh network in order to support all adaptive routing. A data channel is used for d ata transmission; and a routing channel, which travels along the data channel, carries routing and control information. In this study, we investigate two sub network partition techniques. In both techniques, the wavelengths of the data channels are evenly distributed among the sub networks. The routing channels are either shared by all the sub networks (called data channel splitting) or duplicated across the sub networks (called full splitting). Low latency, high density laser sources are required to facil itate the switching of sub networks. In this st udy, we employ VCSEL sources [ 55 ], each of which is a perpendicular emission type of semiconductor laser diode, to power photonic links for fast sub network activation/deactivation operations. VCSEL has the ad vantages of low cost mass production and can achieve high integration density due to its vertical nature. The organization of a VCSEL source is illustrated in Figure 3 2(a). The laser source consists of two D istributed Bragg R ef lector (DBR) mirrors [ 56, 5 7 ] with an active region that contains one or more quantum wells for the laser light generation in between. When a voltage is applied between P DBR and N DBR, the generated current flow drives the p n junction to emit laser light from the bottom of the chip. The VCSEL switching is achieved by applying and removing the forward operating voltage and the DC bias current betwe en the two reflector mirrors [ 57 ]. The VCSEL source can operate at a high speed. Its switching delay is mainly determi n [ 58 ], which is the delay in the emission of light from the laser after applying the driving current [ 17 ]. This delay is typically the time for the
65 driving current to fill the electron up to the laser emission threshold level, which varies fro m 10 to 100 ps, depending on t he type of driving circuitry [ 58 ]. The dissipated power is mainly determined by the average current, which is negligible during this short period. Multiple VCSEL sources can be organized as wavelength division arrays, which mu ltiplex multiple optical signals within a single optical waveguide by leveraging a microlens array and an external focusing lens as shown in Figure 3 2(b). Our design integrates multiple VCSEL arrays within two laser source chips. These VCSEL arrays are su pplied to different sub networks and controlled individually. Assuming that (1) each laser source chip contains four VCSEL arrays used for implementing four 2D mesh sub networks, (2) each VCSEL array consists of 64 wavelength division multiplexing (WDM) la ser sources, (3) the VCSEL cent er to center space is 250 um [ 53 ] and the laser sources are organized as a 328 matrix, the dimensions of a VCSEL array chip is 0.8cm0.2cm0.03cm. Both the optical switch and the network interface need to be modified to supp ort the dynamically partitioned photonic network. In ESPN, each electrical router is connected to a processor with its private L1/L2 caches, as shown in Figure 3 3(a). The routers and processors are located on the processor & cache die, atop the correspond ing optical switches. The electrical router and the optical switch communicate with each other through TSVs and optical/electrical signal converters on the control die. Each electrical router contains four input and four output interfaces. Each output inte rface is shared by two output directions; while an input interface is dedicated to an input direction to support all optical adaptive routing (detailed in next section ).
66 Network Interface : As shown in Figure 3 3(b), the output interface uses a message Fir st In, First Out ( FIFO ) to distribute incoming messages (from local processor) among the active sub networks. Message transmitters are deployed to support optical adaptive routing, each of which serves two sub networks in different output directions. A mes sage transmitter is available when not transmitting data and its connected sub networks are active. The sub network selector assigns messages to the available transmitters in a round robin fashion. When deactivating sub network(s), ESPN employs two mechani sms to avoid destroying in flight messages. First, the laser sources of the deactivated sub network(s) remain on for cycles in order to complete the transmission of traversing messages in network, where is the round trip transmission cycle betwee n two nodes with the maximal distance (We evaluated in 88 network in this study). Second, a message in message transmitter will not be eliminated immediately after being sent to the network owing to potential re transmission. The message is destroy ed only after reaching its destination. Similar to the output interface, the input interface employs a central FIFO to buffer traffic from active sub networks.
67 (a) (b) (c) Figure 3 3. The network components (a) The electrical router (b) The output interface (c) The central controller Central Controller : The state of sub networks is determined by the network pressure, which indicates the ratio of communication demand and available bandwidth. In our design, the network pressure of each output interface is estimated as where is its FIFO data coun t and is the FIFO capacity. This allows our design to better adapt with bursty and un predictable NoC traffic such as the hotspots. The output interfaces periodically send their network pressure to the central controller. ESPN employs an electrical H tr ee network composed of differential transmission lines (T line) [ 51 ] located on the control die to collect network pressure and control the laser source. This H tree network is similar to the common H tree clock distribution network but with reverse signal flow and light load. The central controller aggregates the network pressure, and then identifies the number of active sub networks (via the Network Status Lookup Table), expressed as where is the total number of sub
68 networks, is the current network pressure and is the threshold to activate all sub networks, as shown in Figure 3 3(c). The laser controller generates the laser source control information based on the required number of active sub networks. When the network p ressure fluctuates, the subnetwork status should be adjusted correspondingly. The latency of sub network activation/deactivation consists of the following components: delay in the H tree network, delay in the central controller, delay from the central cont roller to laser, and the laser operation delay. Assuming a 3cm 3cm multi processor chip, the H tree differential T line delay in 22nm is estimated as 8. 04 ps/mm [ 52 ]. So the H tree transmission delay is estimated to be 240 ps. The delay in the central co ntroller is assumed as 3 cycles (600 ps under 5GHz clock). The delay to the laser source chips is determined by the distance between the laser source chip and the processor chip and is assumed as 200 ps. The laser operation delay is 10 ps to 100 ps and we use 50 ps in our simulations. RapidEngy Optical Switch Apart from dynamic network resource allocation, ESPN employs low loss optical switches (i.e. RapidEngy) to further optimize energy efficiency. Note that each optical switch in 2D photonic mesh network requires five pairs of ports to connect the four adjacent switches and the local node. Although a 55 switch can be implemented using crossbar based design in the electrical domain, its photonic implementation is quite challenging. To implement the optica l switch, [ 13 ] adopts a 44 optical crossbar to direct messages to different directions, as shown in Figure 3 4(a). The crossbar connects different input ports to different output ports by electrically tuning the turn resonators to ON or OFF state (shown i n Figure 3 4(c)). Additionally, the optical switch adopts four modulator and four photo detector arrays as the interface between the local node and
69 the beneath crossbar. Each modulator array (EO, WO, SO, NO) modulates messages from the local node to one ou tput direction and each photo detector array (EI, WI, SI, NI) demodulates messages from one input direction to the local node. (a) (b) (c) (d) Figure 3 4. The design of optical switches (a) A 44 crossbar switch with dedicated optical modulator/demodulator array [ 59 ] (b) RapidEngy switch (c) The required turn resonators states for switch in Figure 3 4(a) (ON: the turn resonator is on resonance and signal turns. OFF: the turn resonator is off resonance and signal does not turn) (d) The required turn resonators states for RapidEngy Unfortunately, the modulator and photo detector arrays incur severe pass band attenuation to the traversing messages. For example, the messages from West to East are affected by the WI photo detector array and EO modulator array as shown in Figure 3 4(a). In RapidEngy, we propose to rearrange the modulator and photo detector a rrays
70 to avoid affecting traversing messages, as illustrated in Figure 3 4(b). Now the same messages no longer pass any modulator array and photo detector array. Similar to the 44 crossbar switch, the messages from/to the local node are modulated/demodula ted at corresponding modulator/photo detector array. For example, Figure 3 4(b) shows a message from South converted by the SI photo detector array and a message to West modulated by the WO modulator array. Due to the relocation of the modulators and phot o detectors, RapidEngy introduces additional resonators and waveguide crossings, resulting in signal loss. For instance, the traversing message from West to East experiences two ON resonators in RapidEngy (resonators 9 and 12, as shown in Figures 4(b) and (d)) compared to one ON resonator in the 44 switch (resonator 6, as shown in Figures 3 4(a) and (c)). Also the number of traversing waveguide crossings is increased. Nevertheless, the additional turn resonators and waveguide crossings exert much less impa ct on the signal compared with the modulators/photo detectors. Our simulation results show that in a data channel splitting network consisting of 4 sub networks, the power loss in RapidEngy is 1.61 dB less than that in the switch shown in Figure 3 4(a). A ll Optical Adaptive Routing Although our dynamic resource allocation reduces network power considerably, it could incur performance degradation due to the reduced network bandwidth. Adaptive routing achieves load balance and could compensate for the reduce d network bandwidth; nevertheless existing opti cal based routing algorithms [ 6 ] are mostly static due to the inherent buffer less nature of photonic NoCs. The use of auxiliary electrical network routing [ 16 ] introduces additional hardware and performance o verhead. To overcome these limitations, we propose an all optical adaptive routing scheme by
71 leveraging the low latency optical network to route messages. To our knowledge, this is the first work that explores optical adaptive routing in mesh network witho ut relying on the high power electrical components. Our proposed all optical adaptive routing first establishes an optical circuit switch path between the source and destination node and then transmits messages via that path. To achieve good tradeoff betw een routing complexity and network performance, we adopt minimal adaptive shortes t distance routing algorithm [ 1 ], which searches for the optimal routing path among all the shortest distance paths. The path establishment consists of the following scenarios : A. The source node sends request signals along the shortest path(s) to check their availability. While proceeding, a request signal reserves photonic links along the path for the upcoming message. B. In case the request signal encounters a blocked li nk and fails to reach the destination, the signal carrying the information of the blocked position is transmitted back along its reverse path and releases the reserved links. C. If the request signal reaches its destination, the signal that indicates successful link acquisition is transmitted back to the source node. The message transmission then starts along the reserved links. D. If all the request signals are blocked and fail to reach the destination, the source node retries the next path. Each scenario is described below in detail. A. The Traversal of Request Signal : The request signal travels along the request channel, which is part of the routing channel The routing channel also contains the response channel driven by reverse laser light. T he request channel consists of an even number of waveguides (two waveguides are shown in Figure 3 5). The wavelengths of the request signal are organized as two groups : Path Hops ( PHOP) and Request Hops ( RHOP ). The PHOP stripes across the two waveguides and is divided into several
72 sections (PHOP 1 to PHOP n ), which sequentially records the routing information in n hops Each PHOP x consists of four optical bits ( wavelength s ), which represent four possible turn directions. At each switch, i f the downstream switch is available, the active PHOP bit drives the turn resonator of the corresponding direction to route the request signal and upcoming message. Figure 3 5 illustrates a case in which the request signal traverses through three hops ( go straight, turn left, and finally received by the local node ) Each switch snoops on the PHOP 1 wavelengths t o detect the turn direction of request signal. PHOP 1 is eliminated after the signal is routed through the current switch So PHOP 2 needs to be moved to PHOP 1 to be detected by the next hop. As a result, all PHOP i needs to be moved to PHOP i 1 To achieve this, we apply either physical shift or frequency tran slation mechanism propose d in [ 13 ] to one waveguide, which respectively moves the optical bits to the same or different wavelengths at another waveguide. For example, Figure 3 5 shows that in hop 1, PHOP 3 5 are frequency translated to PHOP 2 4 (different wavelengths) while PHOP 2 4 6 are physical ly shifted to PHOP 1 3 5 (the same wavelengths). Our proposed design implements the all optical adaptive routing by leveraging the RHOP signals and response channel rath er than static X Y routing in [ 1 ]. B. Path Availability Identification: After sending the request signal, the source node needs to be notified on path availability. If the requested path is currently blocked, the source node will be notified about the block position and then plan an alterna tive path, which is achieved using RHOP and REPLY. The RHOP is part of the request signal and is duplicated across the r equest waveguides. An active RHOP i indicat es that the current distance to the destination node is i 1 hops. RHOP is decreased at each ho p
73 by frequency translating the activated bit to the next one. For example, in Figure 3 5, in each hop t he RHOP 1 is eliminated and RHOP 2 6 is frequency translated to RHOP 1 5 in the other waveguide. When the request signal encounters a blocked link or reache s its destination, the REPLY is modulated by physical shifting the RHOP signal from the request waveguides to the response channel and then transmit ted back to the source node The source node examines the REPLY to decide whether the path has been successf ully established.
74 Figure 3 5 The request signal in routing examination and forwarding
75 Figure 3 6 illustrates an example in the context of a 4 2 2D m esh topology In this example, n ode 1 needs to send a message to n ode 7, while the link between n ode s 3 and 7 is currently blocked The source n ode simultaneously generate s up to two request signals (on a minimal adaptive routing basis along both coordinates) to accelerate link establishment. In this example, node 1 gene rates two request signals to its South ( request A) and East ( request B) output ports. The r equest A is frequency trans lated from RHOP 3 to RHOP 2 at node 2, indicating the message proceeds. Due to the blocked link, n ode 3 modulates the REPLY by physical shif ting RHOP to response channel and then eliminates request A. The request B successfully reaches its destination, n ode 7 T hus n ode 1 receive s two REPLYs from the two response channel s and identif ies the REPLY from the East port as a successful link establishment. During the transmission of request signal, i n case that several request signals co ntend for one output port, the highest priority will be given to the one that is closest to its destination by activating the corresponding RHOP signals. Such distance class ordering mechanism ensures a deadlock free network [ 1 ] C. Data Transmission : The source node starts transmitting data along the reserved links once the REPLY indicate s th at a path has been established When the data tran smission completes an optical pulse traverses back along the path to tear down the link s D. Alternative Path Selection : In case that all the request signals are blocked and fail to reach the destination the source node will plan an alternative path. The source node first examines the returned REPLY and locates the block ed position. It then retries the next path using the unblocked links plus a detour for the blocked links
76 That path is still one of the shortest distance paths between the source and the destination. For example, in Figure 3 6 if both requests fail to reach the destination, a possible alternative to request A is link s 1 2, 2 6 and 6 7.
77 Figure 3 6 An example of blocked link in adaptive routing (activated RHOP bit highlighted)
78 Experimental Methodology Machine Configuration and Workloads Our evaluation is performed using a simulator developed from S imics/GEMS [ 41 ] framework. We used GARNET [ 37 ], a detailed cycle accurate on chip network model incorporated within the GEMS framework, and extended it to support our proposed optical NoC architecture. All simulations are performed on an 88 mesh network as listed in Tabl e 3 1. We explore different sub network partition schemes while keeping the data channel bandwidth, i.e. the product of the number of wavelengths per waveguide and the number of bundled waveguides per data channel, constant. The determini [ 16 channels and X Y static routing to establish path, similar to [ 13 adopts all optical ada m m splitting and data channel splitting ESPN respectively. We also compare the performances of the conventional four port switch shown in Figure 3 4(a) (with the prefix processing cores with private L1 and L2 caches fabricated using 22nm processing technology. We assume the interconnect network clock is 5GHz with a supply voltage of 0.5V.
79 Table 3 1. The evaluat ed NoC design NoC Design Link Establishment Routing Scheme Network Division Wavelen gths Data Bus Width (Bytes) Number of Sub networks Optical Switch Type E Deterministic Electrical X Y Static 64 8 1 Four Port O Deterministic Optical X Y Static 64 8 1 Four port Sub O Adaptive Optical Adaptive 64 8 1 Four Port O Adaptive Optical Adaptive 64 8 1 RapidEngy Sub ESPN (F m n ) Optical Adaptive Fully Splitting 64 / m 8 / n m n Four Port ESPN (F m n ) Optical Adaptive Fully Splitting 64 / m 8 / n m n RapidEngy Sub ESPN (P m n ) Optical Adaptive Data Splitting 64 / m 8 / n m n Four Port ESPN (P m n ) Optical Adaptive Data Splitting 64 / m 8 / n m n RapidEngy We used 128 state MMP synthetic traffic (i.e. Bit compliment, Tornado, Bit reverse, and Random) in our simulations. The MMP synthetic traffic generates a time varying NoC utilization by modulating the rate of a Bernoulli injection process on the states of a Markov chain [ 6 ]. The injected messages in MMP synthetic traffic are 64 bytes (the cache line size) and 8 bytes (the invalidation message size) respectively. In addition to synthetic traffic, we use d real world workloads PARSEC [ 60 ] and SPLASH 2 [ 61 ]. We run the PARSEC and SPLASH 2 benchmarks on top of Simics/GEMS and extract the network traffic traces. In order to preserve the traffic characteristic of different benchmarks while stressing the network more than the originally extracted traces, we adopted the e valuation methodology used in [ 47 ]. We normalized the PARSEC and SPLASH 2 traffic traces of all the benchmarks to accommodate the bandwidth of the optical network by setting the normalized average traffic rate to 0.4 times the network bandwidth. This scaling maintains the unbalanced natur e of the traffic load, and stresses the network more than the real traffic load.
80 Power Estimation Methodology W e used the statistics reported in [ 62 63 ] f or the optical network power estimation The electrical energy coupling efficiency of the laser sou rce ranges from 30% [ 63 ] to 50% [ 62 64 ]. W e use d the median value of 40% in this study Another important factor of the total power consumption is the required optical detection power for photo detector s, which is related to the expected Bit Error Rate (B ER) W e adopt ed BER of 10 15 in our study and [ 65 ] shows that each photo detector requires at least 5 power under 5 Gb/s modulation rate B y default all turn resonators are set to OFF state and tuning energy is required when switching to ON state [ 3 ]. Th is energy is assumed to be 100 fJ/bit [ 45 ]. Besides, the power consumed by each modulator is approximately 200 fJ/bit using advanced driver circuits with poly Si carrier lifetimes of 0.1 1 ns and modulation speed of 5 Gb/s [ 45 ]. In a typical photonic No C with auxiliary electrical network [ 16 ], t he total power consumption is the sum of power dissipated by both optical and electrical network s The power consumption of the electrical network is modeled based on [ 22, 44 ] which assumes t he energy required to transmit one bit under 2 2nm technology is 0.83 pJ plus 0.34 pJ/mm link power. Evaluation In this section we explore the design spaces of ESPN and evaluate the performance and power benefits of the proposed techniques. The Optimal Ne twork Power Latency Product (PLP) The threshold to activate all sub networks ( ) determines the tradeoff between network power and latency. We measure the normalized network Power Latency
81 Product (PLP ) metric to determine the opti mal As increases, fewer sub networks are activated, which results in increased network latency and PLP. In contrast, reducing increases network power and PLP. (a) (b) Figure 3 7 The p ower l atency p roduct (PLP) of different n etwork s (a) ESPN (P) (b) ESPN (F) (the average of four synthetic traffic patterns) Figure 3 7 shows the average network PLP of ESPN (P) and ESPN (F) on synthetic traffic patterns with different injection rates. We observe that the optimal varies with network configurations. For example, the optimal on ESPN (P 1 2) and ESPN (P 2 1) is 512 while the optimal on ESPN (P 2 2) is 640. We also observe that increasing traffic injection rate increases PLP by up to 20 times (e.g. a s injection rate increases from 0.35 to 0.40) due to severe network congestion. The optimal is insensitive to the injection rate and remains stable in most cases. In this study, we choose 512 for ESPN (F 1 2), ESPN (F 2 1), ESPN (P 1 2), ESP N (P 2 1), and 640 for ESPN (F 2 2) and ESPN (P 2 2). We apply the same for Sub ESPN( ) and ESPN( ) since the network performance is not sensitive to switch architecture.
82 Network Performance Our proposed all optical adaptive routing reduces both the path request latency and the number of attempts to establish a path. In this section, we evaluate the network performance from these two aspects as well as the overall network latency. The Path Request Latency In all optical adaptive routing the path reque st latency is crucial since the source node may attempt to establish a link multiple times before succeed ing This latency is characterized by the source resonator modulation latency, interim resonator drive latency, destination resonator modulation latenc y and optical link latency. For each hop, the request signals fall into one of the three cases : ( a) frequency translated and then forwarded to the next hop (passed), ( b) transmitted back to the source node through the response channel (blocked), or ( c) re ceived by the local node (received). In case ( a), the request signal needs to be received and driven to the corresponding resonator, the same as in case ( b) except that the to be driven resonator is at the response channel rather than the request channel. In case ( c), the request signal drive s resonators to eliminate itself In all cases, the latency is determined by three factors : ( 1) the time to receiv e PHOP and RHOP signals from the request channel ( 2) the latency of performing a single level CMOS logic to establish a path and ( 3) the time for driving the physical ly shifted and frequency translated resonators. We use the latency parameters from [ 13 35 50 59 ] to estimate the request attempt round trip delay The optical network, inclu ding resonators and peripheral circuitry, operate s at 5 GHz.
83 The Path Establishment Attempts Compared with deterministic routing, the all optical adaptive routing reduces the path establishment attempts by choosing alternative paths, and therefore reduces the average network latency. We compare the number of path establishment attempts with different routing algorithms using synthetic traffic patterns. Figure 3 7 (a) shows the cumulative distribution of the results under low traffic (injection rate = 0.3). W e observe that with all optical adaptive routing, 87% paths are established within 3 attempts, compared to 8 1 % using deterministic routing. (a) (b) Figure 3 8 The number of path establishment attempts under (a) light traffic (b) heavy traffic (the average of four synthetic traffic patterns) I n a congested network the all optical adaptive routing exhibits better efficiency As shown in Figure 3 8 (b), under heavy traffic and with O adaptive, 75 % paths can be established within 3 establishment attempts compared to 64% using deterministic routing By dividing the network into multiple sub networks and supplying with dedicated routing channels ESPN (F 1 2) and ESPN (F 2 2) further increase this ratio to 77 % and 8 1 %.
84 Network Latenc y Figure 3 9 shows the network latency under 1 28 state MMP synthetic traffic Since the switch architecture does not affect network performance, Sub ESPN exhibits the same performance as ESPN E deterministic and O deterministic exhibit the worst performan ce owing to the deterministic routing O adaptive and ESPN (P) benefit from adaptive routing and thus improve performance by 20% 25%. However, due to the reduced bandwidth and subnetwork switch delay, ESPN (P) incurs 1% 3% performance degradation compared to O adaptive. ESPN (F) gain s a 5% 10% performance improvement over ESPN (P) by deploying dedicated routing channels in each sub network. (a) (b) (c) (d) Figure 3 9 Network latency under 128 state MMP synthetic traffic (a) Bit compliment (b) Tornado (c) Bit reverse (d) Random
85 Power and Energy Efficiency Synthetic Traffic Patterns Figure 3 10 shows power breakdown of the investigated NoC design. We observe that among all the sub network partitions, ESPN (P 2 2) shows the best power efficiency which yields 50 % savings on average compared to E deterministic. W hen the number of wavelengths with in each waveguide drops from 64 to 32, a n additional 10 % power saving is observed due to the alleviated optical coupling loss in modulators and photo detectors. On ESPN (F), the power for the routing channel s increases due to the deployment of dedicated resources in each sub network ESPN (F 2 2) reduces power by 46% compared to that of E deterministic. We observe that ESPN consume s 26 % less power than Sub ESPN owing to alleviating the impact of modulators and photo detectors along the traversing path. The power saving s vary s ince the average number of hops that a message traverse s in the 88 network varies with traffic patterns. Figure 3 10 also shows the energy consumed per message. As can be seen, ESPN (P 2 2) and ESPN (F 2 2) save 57 % and 58 % energy respectively compared to the baseline case. T he above two network configurations have different performance and power characteristics but exhibit similar energy per message profile
86 Figure 3 10 Power breakdown on synthetic traffic
87 PARSEC and SPLASH 2 Benchmarks Figures 3 11 and 3 12 show the normalized NoC power and energy efficiency on SPLASH 2 and PARSEC workloads. ESPN (P 2 2) and ESPN (F 2 2) reduce NoC power by 51 % and 48 % compared to the baseline case (E deterministic) In general, the proposed sub network partition techniques achieve more power saving s on benchmarks that manifest higher traffic fluctuation (e.g. fmm and canneal ). This is because ESPN benefit s from traffic fluctuation and is able to de activate the unused sub networks thus decreas ing the overall NoC power. Besides the power efficient architecture, our adaptive routing c ircuit switch reduces the application execution time and therefore further improves the energy efficiency The best case occurs on canneal where ESPN ( P 2 2) saves 68 % of the total energy. On average ESPN (F 2 2) and ESPN (P 2 2) reduce the execution tim e by 22 % and 16 % compared to the baseline case, resulting in 60 % and 5 9 % of the total energy saving respectively. We also observe that similar to synthetic traffic, the energy consumption of ESPN (P 2 2) and ESPN (F 2 2) are very similar (less than 2 % )
88 Figure 3 11 The normalized power consumption on SPLASH 2 and PARSEC Benchmarks Figure 3 12 The normalized energy consumption on SPLASH 2 and PARSEC Benchmark s
89 CHAPTER 4 EXPLORING PHOTONIC INTERFACE FOR OFF CHIP PHASE CHANGE MEMORY SYSTEMS Motivation Current computer systems pose challenges on memory energy conservation, especially on the energy reduction of main memory. For memory intensive applications, the main memory is one of the major power consumers [ 66, 67 ]. Recently, several non volatile memory technologies (e.g. Phase Change Memories, or PCMs) have emerged as alternative to Dynamic random access memory ( DRAM ) solutions by avoiding the power wall with low leakage cells. At current technology nodes, the intensive curre nt injection in PCM cells motivates energy pro portional design. Prior study [ 68 ] breaks the conventional memory ranks into reduction majorly comes from the reduced number of chips and narrower row bu ffers (i.e. sense amplifiers) [ 66 ] involved in each activation and precharge operation. Also, the memory background power is reduced owing to the better utilization of low power mode. Nevertheless, the mini rank design is not natu rally supported in electrical domain due to the potential InterSymbol Interference (ISI) problem caused by the load of multiple ranks p er channel [ 69 ]. Zheng et al. [ 68 ] employs a dedicated interface chip between memory chips and the communication bus at t he cost of power waste and additional data transmission cycles. Besides, the rank performance is constrained by the narrow connections between this chip and memory devices. Other solutions to ISI problem, e.g. the fully buffered dual in line memory module ( DIM M ) [ 70 topology [ 7 1 ], also introduce intolerant transmission latency [ 7 2 ] and thus not applicable.
90 In order to overcome the limitation of electrical mini rank design, we propose OptiPCM, which is an extension to the legacy memory arc hitecture that takes advantage of the recent advances in CMOS compatible nano scale silicon photonic integrated circuitry [ 73, 74 ]. In OptiPCM, the photonic channels connecting PCM arrays are built on the monolithically integrated silicon photonic waveguid es. Far beyond conventional electrical bus, the silicon photonic bus is able to load a large number of memory devices even under very high frequency provided enough injection power. Thus, the ISI effect which limits the electrical mini rank performance is successfully removed by the photonic communication. Besides, OptiPCM is able to provide increased memory level parallelism to hide the long PCM access latency by taking advantage of the large number of devices per channel. Interestingly, unlike the predete rmined electrical links, the optical paths can be easily reconfigured, which enables the traffic aware bandwidth allocation. State of the art Double D ata R ate ( DDR ) protocol does not provide support for non volatile memory interface. Protocol such as Low Power Double Data Rate Non Volatile Memory (LPDDR2 NVM) [ 69, 75 ] provides support for PCM memory interfacing, but lowers overall performance as compared to DDRx. To recuperate such performance loss and to retain low power consumption, we use high bandwidth, low latency and low power photonic communication links to connect PCM chips and follow LPDDR2 NVM based protocol. In summary, the following are the contributions of this section : 1. We propose to apply photonic links to connect large number of independent PCM chips. Apart from the widely recognized high bandwidth and low power characteristic, our design takes full advantage of the high load capacity of optical
91 chan nels, which provides support for the mini rank design and hides the long PCM access latency. 2. We apply the fixed channel division technology to amortize the rank to rank turnaround time, which is critical for OptiPCM design with wide bus and numerous r anks. 3. We introduce the dynamic channel division to compensate for the potential low channel utilization in fixed channel division technology. Dividing the channel based on the traffic utilizes the channel bandwidth more efficiently while amortizing the rank to rank turnaround penalty in heavy traffic Background Phase Change Random Access Memory Currently, PCM is being considered as one of the most promising technologies for next generation non volatile memory. The emerging PCM technology has many advantages such as random access, non volatility, superior scalability, fast read cycle and manufacturing compatibility with existing CMOS process. PCMs differ from the DRAMs in the organization of the cell. Each PCM cell employs reversible phase change m aterials to store information. These materials are usually made using a chalcogenide alloy of germanium, antimony and tellurium (GeSbTe) called GST. Figure 4 1 shows the basic structure of a PCM cell, which consists of a standard NMOS transistor and a phas e change device. PCMs leverage the differences between the two states in the electrical resistivity of GST to store information. In the amorphous state, the high d material to a high temperature threshold using electric al pulse generated Joule heat [ 76 ].
92 Figure 4 1. A single PCM cell
93 Figure 4 2. The organization of a rank of memory device The Memory Devices Organization and LPDDR2 Protocol Figure 4 2 shows the contemporary PCM chip organization. The organization of PCM is highly similar to that of DRAM. In this example, eight PCM chips are ganged to gether and work in lockstep to respond to the commands issued by the memory controller. The links used to communicate between the ranks and memory controller is called channels and the ganged chips form a rank We employ the channel following LPD DR2 NVM p rotocol in our study [ 69 ]. The original LPDDR2 is proposed to become the technology of choice for embedded and mobile applications thanks to its low power charact eristics, e.g. Malladi et al. [ 77 ] leverages LPDDR2 based DRAM in the data centers. LPDDR2 sav es a significant proportion of power ( over 50% ) under similar device density and performance conditions. This saving majorly comes from the reduced working voltage (1.2V), the removal of certain circuits like the Delay Lock Loops (DLLs), the advanced power management, and the shrunken pin count. LPDDR2 NVM officially provides support for non volatile memory such as flash memory and PCM. In LPDDR2, t he signals between
94 memory device and memory controller fall into four categories: the command signals, the add ress signals, the data signals, and the miscellaneous signals. The command and address signals are unidirectional and extend from memory controller to the memory They provide the commands and bank/row/column address to the memory chips. The data bus is a bidirectional bus whose bandwidth is the aggregation of each memory device. For example, Figure 4 2 shows an example of eight 8 bit chip s forming a 64 bit data bus. Data bus also contains the data strobe signals (DQS) used for the data alignment. Apart fro m the four types of signals, the LPDDR2 NVM protocol also contains miscellaneous signals such as the clock signal and temperature sensor signals. Each PCM chip is organized into multiple banks A bank is an independently controllable unit and is composed of several PCM arrays The PCM array consists of the cells which are organized into 2D arrays and accessible th r ough a row address and a column address. In contemporary design, each read/write access to one rank will activate one row specified by the row address across all PCM arrays in one bank and loads data from the cells in that row to row buffers. The column address specifies one bit from the row buffer of each PCM array, and multiple PCM a rrays provides multiple bits. Contemporary memories used in CMP system usually provide the data from several columns in a burst to meet the size of a single cache line.
95 OptiPCM System Organization Figure 4 3. The example of 16 mini rank prototype design of the OptiPCM system OptiPCM employs a set of non volatile memory chips and DIMM (Dual In line Memory Modules) following LPDDR2 NVM protocol as shown in Figure 4 3 OptiPCM replaces the conventional electrical channels in conv entional memories with optical channels. A VCSEL chip is employed as the external laser source and provides laser lights for the optical channels. The Dense Wave length Division Multiplexing (DWDM) silicon photonic technology provides highly aggregated pin bandwidth density, which improves the bandwidth density by two orders of magnitude than that of electrical buses. The communication channels in OptiPCM consist of two bundles of unidirectional waveguides (in purple and blue in Figure 4 3 ) T he outbound cha nnel carries 64 byte wide data, addresses and commands from the memory controller to the memory while the return data travels through inbound channel. The modulator arrays Figure 4 3 ). The memory controller modulates the laser lights in the outbound channel by properly co ntrolling the modulators arrays. OptiPCM adopts connected outbound/inbound channel where the laser source only injects laser lights into the
96 outbound channe l. The connected channel design is based on the half duplex LPDDR2 data bus where the data can only be transmitted in one direction at any instant, and is distinct from previous optical memory designs with separate outbound/inbound channels [ 78 ]. Th e conne cted outbound/inbound channel shares one laser source hence nearly halves the static photonic power in separate outbound/inbound channel To support the photonic channel, the conventional electrical DIMM is redesigned as a CMOS compatible integrated photo nic interface (PI) in place of the conventional electrical pins. PI converts the optical signals from outbound channel to electrical signals for write commands. For read commands, PI recaptures laser lights from outbound channel to inbound channel and then modulates the laser lights to carry the return data. The memory controller generates and distributes optical clock through outbound channel to each memory rank in order to avoid the need for centralized signal retiming unit and global timing synchronizati on among memory ranks. The clock wavelength parallels the data wavelengths, with the clock signal traveling with the data signals. In contemporary design, multiple PCM chips in a rank work in a tandem and share the DIMM timing synchronization circuit and power supply circuit. OptiPCM breaks the PCM chips from one rank into multiple mini ranks, each of which is a single PCM chip with its own power supply and timing synchronization circuits. Unlike the original mini rank design, which breaks each rank up to eight mini ranks, OptiPCM breaks each rank into 16 or even more smaller memory ranks. In OptiPCM, eight PCM chips are placed on one memory board as contemporary memory. The increased ranks require the laser source to inject enough laser power for the comp ensation of waveguide propagation
9 7 loss. The inject ion power is constrained at the first modulator which must be below the threshold ( around 10 20 mW [ 79 ]) that induces nonlinear effects According to our evaluation, though 10 mW modulation power is suffic ient for 64 ranks, more than 32 ranks starts to the performance owing to frequent inter rank switch and yields overwhelming manufacturing cost. Sub channel Division Technology OptiPCM surpasses the original electrical LPDDR2 NVM systems in terms of memory level parall eli sm and power consumption. Nevertheless the prototype design could be further optimized to reduce the rank to rank switch penalty which is non trivial when the number of ranks is large In this section, we leverage both fixed and dyn amic channel division to overcome the rank to rank turnaround penalty Fixed Channel Division The Rank to Rank turnaround penalty exists in the high frequency global synchronous memory system. Unlike the commands to same open memory bank that can be issued and pipelined back to back, the consecutive commands to different memory ranks relies on the system level synchronization mechanisms [ 71 ]. Thus, the shared data bus must be idle for some period of time between data bursts from different ranks. In the elec trical design, the synchronization circuits are used to align the strobe signal DQSs to DQs on data bus Modern LPDDR2 protocol removes the DLL circuits that are usually implemented in the DDRx for synchronization; however, LPDDR2 also suffers from t h e lat ency of synchronization circuit This latency dominates the rank to rank synchronization penalty which depends on the system level synchronization mechanisms and usually costs 1 to 3 cycles [ 69 ]. Due to the unpredictable photodetector/modulator delay and the requirement for optical clock synchronization,
98 the synchronization circuit is also required in the optical domain and the ranks suffer from rank to rank turnaround penalty. It is possible to increase the number of DQs aligned to one DQS. In electrical domain, eight DQs are aligned to one DQS so that the physical designers can easily route the wires. In optical domain, the wavelengths travels within same waveguide demonstrate highly similar physical characteristics, so OptiPCM aligns all data signals of one mini rank to one DQS. The rank to rank turnaround penalty in conventional LPDDR2 NVM with the help of command reordering is shown in Figure 4 4 (a) In this example, t he data burst ( tBL ) and synchronization (tRT RS) operation interleave on data bus. In contemporary CMP system, the typical tBL length is 4 cycles (64 byte s cache line), and the tRTRS rises with increasing frequency of bus clock (e.g. appro ximately 3 cycles in DDR3 [ 71 ]). In OptiPCM, the tBL is reduced to 1 cycle in the 64 byte wide data bus and the number of ranks is large, which could incur more significant bandwidth waste.
99 (a) (b) Figure 4 4 The timing penalty caused by rank to rank switch (a). Memory access timing without channel division for consecutive write to three ranks (PAn = Preactivate rank n, ACTn = Activate rank n, WRn = Write rank n, RAHn = Raw Address High bits, RALn = Raw Address Low bits. [tRP = 5, tRCD = 4, tWL = 1, tBL = 4, tRTRS = 3]) (b). Memory access timing with channel division for consecutive write to four ranks [tRP = 5, tRCD = 4, tWL = 1, tBL = 16, tRTRS = 3]) In the fixed channel division the data channel is equally divided into several sub channels Each sub cha nnel is dedicated to one rank. The memory controller allocates one sub channel rather than the whole data channel to one memory access. Although the width of each sub channel decreases, t he sub channel division amortize s the rank to rank synchronization pe nalty as shown in Figure 4 4 (b). In this example, the narrow sub channel extends the original tBL from 4 to 16, but inter rank switch penalizes only one sub channel and thus quarters the overall rank to rank switch overhead
100 Dynamic Channel Division Although t he sub channel division benefits the system performance, it may fail to utilize the provisioning channel bandwidth sufficiently especially under non memory intensive applications To address this issue, we propose the dynamic channel division, wh ich dynamically adjusts the sub channel width (i.e. number of wavelengths) based on the incoming traffic. When there is only one waiting command in the command queues, the memory controller allocates the whole channel to it. When two or more commands in th e command queues compete for the channel, the memory controller seeks to equally divide the channel width among these commands using the current wavelengths availability information. However, i f the equal division is unobtainable, one or more request(s) wi ll be delayed to next assignment. In the non memory intensive applications the dynamic channel division features low latency data channel; while the rank to rank switch penalty is amortized in the memory intensive applications. The dynamic channel division requires extra design module for memory device and the memory controller as we will discuss in next section The Structure of PIs In OptiPCM, the PIs are deployed in place of the c onventional electrical DIMMs as shown in Figure 4 5 (a). PI direct s photonic signals to appropriate ranks and convert between electrical/optical signals. One RW signal in each sub channel travels along with the data signals to indicate the current direction of one sub channel. As shown in Figure 4 5 (a), all RW bits from ou tbound waveguides are first directed by R1 to the vertical waveguide, and then separated by R2 resonators as the pulse control signals of R3 [ 15 ] by injecting or canceling the free carriers The R3 resonators then direct the signals to either the ranks (or optical crossbar in dynamic channel division) or the
101 inbound waveguides based on their status. In the prototype design, t he signals are directed to the ranks and the built in photodetectors c onvert the optical signals to electrical signals. (a) (b) Figure 4 5 The structures of important photonic components (a). The p h otonic interface (PI) (b). The o ptical c rossbar in d ynamic c hannel d ivision
102 To support the fixed channel division the PI simply direct s the signals in one sub channel to the corresponding rank using passive turn resonators ( which is not shown in Figure 4 5 ). For t he dynamic channel division the PI uses a group of optical bits name d the connectivity bits and encodes the connection information into them. An optical crossbar under the control of the connectivity bits (shown in the r ed dashed dotted box in Figure 4 5 (a)) switches the data betwe en the sub channels and the ranks. A n n w optical crossbar is required to switch the data between n ranks and w wavelengths wide data bus The structure of the crossbar is depicted in Figure 4 5 (b). The optical crossbar (shown in the r ed dashed dotted b ox in Figure 4 5 (b)) contains n w basic unit. One basic unit contains one passive resonator used to extract one connectivity bit and one optical tuning resonator that directs the data from one sub channel to one rank For example, an 8 sub channel with 6 4 bytes wide data bus requires 512 8 = 4096 such components. The PI requires no crossbar like components for the read operation. As Figure 4 5 (a) shows, the modulator arrays from different ranks modulate data to the same waveguide at different wavelength s to avoid interference. Increased ranks and sub channel division requires extra photonic components and incurs extra limitation and overhead. The extra resonators and increased length of waveguides attenuate the traversing laser lights. However, the maxi mal power that the modulator could support must be below the threshold at which nonlinear effects are induced which is typically 10 20 mW. The optical power experiences up to 1 filter drop loss, 4095 filter through loss, 1 modulator insertion loss, and 1 photodetector loss when the system employs dynamic wavelength assignment with 8 ranks and 64 bytes
103 wide da ta bus. This part of loss is around 6.1 dB. Current technology node presents the waveguides which have a p ropagation loss of 0.3 dB/cm [ 80 ] Assuming 10 mW modulation power, the length of waveguide could be up to 89 cm according to the calculation, which is sufficient to route waveguides to memory boards. The OptiPCM uses the internal row buffers within PCM chips to convert different widths of sub channel and internal data. The data from the sub channel fill the row buffer i n one or more cycles and is written into the memory cells in one batch The data read out from memory cells is also temporarily stored in the buffers and then consecutivel y sent to the sub channels. The overhead of the dynamic channel division mainly comes from the optical crossbar and the extra internal width conversion circuit. The calculation results from optical latency model [ 5 73 81, 82 ] indicate that the optical c rossbar incurs less than 100 ps latency, which is negligible under 533MHz clock (the highest frequency supported by Joint Electron Devices Engineering Council ( JEDEC ) LPDDR2). We synthesize th is circuit in Synopsys Design Compiler [ 83 ] and find that the cr itical path latency is also within one cycle. We also include its power consumption information under different traffic rates. The Design of Memory Controller
104 Figure 4 6. The structure of a memory controller
105 Figure 4 7 Finite state machine in the Enhanced Wavelength Assigner Note that the memory controller needs to be modified to support the dynamic channel division. OptiPCM uses the First Ready First Come First Serve (FR FCFS) memory controller [ 84 ]. In FR FCFS memory controller, the transaction arbiter accepts requests ( i.e. transactions) from multiple processors or I/O devices and arbitrates for them Once a transaction wins arbitration and enters into the memory controller, it is
106 decomposed to a sequence of memory c ommands and mapped to a command queue. The command queues are arranged in such a way that there is one queue per rank. Then, commands are scheduled to the memory devices through the optical signaling interface depending on the command scheduling policy. Op tiPCM uses the same transaction scheduling and address mapping mechanism as conventional memory co ntroller, while enhancing the command scheduler to support dynamic channel division as shown in Figure 4 6 The enhanced FR FCFS command scheduler used in t he prototype design and fixed channel division is highly similar to conventional command scheduler. The workflow of enhanced command scheduler is depicted in Figu re 4 7 The memory commands in the command queue fall into five categories, i.e. activate, pre activate (in place of the precharge command of volatile memories ), read, write, and miscellaneous commands (e.g. power down). The command scheduler round robinly checks the command queues and then allocates the active, preact ive and control command in the conventional way. O nce the scheduler finds an issuable write/read command from one command queue, it checks the commands at the top of each rank command queues. The scheduler then stores all the read and write commands into a pool and seeks to equally divi de the available channel width among them provided that two or more commands are in the pool. The channels are divided in the unit of one byte, and the total channel width is divisible by this allocated sub channel width. T he memory controller send s unmod ulated laser lights to memory devices for read command and modulates the optical signals carrying the data to the memory devices for write command The enhanced command
107 scheduler controls turn resonators by varying the electrical signals. In the fixed chan nel division, the turn resonators could be implemented as passive resonator and needs no additional control. In contrast, the memory controller controls the direction of data channels in prototype design and controls the turn resonators to dynamically adjust the sub channel width and connectivity in the dynamic channel division. Experimental Setup Simulation Method ology We evaluate the power consumption and performance improve ment of OptiPCM using Simics [ 41 ], a multi processor sy stem simulator, and DRAMSim2 [ 85 ], a cycle accurate memory system simulator. In our simulation, we mix several benchmarks from single t hre aded SPEC2006 [ 86 ] benchmark suits to generate various memory stress We also choose four multi threaded benchmarks from PARSEC benchmark suit [ 60 ] All the benchmark configurations are listed in Table 4 1 All the tests are executed on a quad core system (2 threads / core) with 1GB memory as mentioned in Table 4 2 We test the following simulation scenarios as shown in Table 4 3 In our study, each memory board acco mmodates 8 memory chips and we assume that the E rror C orrecting C ode ( ECC ) bits are stored a long with their associated data bits in the sa me page [ 68 ].
108 Table 4 1 Simulation b enchmarks Scenario Workloads Traffic(Mbps) wr rd SPEC 1 bzip 2, gcc 2, sjeng 2, lbm 2 399.9 1027.7 SPEC 2 Milc, bzip 2, gcc 2, sjeng 2, lbm 772.1 1182.3 SPEC 3 Milc, GemsFDTD, bzip 2, gcc 2, sjeng, lbm 822.0 1181.0 SPEC 4 Milc, GemsFDTD, mcf, bzip 2, gcc, sjeng, lbm 804.9 1967.3 SPEC 5 Milc, GemsFDTD, mcf, cacbusADM, bzip, gcc, sjeng, lbm 963.2 1842.0 SPEC 6 Milc, GemsFDTD, mcf, cacbusADM 2, bzip, gcc, sjeng 798.8 2039.8 SPEC 7 Milc, GemsFDTD, mcf 2, cacbusADM 2, bzip, gcc 608.6 2590.8 SPEC 8 Milc, GemsFDTD 2, mcf 2, cacbusADM 2, bzip 595.9 2556.5 SPEC 9 Milc 2, GemsFDTD 2, mcf 2, cacbusADM 2 654.2 2831.7 blacksch Financial Analysis, 65,536 options 2.1166 5.1035 swaption Financial Analysis, 16 swaptions, 20,000 simulations 7.8720 2.0904 freqmine Data Mining, 990,000 transactions 24.978 16.141 x264 Media Processing 128 frames, 640 360 pixels 114.08 138.40 Table 4 2 Machine configuration Parameter Configuration Processor 4 cores, Pentium 4, 1.0 GHz, In Order, 4 IntALU, 2 FPALU Width 4 wide fetch/issue/commit TLB 128 entries(ITLB), 256 entries(DTLB), 4 way, 200 cycle Branch Pred. 2 K entries Gshare, 10 bit global history, 32 entries RAS I/D L1 Cache 64 KB, 8 way, 64 Byte/line, 2 ports, 3 cycle Integer ALU 4 I ALU, 2 I MUL/DIV, 2 Load/Store FP ALU 2 FP ALU, 2 FP MUL/DIV/SQRT L2 Cache 512 KB, 8 way, 64 Byte/line, 12 cycle Data Channel Width Electrical: 64 bit/channel, Optical: 64 byte/channel ;533MHz double data rate Memory Capacity 1 Giga Bytes
109 Table 4 3 Simulation s system) BASE The conventional electrical LPDDRx compatible channel with eight chips in one rank (baseline case) MINI n The mini rank configuration electrical LPDDRx c hannel with small ranks as in [ 68 ] PRT n Prototype OptiPCM design FCD n Prototype OptiPCM design with fixed channel division DCD n Prototype OptiPCM with dynamic channel division Power Model of the Communication Bus Figure 4 8 The power modeling of the LPDDR2 NVM (the equivalent driver impedance R ON is equally devided into two parts: R ONPU and R ONPD The value is typically ) Table 4 4 Optical l oss in v arious c omponents Optical c omponents Attenuation Optical c omponents Attenuation Optical coupler 1 dB Optical splitter 0.2 dB Interlayer coupling loss 1 dB Filter through 1 4 ~1 2 dB Filter drop 1.5dB Photo detector 0.1 dB Waveguide loss 0 .3dB/cm Bending loss 0.5 dB Non linear loss 1 dB Modulator i nsertion loss 0 ~ 1dB Waveguide crossing 0.05dB
110 Electrical Links: The electrical link between memory and mem ory controller is modeled as [ 74 ]. The transmission line between the PCM and the processor could be characterized using a simple RC model. In LPDDR2 NVM standard, DQ termination may not be used to conserve on power dissipation or board space. LPDDR2 NVM standard also uses the Low Voltage Complementary Metal Oxide Semiconductor ( LVCMOS ) logic level rather than Stub Series Terminated Logic ( SSTL ) in DDRx. Figure 4 8 shows the LPDDR2 NVM communication bus model The power consumed on the DQ bus c ould be calculated as: where the DQ data rate frequency is twic e the system clock frequency [86 ]. For the differential transmission line, the voltage supplied to the DQ bus is 0.5 times Optical Links: The power consumed on the optical channels is an aggregation of both the static power and the dynamic power. The static power consists of the required detection power for each resonator, the tuning power of the resonators when they are tuned to be ON stat e, the traversing optical power loss, and the power consumed by the heater of the resonator. The dynamic power is consumed by the modulators and photodetectors when modulating and detecting optical data. The important factor that affects the total static p ower consumption is the required optical detection power for a singl e photodetector. Prior study [ 65 ] shows that the power consumption of the photodetectors is related to the BER. We adopted expected BER of 10 15 [ 5 23 ] to ensure reliable end to end commu nications which required photodetector [ 65 ]. T he power consumption for different photonic components is summarize d in Table 4 4 [ 7 11 ]. By default, all the ring resonators are set to OFF state. The energy is required when they are tuned to ON state [ 3 ] and this in plane Poly Si
111 energy per reson ator is assumed to be 0.5 mW [ 18 ]. Assuming advanced driver circuits with poly Si carrier lifetimes of 0.1 1 ns, the power consumed by each modulator is approximately 200 fJ/bit [ 45 ]. The ener gy coupling efficiency of the laser source ranges from 30% [ 63 ] to 50% [ 7 ]. We use the median value of 40% in our power model. PCM Devices: The main difference between the DRAM and PCM is the organization of the cells. DRAM employs the 1T1C cell while PCM employs the 1T1R cell (shown in F igure 4 1 ). In the non volatile memory, the preactive commands that load the row address buffer replace the precharge command in the volatile memory. Figure 4 9 The power consumption under different memory states [88 ] per memory chip (P: dynamic power consumption of the peripheral circuits; C: power consumption of the cells; L: leakage power consumption; units: mW; n: number of modified bits per row) The PCM power has two major consumers: the peripheral circuits and the cells. We adopt the power consum ption profile from CACTI 6.5 [ 89 ] for the peripheral circuits,
112 and the power data of the PCM model extended from [ 78 ] in our study. The PCMs work in different states when operating, as shown i n Figure 4 9 Its timing parameter is shown in Table 4 4 In reading and writing states, the memory returns the latched data from the row buffers in response of the column access command. We obtain the sense amplifier power and the row buffer power data fr om CACTI. The preactive operation is analogous to the precharge in the DRAM accesses, which resets the row buffer to the idle state once the minimum p reactive latency (tRP) is satisfied In the preactive operation, the data stored in the row buffer is writ ten back to the memory cells using partial write where only the modified bits will be written [ 66 ]. In the active state, a row of data from the PCM cells are sensed, amplified and then latched into the sense amplifiers. All but power down states consume le akage power. The idle PCM device in power down state could save the leakage power however incurs longer exit latency going back to the idle or active state to serve incoming requests. By leveraging the mini rank design, it is possible to fine tune the stat e of each rank without impeding the data operation in other ranks. In power down state the memory device is still supplied with power however most of the peripheral circuits like input / output buffers are deactivated [ 90 ] The LPDDR2 NVM protocol supports the power down state with stopped clock The only overhead of resuming the clock is a NOP command before the next access command could be applied. T he deep power down state eliminat es power to both the peripheral circuitry and memory arr ay and will be supported in future LPDDR x protocol. In this study we choose to use the power down state with stopped clock Applying this state effectively reduce s the power consumption of the PCM device, while incurring limited entering and
113 exiting overhe ad. We use CACTI to estimate the leakage power consumed by the peripheral circuits. PCM s belong to the class of non volatile memory h ence there is almost no leakage power consumed by the cells [ 78 ]. The data f rom Micron P8P PCM datasheet [ 88 ] shows that l ess than 100 uA current per memory chip will be consumed in the low power state. So we assume that the power consumption in power down state is negligi ble, which is consistent with [ 9 1]. Performance Evaluation Power Consumption Breakdown T he PCM device po wer consumption could be categorized into three groups using the power analysis above : background operation, and read/write power The background power is consumed all the time when the memory chip powers on except for the power down state The device consumes operation power when performing activation or preactive operations. The read/write power is consumed when the device reads or writes data. We record the power in Figure 4 10 and find that most of the overall power consumption is decre ased when more ranks are deployed in OptiPCM. For example, the PRT 64, FCD 64 and DCD 64 reduce the overall power by 34.8%, 27.8% and 21.8% respectively compared with the baseline case. Due to the limitation of pages, we only show the results from half of the benchmarks; the remainders exhibit similar behavior
114 Figure 4 10 The breakdown of power consumption in OptiPCM The overall power reduction comes from the different groups. The memory cell R/W power is caused by the write/read operations. This p art of power is majorly proportional to the number of memory accesses and increases with the reduced execution time. For example, the PRT 64, FDC 64 and DDC 64 increase the power by 10.1%, 26.3%, and 40.3%. The memory operation power occupies a large propo rtion of the overall power consumption owing to the low power LPDDRx interface and low standby power. The operation power reduces significantly by 84.8%, 83.5% and 82.1% in PRT 64, FDC 64 and DDC 64 modes due to the reduced width of activated sense amplifi ers in smaller ranks. The memory background power is also preserved owing to the better utilization of low power state. The smaller rank will be staying at idle state more frequently than original design; the negligible cell leakage power helps saving the power consumption in that state as well. Thus, the PRT 64, FDC 64 and DDC 64 consume 42.7%, 40.2%, and 36.7% less background power on average. Moreover, the photonic channel saves 44.1% power on average compared with electrical channels though its power in creases with number of ranks.
115 Latency Evaluation under Different Memory Configurations Figure 4 11 The latency behavior in different test scenarios Figure 4 11 shows the latency improvement under different network configurations. Overall, t he SPEC 2006 benchmar k suit exhibits much higher latency than that of PARSEC owing to the significantly larger memory accesses. Among the network configurations, w e observe that the conventional mini rank design incurs an average of 1 6.9 % performance degradation, especially in the low traffic PARSEC benchmarks (on average of 27.3%) due to the insufficient channel utilization. After employing the photonic channels, the memory latency is reduced ( i.e. PRT 32 ) by 13.9% compared with the BASE This is caused by the increased ranks that hide the long PCM access latency and the high bus width that photonic waveguides provide The fixed channel division (FCD 32) improves the latency by 30.6% on average of SPEC and 3.6% on average of PARSEC benchmarks. The rank to rank turnaround penalty is significantly amortized in the high traffic benchmarks (SPEC) owing to fixed channel division; however, the slight performance improvement is mainly due to the increased bus width.
116 The dynamic channel division ( DCD ) compensate s for t he performance degradation under low traffic since it shows similar behavior with the PRT in low traffic while amortizes the rank to rank turnaround penalty when the traffic is heavy The dynamic channel division (DCD 32) gain s an average performance impro vement by 32.0% and 30.9% in SPEC and PARSEC compared with baseline case. The performance improvement comes from three aspects: 1) the increased ranks provide better parallelism 2) the increased width of photonic communication channel 3). The dynamical all ocation of channel width reduces the rank switch penalty while preserving the low latency when the traffic is low. System Throughput under Different Number of Ranks We evaluate the system throughput under different number of ranks. Increasing ranks could provide better memory l evel p arallelism to hide the long latency; however, the width of each sub channel may be re duced. We observe from Figure 4 12 that in most cases the system throughput improves with the increased ranks. The memory intensive benchmark s exhibit more significant throughput improvement than the non memory intensive benchmarks. For example, the DDC 32 and FCD 32 exhibits 28.1% and 39.7% improvement compared with BASE on average of all the SPEC benchmarks, compared with 3.0% and 15.5% impro vement on average of PARSEC benchmarks. This performance variation is due to the traffic difference; the increased number of ranks can hide the latency difference better when the traffic amount is high. Figure 4 12 shows that more ranks do not always help the system performance much. For example, the FCD 8, FCD 16, FCD 32, and FCD 64 promote the system performance by 19.8%, 26.2%, 28.1% and 28.7% respectively; the DCD 8, DCD 16, DCD 32, and DCD 64 improve the system performance by 30.0%, 37.3%, 39.8%, and
117 40.4% respectively. In both cases, 32 configuration is sufficient for parallelism, so 64 configuration demonstrates quite slight performance improvement while introduces in additional power consumption and manufacturing cost. On PARSEC benchmarks, the in creased number of ranks even hurt the performance in FCD due to the reduced sun channel width. Figure 4 12 Normalized memory throughput under different rank number (All results are normalized to BASE) FCD exhibits better performance compared with DCD especially on PARSEC benchmarks. For example, in 32 and 64 configurations, the DCD demonstrates 9.1% and 9.2% improvement on FCD in SPEC and 12.1% and 13.4% improvement on PARSEC. The DCD performs better on PARSEC since in could utilize the channel w idth better under light traffic compared with FCD. Channel Width Impact on OptiPCM
118 (a) Average latency of SPEC benchmarks (b) Average latency of PARSEC benchmarks Figure 4 13 The latency under different data bus widths We also evaluated the system performance under different data bus widths, from the contemporary 8 bytes to a whole cache line (64 bytes). The BASE following contemporary electrical channel supports up to 8 bytes wide data bus. We simulate the BASE confi guration, though unimplementable, with up to 64 bytes wide bus (shown i n dash in Figure 4 13). Figure 4 13 (a) shows that wide bus helps improving the performance of all the configurations. In PRT, FCD and DCD, the 64 byte bus improves the performance of 8 byte bus by 18.1% in SPEC and 24.0% in PARSEC. The 64 byte FCD improves 8 byte FCD performance by 31.1% in SPEC and 30.2% on PARSEC, which is the highest among the three network configurations. The FCD suffers from high latency especially in the low traf fic benchmarks due to the very narrow sub channel. I n th e memory intensive applications with FCD configuration, the memory accesses are able to fully utilize the sub channels and the system achieves better performance. Figure 4 13 (b) also shows that on the low traffic benchmarks (PARSEC), the increased ranks exhibit limited impact on the performance. For example, the FCD 32 exhibits a 4.0% improvement on FCD 16 on average of the PARSEC benchmarks, but
119 DCD 32 improves DCD 16 by only 0.9%, compared with 6.3% and 9.1% on SPEC. The latency of the channel affects the performance considerably in low traffic benchmarks. For example, the 64 bytes width FCD shows 27.7% performance degradation compared with 8 bytes width FCD.
120 CHAPTER 5 REL ATED WORKS Recent advances in silicon based photonic devices have inspired the possibility of realizing photonic NoCs that satisfy the communications requirement of future multi core processors. There are many researches focusing on the critical components of photonic network. A common factor among the above topologies is that they rely on high speed modulators, switches, multiplexers, de multiplexers, or detectors which utilize ring resonators as building blocks. The thermal reliability of ring resonators is left unaddressed. Temperature aware routing was proposed for electrical NoCs using ThermalHerd [ 92 ], a distributed run time thermal management scheme. ThermalHerd tries to mitigate the hotspot s generated at a router by reducing its workload using different routing schemes. High speed modulators have been demonstrated using two different structures: Mach Zehnder interferometers [ 93 ] and resonant structures [ 32 ]. Among those, Mach Zehnder interfe rometers h ave better thermal stability [ 20 ]. However, they are bulky and consume greater power. Ring resonators are smaller in size but highly sensitive to temperature variations [ 20 ]. The effect of temperature variations on the operation of ring resonator s has been mitigated by the use of metal heaters placed on top of waveguides. However, this method consumes significant power and is cumbersome to implement for large number of ring resonators. Improvements on metal heaters have been pr oposed by Manipatrun i et al. [ 25 ] by using localized heating due to PIN junction DC bias current. Nawrocka et al. [ 21 ] have also observed the undesirable shifting of resonance frequency due to temperature variations. To mitigate the thermal impact on ring resonators, Biberama n et al. [ 94 ] used integrated
121 wavelengths. However, for a chip with thousands of ring resonators, it is difficult to monitor and control each ring separately. Another app roach taken by [ 95, 96 ] is to induce pressure in the resonator by using oxidation methods to compensate for the effect of temperature variations on the resonance frequency shift. Guha et al. [ 97 ] proposed passive temperature compensation by coupling a ring resonator to a Mach Zehnder interferometer. A laser source can be tuned to follow the resonant frequency shift of the rings to compensate for temperature effects. Tunable lasers are commonly used in communicat ions and testing; Wang et al. [ 98 ] used a tuna ble laser with steps of 0.01 nm over the wavelength range of 1520 1580 nm to characterize micro ring resonators. Although all the above techniques have been demonstrated for a single or small number of resonators, they are difficult to implement in large s cale photonic NoCs because of implementation difficulty or the unwieldy number of terminals to control the heaters. Until a proper solution at the device level is found, architecture and OS level solutions must provide reliable operation under wide thermal variations. To our knowledge, there has been no prior work on designing thermal resistant photonic NoCs at architecture and system levels. Many researchers adopted the photonic techniques in the interconnection among the processing cores targeting at the reduction of the overall chip power consumption. However, prior solutions usually impose new challenges such as the auxiliary electrical network or oversimplified routing method. Shach am et al. [ 16 ] have proposed a 2D folded torus topology utilizes a circu it switched network. The electrical packet switched network is charged with the setup and breakdown of an optical path before transmitting
122 a data packet; the electrical network, however, incurs additional power and latency cost. Our ESPN architecture emplo ys a single optical network and thus achieves a low power high speed network. Vantrease et al. [ 15 ] proposed a fully connected high bandwidth low latency optical crossbar architecture. Every node possesses a dedicated channel through which other nodes can transmit data. The arbitration of the channels is performed through requests to the token channel. Although it can achieve a significant performance improvement, a system with N nodes requires at least N + 1 serpentine waveguides, which causes layout diffi culty and dramatic static power. P an et al. [ 14 ] proposed a clustered hybrid electrical/optical architecture to address the scalability problem in [ 15 ]. The crossbar is partitioned into multiple crossbars in order to localize arbitration. In [ 47 ], a flexib le crossbar is proposed to minimize the static power consumption by fully sharing a reduced number of channels across the network through doubling the modulators and photo detectors. It copes with spatial traffic imbalance among cores but does not address the temporal traffic imbalance. Cianchetti et al. [ 13 ] proposed a switch based on chip photonic network that uses source based routing and reconfigurable optical switches. It introduces electrical buffers into the network design to address the contention p roblem. However, the power efficiency and network performance in heavy traffic degrade significantly due to the frequent accesses of electrical buffers. Tan et al. [ 99 ] propose a micro ring resonator based generic wavelength routed optical router. They con struct the optical routers by minimizing t he number of passive micro ring resonator s used Ye et al. [ 100 ] takes advantage of both electrical and optical routers and interconnects in a hierarchical manner. It employs adaptive power control mechanism, low l atency control protocols, and hybrid optical
123 electrical routers However, the power consumption and performance is still restricted by the electrical network. In this dissertation our ESPN architecture avoids the use of electrical buffers by leveraging op tical routing to establish routing paths. Some other researchers start working on the photonic connected memories; but most of the previous works are targeting at the DRAM based memory architecture. They usually focus on leveraging the photonic links to su pport the memory device in order to preserve power and achieve higher performance. For example, Beamer et al. [ 78 ] propose to use photonically interconnected memory by using a monolithically integrated silicon photonic technology to replace replacing electrical I/O and the electrical links across the memory chips. Hadke et al. [ 101 ] leverage the low latency optical links to replace the multi hop store and forward network in the fully buffered DIMM [ 7 0 ] and successfully support up to 32 DIMMs Hendry et al. [ 102 ] propose the photonic network on chip with integrated memory I/O interfaces to build a uniform photonic network on chip communication system using the low latency and low power photonic links. Udipi et al. [ 103 ] propose a 3D stacked me mory architecture by adding an interface die to handle the conversion between optics and electronics. The majority of p rior photonically connected memoriy architectures focus on leverag ing the photonic channels to reduce the power consumption on communicat ion channels The power saving on communication channels however, is very limited in LPDDR2 NVM environment where the channel consumes less than 20% of the memory system power [ 67 ]. Malladi et al. [ 77 ] leverages LPDDR2 based DRAM in the data centers. They architect server memory systems using mobile DRAM devices, trading peak bandwidth for lower energy The fundamental difference between
124 our work and prior work s is: in our design the major part of the power saving and performance improvement are gained from more effective management of PCM, not just from the channels Our design substantially improves the PCM performance and achieves significant power saving from the management of PCM power consumption thanks to the large number of ranks enabled by photonic channels
125 CHAPTER 6 CONCLUSION Photonic NoCs provide many benefits over their electrical counterpart such as low latency, high bandwidth density, and repeater less long range transmission. Recent advances in silicon photonics and integration with microelectronics present an opportunity to exploit the benefits of photonic NoCs in future multi core processors. To implement photonic NoCs, high speed modulators, switches, multiplexers, demultiplexers, and dete ctors must be integrated into the chip. Recent work has demonstrated that ring resonators can be used as versatile devices to build photonic NoCs. However, architecting photonic NoCs presents new challenges. In particular, thermal effects are a major conce rn. For example, temperature variations cause a shift in the resonance frequency prompting the resonator to respond to a different frequency. By responding to a different frequency, unintended operations occur or data is corrupted, thereby introducing high BER or even causing faulty operation in a photonic NoC. Thermal effects deteriorate the reliability and performance of photonic NoCs. Therefore, the successful implementation of photonic NoCs hinges on the ability to overcome thermal challenges. Current d evice level methods to counter the effect of temperature variations are difficult to implement in CMOS processes or not suitable for large scale on chip networks. W e address the effect of temperature variations on photonic NoCs and propose cross layer solu tions targeted at the circuit, architecture, and OS levels to mitigate their effect and improve reliability. At the circuit layer we maintain the original temperature by varying the bias current through ring resonators. At the architecture layer, we re rou te messages away from hot regions and through cooler regions to their destinations. At the
126 OS layer, we employ thermal/congestion aware co scheduling to relocate workloads to the outer cores where heat dissipation is more efficient. This encourages message s to use the cooler center of the processor. The solutions can be integrated with each other to further reduce the BER and improve reliability. We have shown that the average BER and MER are reduced by 94% (96%) and 76% (84%) for SPF+OS (and TF+OS) techniq ues respectively. We have also shown that the power consumption can be reduced by up to 36% by applying the three techniques. Also, substantial static power consumption limits the scalability of photonic NoCs. W e presented an energy efficient photonic NoC architecture t o enable traffic aware dynamic allocation of network resources. We showed that dynamical coupling of communication resources and network traffic could save up to 50% of the total optical NoC power. We also employ an optical based adaptive ro uting circuit switch, which reduces the application execution time by 22%. By integrating these technologies, our structure achieves 51% and 60% power and energy savings compared to the baseline case. We also explored a high performance and energy efficien t memory communication infrastructure for PCMs. The widely used LPDDRx NVM protocol provides an implementation of energy efficient memory interface for non volatile memories, however, degrades the system performance. We propose to leverage photonic channel s in place of the conventional electrical channels. The photonic channels save energy from both the memory organization and communication channel. Meanwhile, our OptiPCM structure supports large number of memory devices on photonic channels and is able to process a large number of concurrent accesses and
127 thus improves the system performance. The channel division provides better channel utilizations and further improves the system efficiency. Our simulation results show that PRT, DCD and FCD design saves up to 34.8%, 27.8%, and 21.8% overall power consumption while increases the system performance by 22.3%, 28.7%, and 40.4% respectively.
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138 BIOGRAPHICAL SKETCH Zhongqi Li was born in Chengdu China. He received his B.S. and M.S. degree in e lectrical e ngineering from University of Electronic Science and Technology of China Chengdu China, in 200 6 and 2009 respectively. He received his PhD degree in electrical and computer engineering from the Univers ity of Florida, Gainesville, FL in 201 2 He is a recipient of UF Alumni Graduate Fellowship He has interned as a processor modeling engineer at Marvell Semiconductor in 2012. His current re search interests include the CPU/GPU architecture and the interconnect network of the heterogeneous processor.