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Title: Parallel simulation of chip-multiprocessor architectures
Physical Description: Book
Language: English
Creator: Chidester, Matthew C.
George, Alan D.
Affiliation: Intel Corporation
University of Florida
Publisher: High-performance Computing and Simulation Research Laboratory, Department of Electrical and Computer Engineering, University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2002
Copyright Date: 2002
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2002, HCS Research Lab, U of Florida All Rights Reserved


Parallel Simulation of Chip-Multiprocessor Architectures


MATTHEW C. CHIDESTER AND ALAN D. GEORGE


High-performance Computing and Simulation (HCS) Research Laboratory
Department of Electrical and Computer Engineering, University of Florida
P.O.Box 116200, Gainesville, FL 32611-6200





Primary Contact Author

Dr. Alan D. George
High-performance Computing and Simulation (HCS) Research Laboratory
Department of Electrical and Computer Engineering
University of Florida
216 Larsen Hall, P.O. Box 116200
Gainesville, FL 32611-6200

Phone: (352)392-5225
Fax: (352)392-8671
e-mail: george@hcs.ufl.edu





Additional Author Contact Information

Dr. Matthew C. Chidester
Intel Corporation
8104 SW Charlotte Drive
Beaverton, OR 97007

Phone: (503)259-0831
e-mail: matthew.c.chidester@intel.com






2002, HCS Research Lab, U of Florida All Rights Reserved


Parallel Simulation of Chip-Multiprocessor Architectures

Matthew C. Chidester and Alan D. George

Abstract-Chip-multiprocessor (CMP) architectures present a challenge for efficient simulation,
combining the requirements of a detailed microprocessor simulator with that of a tightly-coupled
parallel system. In this paper, a distributed simulator for target CMPs is presented based on the
Message Passing Interface (MPI) designed to run on a host cluster of workstations.
Microbenchmark-based evaluation is used to narrow the parallelization design space concerning
the performance impact of distributed vs. centralized target L2 simulation, blocking vs. non-
blocking remote cache accesses, null-message vs. barrier techniques for clock synchronization,
and network interconnect selection. The best combination is shown to yield speedups of up to 16
on a 9-node cluster of dual-CPU workstations, partially due to cache effects.

Index Terms-Chip-multiprocessor, parallel simulation, MPI, SCI, Myrinet


1 INTRODUCTION

Simulation is an important tool in the development of new microprocessors. A design must be

rigorously validated prior to implementation to ensure that it is both functionally correct and

performs well. A large range of test cases must be explored while many configurations are

compared to optimize the design. As microprocessors become more complex, encompassing both

a larger number of transistors and numerous architectural features for increased performance, the

design space that must be evaluated through simulation explodes.

In the design of parallel systems, simulation times are increased further by the need to

simulate multiple processors, a still wider range of inputs, and larger datasets. One technique for

reducing the simulation time is to scale datasets down in size [8], but this approach introduces

inaccuracies and necessitates a detailed analysis of each workload to determine which parts) can

be safely scaled. Another performance enhancement involves simplified processor models with

an emphasis on accurate memory subsystem and interconnect simulation [24]. However, these

simplified models do not reflect the behavior of modem superscalar, out-of-order processors. A

third method to address simulation explosion is to make use of parallel simulation.






2002, HCS Research Lab, U of Florida All Rights Reserved


Parallel simulation employs multiple processing nodes to increase the simulation rate. A

common approach is to perform separate simulations for different settings of parameters-i.e.

cache sizes, workloads, datasets, etc.-simultaneously on different processors of a parallel

system. While this approach can greatly increase the throughput of the simulations, it does not

reduce the latency required for a single simulation to finish. Often, a designer desires rapid

feedback regarding a specific change in order to guide future decisions. For such situations, low-

latency turnaround time is preferable to high throughput. Parallel simulation of event-driven

models such as logic- and circuit-level evaluation has been successful in reducing these

computationally intensive tasks. However, many of the architectural design choices for modern

uniprocessors must be made early in the design cycle with the aid of a fast "performance model"

that is often written in C or C++ [28]. Parallel simulation has been applied to such systems in the

past [29],[24], but has been limited to modeling loosely-coupled, distributed shared-memory

(DSM) systems and used direct-execution to model the individual processing elements.

Future systems are likely to be composed of multiple processors integrated on a single die

known as chip-multiprocessors (CMPs). The CMP has been the subject of many research projects

[9],[17],[13],[3] as well as commercial implementations such as IBM's Power4 [20] and HP's

Mako [19]. CMPs reduce the impact of interconnect delay on clock frequency [1] and reduce

design time by making use of repeated, regular structures. A CMP also lends itself to parallel

simulation: the processors in a large target CMP can be distributed to different nodes of a host

parallel machine and simulated simultaneously. However, the tight coupling of the processors on

a CMP with a shared L2 cache and the low latency of such accesses serve to increase the

communication demands of a parallel CMP simulator over that of a DSM system.

This paper explores the parallel simulation of a CMP on a distributed host system consisting

of commercial off-the-shelf (COTS) workstations connected with a high-speed network [2]. For






2002, HCS Research Lab, U of Florida All Rights Reserved


portability, the simulator uses the Message Passing Interface (MPI) [23] for inter-node host

communication. Several parallelization schemes are evaluated, including a distributed target

cache model vs. a centralized scheme, blocking vs. non-blocking accesses to the remote cache,

and the use of null-messages vs. barriers to maintain clock synchronization. Microbenchmarks

are used to evaluate the performance of each alternative prior to constructing the full simulator.

Two popular system-area networks (SANs), the Scalable Coherent Interface (SCI) [31] and

Myrinet [4], are also evaluated as the interconnect for the host platform.

In the next section, conventional performance modeling of a CMP and common parallel

simulation methods are introduced. Section 3 describes several parallelization approaches

considered for the CMP simulation. In Section 4, the alternate approaches are studied through the

use of MPI-based microbenchmarks running on the target platform to quantify the tradeoffs

associated with each. Section 5 demonstrates the performance of a parallel CMP simulator based

on the algorithm selected from the microbenchmarks. Section 6 provides a discussion of related

research. Finally, Section 7 contains conclusions and avenues for future research.

2 BACKGROUND

In order to understand the design challenges of a parallel simulator for a CMP, it is important to

consider the state-of-the-art in sequential simulations of uniprocessor and multiprocessor systems.

Also, there are many approaches that can be used to parallelize a sequential simulation. In this

section, both issues are addressed separately.

2.1 Sequential Simulation of a Microprocessor

Traditional, sequential simulation for performance modeling of a microprocessor typically

achieves the fastest simulation rate by employing some form of trace-driven simulation. In trace-

driven simulation, application code is run through a fast, functional simulator or on an existing

machine (perhaps even of a different architecture from that being simulated). A "trace" of







2002, HCS Research Lab, U of Florida All Rights Reserved


important activities is logged, consisting of a sequence of decoded instructions with specified

input and output dependencies, memory accesses, and branch instructions. The trace is then fed

into a timing simulator to determine the execution time of the instruction sequence given stalls

due to dependencies, cache misses, and branch mispredictions.

Because the trace files have a tendency to grow rather large, in practice they can be generated

dynamically at the front-end of the simulator. This approach also allows simulation of

mispredicted instructions, an effect that has been shown to cause cache pollution and significantly

impact performance [32]. Figure 1 shows a block diagram of a sequential simulator such as

SimpleScalar [5].


Trace Generator Instruction Timing Simulator
Trace Execution
Architected Program Counter Execti
Register Instruction Pipeline
Application State
Code [Simulated
Code Simulated Dependency Tracking
Branch Memory Cache
Prediction Space Statistics
Logic Mispredict Clock Cycle Counter
Detection


Figure 1. Trace-driven simulation of a uniprocessor using a sequential simulator.


The front-end of the simulator maintains the programmer-visible register state including the

program counter and the memory space occupied by the simulated application code. In order to

simulate mispredicted instructions, branch prediction is also performed by the trace generator. In

this respect, trace generation performs the same duties as the fetch stage in a pipelined

microprocessor. The back-end consists of a timing simulator which feeds the trace through the

complete pipeline, stalling on register dependencies and cache misses when appropriate. The

timing simulator notifies the trace generator when a mispredicted instruction is retired so that a

new trace can begin from the correct path.

The instruction trace is generated by executing each instruction as soon as it is fetched. By

executing instructions in-order, the outcome of all branches can be immediately known. This






2002, HCS Research Lab, U of Florida All Rights Reserved


technique allows simulation of key boundary conditions, such as perfect branch prediction. More

importantly, it allows the remainder of the timing simulation to know a priori when an instruction

is on a mispredicted path, greatly simplifying the recovery of speculative state when a

mispredicted branch is resolved.

2.2 Sequential Simulation of a CMP

A chip-multiprocessor consists of multiple processors integrated on a single die. Unlike

conventional symmetric multiprocessors (SMPs) that share only a common memory space, the

processors in a CMP interface at a common cache level such as the L2 cache [25]. An example of

a 4-processor CMP is shown in Figure 2.

Branch Prediction Branch Prediction

CPU 1 CPU 2

L1 Cache L1 Cache L2
Branch Prediction Branch Prediction Cache

CPU 3 CPU 4

L1 Cache L1 Cache

Figure 2. Organization of a CMP.


To enable simulation of such a CMP, the simulation of each processor is interleaved on a

clock-cycle basis. In a sequential simulation, the outermost loop simulates a single clock cycle

for each CPU in the CMP before incrementing the clock-cycle counter and repeating the loop.

This approach enforces time-ordered consistency between each processor in the CMP. By

incrementing the cycle counter only when all processors have completed execution of that cycle,

any external effects which one processor may have on another are maintained.

In the CMP configuration shown in Figure 2, the only external effects which one processor

may have on another involve accesses to the shared L2 cache and, by extension, the global

memory. Maintaining coherency of each processor's view of this shared resource is another

major issue in parallel simulation.






2002, HCS Research Lab, U of Florida All Rights Reserved


For the CMP architectures evaluated in this paper, a Modified-Shared-Invalid (MSI) protocol

[18] is used to enforce coherency between the L1 caches and the shared L2 cache. A given cache

line in the modified state indicates that only one L1 cache contains that data and both read and

write access is permitted. A line in the shared state may be present in multiple L1 caches and is

therefore read-only. If a processor desires write access to a shared line, it must first request

exclusive access through the L2. The L2 cache will invalidate any other shared lines and grant

the access. A line that is not present in any cache is said to be in the invalid state.

2.3 Parallel Simulation Techniques

Research in the area of parallel simulation techniques focuses primarily on event-driven

simulation. In a parallel, event-driven simulation, each parallel process maintains a queue for

locally generated events and separate queues for events generated by each remote process that

will influence the local process. Events are processed in-order, with the event having the smallest

timestamp of those in any queue, local or remote, processed first.

The challenge in parallel simulation is insuring that events are processed in globally

consistent order. That is, an event can only be processed by the destination node when it can be

sure that no events with an earlier timestamp will arrive at a later time. One way to ensure such

consistency is to wait until all remote input queues contain at least one event before selecting the

one with the lowest timestamp.

Such a scheme can easily lead to deadlock situations when there are no events generated for a

particular remote process. A common method to avoid this situation is to employ lookahead and

null messages [6]. If a process is deadlocked waiting for an event from one or more remote

processes, it will send a null message to the other processes in the system. The null message

represents an event that does not require any action but indicates the earliest timestamp for which

an actual event may be received from the source node.






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This timestamp is computed by taking the current time step of the source node plus a

lookahead value. The lookahead value is system-dependent and represents any processing time

that would be applied to incoming events before an outgoing event can be generated. An example

of null-message-based synchronization applied to a CMP will be provided in Section 3.3.

The use of lookahead and null messages can result in a significant level of generated null-

message traffic, particularly if the lookahead is small or if the processes infrequently generate

events destined for remote nodes [10]. Another method is to use barrier synchronization [14].

In this approach, all processes process events freely up to a certain timestep and then wait for all

other processes to reach the same timestep. If the barrier interval is selected based on the

lookahead value, each process is guaranteed to have received any remote events that may

influence the current barrier-bounded interval prior to the barrier at the beginning of the interval.

3 PARALLEL SIMULATION APPROACHES FOR A CMP

In this section, the parallel simulation techniques presented in the previous section are combined

with simulation of multiprocessors to produce several alternatives for parallel simulation of a

CMP. For performance reasons, architecture-level simulations of microprocessors are cycle-

driven rather than event-driven. Typically, hundreds of events occur each clock cycle in a

speculative processor, making the overhead of event queues too costly. Therefore, traditional

event-driven parallel simulation is not directly applicable to a CMP.

However, as noted previously, a CMP can be viewed as a collection of independent

processors that can only affect one another through the memory hierarchy. If the architecture of

Figure 2 is assumed, all such external events will occur through the shared L2 cache. While a

single processor may access memory multiple times per clock cycle, modern L1 caches provide

hit rates in excess of 95%, so accesses to the L2 cache are relatively infrequent.






2002, HCS Research Lab, U of Florida All Rights Reserved


In this paper, we adopt a scheme similar to [29] where the processor and L1 caches are

modeled with conventional, cycle-driven simulation techniques, but parallel, event-driven

simulation is applied between each L1 cache and the shared L2. However, the processor model is

more detailed along the lines of those in [5] or [26] than the direct execution used in [29].

Because L2 accesses are infrequent and have a high latency in terms of the simulated timespace,

the simulation can be parallelized using a message-passing approach, allowing use of cost-

effective and scalable distributed systems as the underlying simulation platform.

The following discussion assumes that the parallel simulation will be divided into one or

more threads for concurrent execution. The mapping of threads to nodes on the simulation

platform will be considered later.

3.1 Centralized vs. Distributed L2 Parallelization

One design choice for parallelization involves the shared L2 cache. The most straightforward

approach is to simulate the target's L2 cache in a dedicated thread on the host and each target

processor in a separate thread, as shown for ap-processor CMP in Figure 3a. The dotted lines in

this figure indicate that each target processor and associated L1 cache are simulated in a

processor thread while the L2 and memory subsystems are handled by the L2 thread. The L2

thread also models the desired interconnect and contention between processors. It is possible for

each processor thread to handle more than one target processor/L1 pair when p exceeds the

number of processor threads, N, although in this typical example, N= p and the total number of

threads isp + 1.

The centralized approach has several disadvantages. First, by requiring a thread dedicated to

L2 transactions, the parallel efficiency is reduced. Parallel efficiency is defined as the speedup

divided by the number of processors in the simulation platform, where speedup is the execution

time of a sequential simulation divided by the execution time of the parallel simulation. Since the






2002, HCS Research Lab, U of Florida All Rights Reserved


speedup is limited to N, the parallel efficiency is at most NI (N+I) which is poor for small N. For

large N, the centralized L2 becomes a bottleneck. To alleviate the first problem, one of the

processor threads can take on the dual role of simulating one or more target processor/L1 pairs

and the L2 cache. However, this approach limits the performance of the target processor

simulations of that thread, requiring the other processor threads to wait on the slower thread.

Processor Threads Processor Threads
---II -II- I -- II--- II---- I----
Po PI P2 ----- P Po P P2 ----- PP I
I, I I I~P

LLL1 L1 L1 L1 I L1i L1i


L2 L2 L2 L2 I L2
L2 Thread I i ----
Memo Mem Mem Mem Mem
I----------------------------
a) Centralized L2 simulation b) Distributed L2 simulation
Figure 3. Centralized vs. distributed simulation of the L2 cache.


An alternative approach is to distribute the simulation of the L2 cache across all processor

threads and eliminate the L2 thread as in Figure 3b. While this figure assumes p = N, each host

thread can simulate multiple target processor/L1 pairs as well as part of the L2 cache when p > N.

The memory space is interleaved on a cache-block basis with each thread having responsibility

for all accesses to a particular bank of memory. If lower-order interleaving is used, the accesses

should be relatively evenly distributed, allowing the performance to scale as Nis increased.

One drawback to a distributed L2 simulation is the potentially high cost to simulate

contention. If the number of target L2 cache ports must be limited to fewer than N or if modeling

a non-banked cache is required, each portion of the L2 must communicate with the other portions

to arbitrate for access. Another drawback involves maintaining a global view of the clock. With

a centralized L2 simulation, all-to-one and one-to-all communication can be used to maintain

clock synchronization, but a distributed L2 simulation requires costly all-to-all communication.






2002, HCS Research Lab, U of Florida All Rights Reserved


The performance of either approach depends on the frequency of cache accesses and clock

synchronization events in the target system as well as the number of threads tasked to the

simulation on the host. These tradeoffs will be evaluated in greater detail in Section 3.3.

3.2 Blocking vs. Non-blocking L2 Requests

Another design decision with a large potential effect on parallel simulation performance involves

the accesses to the target L2 cache. Traditional, sequential simulators applying a trace-driven

approach as in Figure 1 decouple the memory access from cache hit checking. In simulations of

CMP architectures, the cache access is integral to the data access due to the need to maintain

coherency. Therefore, the trace generator must perform memory transactions through the cache

logic. With either centralized or distributed L2 parallelization, a remote access is required on the

host to obtain the data for every L2 access in the target.



Fetch Decode Rename Issue Execute WB Retire


L2 Access

a) Blocking L2 Access


Fetch Decode Rename Issue Execute WB Retire
--- L2 miss cycles -
L2 Request L2 Response
Made Required

b) Non-blocking L2 Access

Figure 4. Blocking vs. non-blocking accesses to the L2 cache.


Supporting trace-driven cache accesses requires the accesses to be blocking. That is, each

target memory reference is processed in order by the front-end of the simulator to satisfy

dependencies before moving on to the next instruction. Figure 4a demonstrates the placement of

the L2 accesses of such a simulation on a processor with 7 pipeline stages similar to that which






2002, HCS Research Lab, U of Florida All Rights Reserved


will be used later in this paper. Trace generation occurs during the fetch stage of each instruction,

so the processor thread must block until a response from the remote L2 is received.

As mentioned in Section 2.1, trace-driven simulation greatly simplifies the task of handling

mispredicted instructions. However, requiring blocking access to the L2 cache in a parallel

simulation is a significant disadvantage. The L2 transactions now initiate a remote transaction

and therefore have a much higher latency than in a sequential simulation. While the transaction is

outstanding, no other instructions in the simulated processor can be fetched, limiting performance.

Another disadvantage to trace-driven simulation is that it introduces inaccuracies in modeling

parallel systems due to imprecise timing of events. Such inaccuracies can profoundly impact the

outcome of shared accesses such as lock contention. Depending on the desired level of detail in

the target platform, such a scheme may be undesirable from a purely functional standpoint.

An alternative approach is to allow non-blocking accesses to the L2 cache as in Figure 4b.

This approach more closely resembles the timing and execution of instructions in hardware than

the trace-driven approach. The request is performed when all input dependencies are satisfied and

the instruction is issued. Since an L1 miss will have a latency of several cycles in the target

processor even if it hits in the L2, the simulation can continue for a number of cycles before the

response from the L2 is required in the writeback stage. In this manner, the host communication

latency for the remote L2 transaction can be hidden.

The major disadvantage of a non-blocking approach is added simulator complexity. Trace-

driven simulation is no longer employed and branch mispredictions are not readily identifiable.

3.3 Null-message vs. Barrier Synchronization

Both the null-message and barrier synchronization approaches described in Section 2.3 can be

straightforwardly applied to the interface between the target L1 and L2 caches in a parallel CMP






2002, HCS Research Lab, U of Florida All Rights Reserved


simulator. In both cases, the timestep for synchronization is based on the target L2 cache access

time in terms of simulated clock cycles.

For example, in a CMP system where events are generated between the target processors and

the shared L2 cache with directory-based coherency, the lookahead interval would be the target

L2 cache access time, L, where "cache access time" refers to the minimum number of cycles from

the time an L2 request is issued to the time that the L2 returns the data on a cache hit without any

coherency misses for the desired target system. For simplicity, it is assumed that a coherence

miss will not generate any traffic (i.e invalidate requests, state downgrades, etc.) before this time,

but systems such as that of a snoopy bus could also be modeled if the lookahead is adjusted

accordingly. If the L2 has processed all events up to and including time t, the earliest time that

an event destined from the L2 to a processor (e.g. the data returning or a cache line invalidation)

can be generated is t + L. The L2 could send a null message to each host processor thread

indicating that it is safe to proceed up to time t + L without the possibility of receiving an event

with an earlier timestamp from the L2.

Similarly, the barrier synchronization approach requires all host threads to wait at a barrier

every L clock cycles. In this manner, requests issued prior to a barrier can only affect events after

the barrier. As long as messages are received in-order and the L2 thread processes all outstanding

requests before reaching the barrier, consistency is preserved between all host threads.


4 EVALUATION OF SIMULATOR DESIGN ALTERNATIVES

In order to select the best parallel simulator design from the alternatives presented in Section 3,

each design decision must be studied with regard to performance tradeoffs. Developing a

simulator for each alterative would be very costly in terms of design time and complexity.

Instead, several MPI-based microbenchmarks were developed to enable study of the tradeoffs






2002, HCS Research Lab, U of Florida All Rights Reserved


associated with the parallel algorithm alternatives when executed on the target system. Key

components of the full-fledged simulator are abstracted and provided as parameters to a much

simpler parallel program that, in essence, simulates the behavior of the final application. The

approach is much simpler than the analytical models of WWT found in [12], trading accuracy for

rapid prototyping.

4.1 Evaluation Platform

The purpose of the microbenchmarks is to model the expected communication behavior of the

parallelized simulator with reasonable accuracy. In this section, we consider the parameters of the

simulator necessary to describe its communication pattern, introduce the design space that will be

explored, and describe the experimental platform on which the microbenchmarks and, ultimately,

the parallel CMP simulator will be executed.

4.1.1 Simulation Parameters

As in most parallel applications, the performance of a parallel CMP simulation is largely limited

by the communication. The communication consists of two components: data-value

communication between the separate L1 and shared L2 caches of the target system and clock-

cycle synchronization. The latter is largely determined by the parallel algorithm and is a key

component for microbenchmark evaluation. The former is a product of the simulated application

and the timing simulator. For simplicity, we will abstract the cache-to-cache communication

pattern using parameters measured from a sequential version of the simulator.

The simplest model for the cache transactions involves only two parameters: the time

required to simulate a single clock cycle for a single processor of the CMP, tc, and the average

number of cache transactions generated each cycle, A. We define tc in terms of the rate of a

uniprocessor simulation in cycles/sec, fc. The microbenchmark simulates the full application by

simply delaying for t, seconds per simulated clock cycle. Since experimental analysis shows A to






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be less than one, the microbenchmarks generate cache requests in a given clock cycle only when a

randomly chosen value between zero and one is found to be less than A.

Additional parameters to the microbenchmarks include the number of processors in the target

CMP, p, and the number of host processor threads in the parallel simulation, N. The clock

synchronization frequency depends on the lookahead, in this case the L2 cache access latency,

and is defined as L. The parameters and their default values, taken from a sequential version of

the CMP simulator, are summarized in Table 1.

Table 1. Parameters to microbenchmark simulations.
Parameter Values Description
t, 1 /fc seconds Simulation latency per clock cycle
fc 100000 cycles/sec Simulation rate of clock cycles
A 0.045 accesses/cycle L2 cache accesses generated per cycle
L 12 cycles Lookahead for parallel simulation
p 2 32 processors Number of processors in target CMP
N 1 16 threads Number of processor threads in host


The t, value reflects the performance of the sequential simulator. Smaller values indicate a

faster sequential simulator, therefore more difficult parallelization due to higher communication-

to-computation ratio. The parameter A is comprised of four access types: L1 instruction-cache

misses, L1 data-cache misses, L1 data-cache writebacks, and L1 coherence misses. Including a

term for the variance of the A parameter would enable more accurate workload characterization,

but it will be shown that even a simple model without variance has sufficient accuracy.

4.1.2 Alg t il/nl Alternatives

Using the microbenchmark tests, each of the three major design options presented in Section 3

will be explored: centralized vs. distributed L2 cache simulation, blocking vs. non-blocking L2

cache accesses, and null-message vs. barrier clock synchronization. Table 2 illustrates the naming

convention we will use in the remainder of this section to distinguish between design alternatives.






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Table 2. Parallelization alternatives.
Name L2 Simulation L2 Accesses Clock Synchronization
CB Centralized Blocking
CBN Centralized Blocking Null-message
CBB Centralized Blocking Barrier
CN Centralized Non-blocking
CNN Centralized Non-blocking Null-message
CNB Centralized Non-blocking Barrier
DN Distributed Non-blocking
DNB Distributed Non-blocking Barrier


Note that not all possible combinations are present in the table; when appropriate, the design

space has been narrowed through the tradeoff analysis presented later in this section. Also, three

incomplete designs are examined: CB, CN, and DN. These configurations do not conduct any

form of clock synchronization and therefore could not be used in the full parallel simulator. They

are presented as a baseline to establish the impact of clock synchronization communication

relative to the data communication.

4.1.3 Experimental Platform

In the parallel simulator, the vast majority of messages are either target cache requests, cache

responses, or clock synchronization messages. A cache request contains an address field, a source

processor identifier, and a timestamp for a total payload of 32 bytes. Messages that are

considered cache requests include line fill and upgrade requests from an L1 to the L2 cache or

line flush and downgrade requests from the L2 to an L1 cache. Cache responses consist of the

same identifying fields as the cache request plus the associated data. Cache lines are 32 bytes,

giving cache response messages a total payload of 64 bytes. Cache responses include both line

fills from the L2 to a requesting L1 and writebacks from an L1 to the L2. Clock synchronization

messages for null-message-based synchronization contain only a 16-byte timestamp, while barrier






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synchronization allows use of zero-byte messages. The type of message is identified through the

MPI_TAG field and therefore does not add to the payload length.

The SCI testbed consists of 9 nodes connected in a 3x3 unidirectional torus. Each node

contains two 733-MHz Pentium-III processors using a Serverworks LE chipset and 256 MB of

PC133 SDRAM. The SCI adapters are from Dolphin and Scali [30] and feature a link speed of

4.0 Gbps with a 32-bit, 33-MHz PCI interface. Version 2.1.2 of Scali's implementation of MPI,

ScaMPI, provides the messaging layer.

The Myrinet testbed consists of 9 nodes connected through an M2L-SW16 16-port switch.

Each node features dual 600-MHz Pentium-III processors and an i840 chipset with 256 MB of

PC100 SDRAM. The Myrinet adapters have a link speed of 1.28 Gbps with a 64-bit, 66-MHz

PCI interface. GMPI 1.2.3 is used for the messaging layer.

The disparity in processor clock frequency between the SCI and Myrinet testbeds is negated

by the fact that the per-cycle computational delay, tc, is measured in absolute seconds. Therefore,

both platforms assume the same uniprocessor simulation rate. Any differences measured in

performance will be due to the communication delay alone.

Figure 5 compares the latencies of SCI and Myrinet. The plotted data is for one-half of the

round-trip time (RTT) of an MPI message of specified size. SCI provides a much lower latency

than Myrinet, particularly for small packet sizes. The 32-byte cache request messages have a

latency of about 7.1 |ts under SCI and almost 19 |ts with Myrinet. The larger cache response

messages require latencies of 10.7 |ts for SCI and 19.1 |ts for Myrinet. The small clock

synchronization messages have an extremely low latency with SCI at 5.2 |ts for an 8-byte

message in the null-message scheme or 4.5 |ts for a zero-byte barrier message. Myrinet shows no

advantage to very small messages, requiring about 17.6 |ts for either size.







2002, HCS Research Lab, U of Florida All Rights Reserved


35
30 --M
25
2 20
S15
10
5
0
0 64 128 192 256 320 384 448 512
Payload Size (bytes)
SCI -x-Myrinet

Figure 5. RTT/2 latencies for MPI messages over SCI and Myrinet.


4.2 Microbenchmark Results

In this section, the microbenchmarks are used to determine the best design alternative from those

discussed above. First, the combinations of L2 simulation, access approach, and clock

synchronization protocol will be examined through experiments run on the SCI testbed. Then, the

performance of the optimal scheme will be compared against the same on the Myrinet testbed.

In the following experiments, it should be noted that N refers to the number of host processor

threads. For the distributed L2 simulations, there are N total threads in the parallel simulation.

For the centralized L2 simulations, an additional thread is used for the L2 cache requiring N + 1

total threads. When parallel efficiencies are provided, the efficiency is in reference to the total

number of threads as appropriate for the L2 simulation.

The mapping of host threads to nodes on the testbed is one-to-one except when N = 16.

Because the SCI testbed is limited to 9 nodes, two processor threads are run on each SMP node,

sharing a single SCI interface. When a centralized L2 is required, the L2 thread is run alone on

the ninth node. The same configuration is used on the Myrinet testbed.

4.2.1 Parallelization Schemes

The first set of microbenchmark experiments makes use of a centralized L2 cache. Figure 6a

shows the performance with blocking L2 accesses while Figure 6b demonstrates non-blocking







2002, HCS Research Lab, U of Florida All Rights Reserved


performance. Both figures compare the speedups obtained with null-message and barrier

synchronization. Two baselines are provided: an ideal speedup and the speedup when no clock

synchronization is performed. The ideal speedup is defined as the number of processor threads.


16 16
14 14
12 12

B l -
> 6 6

2 2 -
0 0
1 2 4 8 16 1 2 4 8 16
# of processor threads # of processor threads
-.- Ideal -- CB -*- CBN -x- CBB --Ideal CN -- CNN x-CNB
a) blocking L2 accesses b) non-blocking L2 accesses

Figure 6. Microbenchmark performance predictions for centralized L2 cache schemes on SCI
testbed (p =N,fc = 105, A = 0.045, L = 12).


The first trend evidenced in Figure 6 is that the CB and CN approaches both track well with

the ideal. Furthermore, there is very little difference between CB and CN because the L2 requests

from each processor are very infrequent. However, a large reduction in performance results when

clock synchronization is added. This reduction is due to the fact that host processors must wait

even when they do not have a target L2 request of their own when other target processors have

requests. Asp increases, the probability of at least one target processor performing a request and

therefore requiring the others to wait is increased.

With null-message synchronization taken into account, Figure 6 shows a maximum speedup

of about 2 for both CBN and CNN schemes with N = 16. Barrier synchronization fares much

better, particularly with non-blocking accesses, showing a speedup of 3 for CBB and 5 for CNB.

The benefit of non-blocking accesses in the barrier approach is that all host processors can

proceed to the barrier even if there are outstanding requests. With blocking accesses, any

processor making an L2 request will arrive at the barrier much later than those that do not make a







2002, HCS Research Lab, U of Florida All Rights Reserved


cache access. Once again, asp is increased, the probability of a single processor slowing the rest

increases.

The difference between the performance with and without clock synchronization and the

difference between the two types of synchronization can be explained by examining the type and

number of messages that the L2 thread must handle. Figure 7 shows the number of messages sent

and received by the L2 thread per simulated clock cycle. Figure 7a shows the number of cache

requests, cache responses, and null messages processed in the CNN scheme, while Figure 7b

shows the requests, responses, and barrier messages for CNB. In the simplified microbenchmark

approach, the number of messages sent by the L2 equals the number received for each type. Also,

the messages are evenly distributed between each of the processor threads.

60 60
50- 50-
S40 40
a)
30 30
20 20
1 0 10
00 C== 00
1 2 4 8 16 1 2 4 8 16
# of processor threads # of processor threads
Requests 0 Responses O Null Messages 0 Requests 0 Responses 0 Barriers
a) CNN b) CNB

Figure 7. Messages handled per simulated clock cycle by centralized L2 cache with non-blocking
accesses on SCI testbed (p =N,fc = 105, A = 0.045, L = 12).


As Figure 7 illustrates, the amount of traffic for clock synchronization greatly exceeds the

cache traffic, particularly for large values of N. As described in Section 4.1.3, SCI provides a

latency of 10.7 |ts for the large cache responses and lower latencies for the other messages. If all

messages were of the larger variety, up to 93000 messages could be serviced by the host per

second or 93000 If = 0.93 messages per target clock cycle. Thus, the performance becomes

communication-bound at N > 8 where more messages per cycle are required.






2002, HCS Research Lab, U of Florida All Rights Reserved


Figure 7 also illustrates a large difference between the number of null messages generated in

CNN verses the number of barriers in CNB. The number of barriers scales linearly with N and is

also dependent on L. Since the cache traffic also scales with N, the barrier messages account for a

constant 65% of the total traffic for all values of N for the combination of L and A used.

By contrast, null messages are sent whenever a thread reaches a limit based on the lookahead.

To understand why the number of null messages grows faster than N, consider a simple system

with two processor threads, both initially at simulated clock cycle to. The target processors in the

first thread do not make any cache accesses and proceed to cycle to + L before the host sends a

null message. A target processor in the second thread makes a cache request in cycle to + 1. The

L2 must wait until the first thread sends its null message before deeming it safe to process the

second thread's request. At that time, it can respond to the first thread's null message with

permission to proceed to cycle to + 1 + L (this is because a request may still arrive from the second

thread with time stamp to + 2 that may affect the first thread). Therefore, the null message grants

the first thread permission to simulate only one additional clock cycle before another null

message will be required.

As the number of threads in the system is increased, the probability of at least one thread

making a request in any given clock cycle increases. Therefore, due to the effect described above,

the null messages in CNN effectively grant permission for fewer clock cycles of simulation for

increased N. Since the number of threads sending null messages increases and the null messages

become more frequent, the number of null messages increases faster than N. For the data in

Figure 7, null messages account for only 58% of the total traffic in a 2-thread system. In the 16-

thread system, the null messages account for 86% of the total traffic.

The results from Figure 6 indicate that the combination of non-blocking L2 accesses and

barrier synchronization perform best. In Figure 8, the distributed L2 approach is compared to the







2002, HCS Research Lab, U of Florida All Rights Reserved


centralized L2 by using DNB and CNB schemes, respectively. For a baseline comparison without

clock synchronization, DN and CN performance is also shown. Figure 8a illustrates the parallel

efficiency while Figure 8b shows the speedup.


100% 14
12
S80% 0

SD -- DB -A- -
60 8
CL 6
40% -5



1 2 4 8 16 1 2 4 8 16
# of proceor threads # of procedistributed between all or threads
thereby slowing the target processor simulatioDNB The CN scheme, CNB
a) parallel efficiency b) speedup
Figure 8. Microbenchmark performance predictions for centralized vs. distributed L2 with non-
blocking accesses and barrier synchronization on SCI tested
(p = N,f = 105, A = 0.045, L = 12).


The efficiency of the distributed approach declines as the number of threads is increased,

even for the DN scheme that does not perform clock synchronization. This decline in efficiency

is due to the fact that the L2 processing task is distributed between all of the processor threads,

thereby slowing the target processor simulation. The CN scheme, by contrast, increases in

efficiency due to the N / (N + 1) scaling effect described earlier.

When barrier synchronization is added, the efficiency of the distributed approach decreases

rapidly. This effect is due to the need to communicate a barrier message with every other thread

in the system in DNB rather than with only the centralized L2 thread in CNB. This all-to-all

synchronization requires Ntimes as many transactions as the centralized approach.

The resulting conclusion is that the distributed L2 is slightly more efficient for a small

number of host threads, N < 4. At larger thread counts, the centralized L2 has a slight

performance and efficiency advantage. This outcome, coupled with the ability to efficiently






2002, HCS Research Lab, U of Florida All Rights Reserved


model contention effects allowing more flexibility in exploring target architectures, gives the edge

to a centralized L2 design.

4.2.2 Interconnect Selection

With the conclusion that the CNB approach shows the greatest performance potential, we now

turn to a comparison of the underlying network for the parallel simulation platform. In Figure 9,

the parallel efficiency and speedup of the CNB-based microbenchmarks are shown for both the

SCI and Myrinet testbeds. The CN results are also provided as a baseline.


100% 14
12
8 0% 8
10-
60% 800o





1 2 4 8 16 1 2 4 8 16
# of processor threads # of processor threads
[--CN(Myrinet) --CNB(Myrinet) -*-CN(SCI) -x-CNB(SCI)
a) parallel efficiency b) speedup

Figure 9. Microbenchmark performance predictions for SCI vs. Myrinet testbed with centralized
L2 and non-blocking accesses (p = N,fc = 10, A = 0.045, L = 12).


The performance of both interconnects with the CN baseline is similar up until N = 8, where

the Myrinet platform shows a decrease in parallel efficiency. From Figure 7, it can be seen that at

this system size, the L2 must process 0.36 request and response transactions per simulated clock

cycle. As previously discussed, the 10.7 |ts latency of SCI allows the host up to 0.93 transactions

per simulated target clock cycle. Myrinet, however, shows a latency of 19.1 |ts, allowing only

0.52 host transactions per simulated cycle. Though this appears to be sufficient capacity for the

required target cache traffic, in practice the effective latency is increased due to contention when

different hosts make multiple requests simultaneously. At 0.36 transactions per clock cycle, the






2002, HCS Research Lab, U of Florida All Rights Reserved


load on the Myrinet network exceeds 50% and contention is frequent. The SCI performance trails

off similarly at N= 16 where 0.72 cache transactions are made per target clock cycle.

When barrier synchronization is added, the performance advantage of SCI becomes even

more pronounced. Not only can the SCI network handle a higher load due to its decreased

latency, but the majority of the traffic consists of zero-byte barrier messages. As shown in

Section 4.1.3, ScaMPI provides a 4.5 |ts latency for these messages while Myrinet requires 17.6

hts. The result is a potential speedup at N= 16 of 4.9 using SCI and only 2.7 with Myrinet.


5 PARALLEL SIMULATOR RESULTS AND ANALYSIS

Based on the results from the previous section, the SimpleCMP simulator [7], a derivative of

SimpleScalar [5], was parallelized using the CNB scheme and subsequently executed on the SCI

testbed. In this section, the simulation platform is described, parallel performance is explored,

and the measured performance is compared to the microbenchmark predictions from the previous

section to demonstrate the validity of the microbenchmark approach.

5.1 Simulation Platform

SimpleCMP performs trace-driven simulations of a 32-bit, MIPS-like instruction set [27],

complete with register renaming, out-of-order execution, superscalar issue, and detailed cache and

branch prediction logic. In addition to the CNB parallelization scheme, the simulator was also

modified to more closely resemble the hardware of an Alpha 21264 [21] by replacing the register

update unit (RUU) with a reorder buffer and rename logic. Separate issue queues are provided for

integer and floating-point instructions, and the pipeline depth matches the design of the 21264.

The dual, synchronized register-file of the 21264 was not implemented. The cache subsystem is

directory-based with point-to-point links from each L1 to the shared L2. Table 3 shows the key

architectural parameters for each individual processor in the simulations.






2002, HCS Research Lab, U of Florida All Rights Reserved


Table 3. Processor architecture parameters.
Parameter Value
Fetch/Issue/Retire Width 4 instructions
Rename Registers 128
Integer Issue Queue 64 instructions
Load/Store Issue Queue 32 instructions
Integer ALUs 4
L1 cache ports 2
Branch Predictions/cycle 1
Global: 12-bit history, 4k x 2-bit saturating counters
Branch Predictor Local: 1k x 10-bit history, 1k x 2-bit saturating counters
Select: 12-bit global history, 4k x 2-bit saturating counters
Branch Target Buffer 2048 entries, 2-way associative
Return Address Stack 32 entries
Minimum Misprediction Latency 8 cycles
L1 I-Cache 64 kB, 2-way associative, 32-byte lines, 1-cycle latency
L1 D-Cache 64 kB, 2-way associative, 32-byte lines, 1-cycle latency
Unified L2 Cache 8 MB, 4-way associative, 32-byte lines, 12-cycle latency
Memory Access Latency 100 cycles, 8 bytes/cycle

Table 4. Selected SPLASH-2 benchmark components.
8-processor Parameters
Benchmark Input Dataset Instructions 8-proc r
fc A
LU 256x256 matrix, 16k blocks 66 M 131k 0.0147
Ocean 258x258 grids, 4 time-steps 330 M 102k 0.0463
Radix 256k integers, radix = 1024 16 M 131k 0.0355
Raytrace teapot.env 282 M 98k 0.0103
Average _174 M 107k 0.0267


In order to evaluate the parallel simulator under a variety of workload conditions, a

representative subset of the SPLASH-2 benchmarks [34] was selected. SPLASH-2 provides a

suite of shared-memory benchmarks for parallel systems and includes several kernel and

application components. Table 4 lists the four benchmarks used, two kernels and two

applications: LU, ocean, radix, and raytrace. The table also provides the measured values forfc

and A when executed on an 8-processor CMP using the sequential version of SimpleCMP. These

values vary slightly for different degrees of parallelism, with fc generally decreasing and A

increasing with higher target processor counts.







2002, HCS Research Lab, U of Florida All Rights Reserved


All statistics are gathered only from the parallel portion of the benchmarks, neglecting any


sequential start-up code and generation of working-set data. This common practice assumes that,

with realistic workloads, the initialization code is insignificant compared to the overall execution


time. In practice, the non-parallel part can be sped up considerably by using only the front-end of

the trace generator and not the timing simulator, or even eliminated entirely through


checkpointing. Table 4 provides the approximate number of instructions that comprised the

parallel portion of each benchmark.

11000
10000
9000
8000
E 7000
S6000
5000
4000
S3000 _
2000
1000
0
1 2 4 8 16 32
# of processors in CMP
--LU -x- Ocean ---Radix -- Raytrace ---Average

Figure 10. Sequential execution time for SPLASH-2 benchmarks.


As a baseline for comparison, Figure 10 shows the execution time for the sequential version

of SimpleCMP for each benchmark and the overall average measured on one of the SCI testbed


nodes. The execution time remains relatively constant up to an 8-processor CMP despite an

increase in target complexity because fewer cycles are necessary due to the benchmarks' near-

linear speedup. A sharp increase for 16- and 32-processor CMPs occurs as the simulator working


set exceeds the 256 kB L2 cache of the host systems for large target processor counts.

The sequential simulation scheme described in Section 2.2 results in highly reduced locality

at large processor counts due to the sequential iteration over each processor for every simulated

clock cycle. In addition, the capacity required to hold all frequently used data increases. The key

data elements for each iteration are the decoded instructions which are 236 bytes in length. Each






2002, HCS Research Lab, U of Florida All Rights Reserved


processor can have up to 128 such instructions in-flight, for a total of 29.5 kB of data. With eight

processors in the simulated CMP, 236 kB is required to hold just the decoded instructions. In

addition, the L1 caches and branch predictors are also frequently accessed.

A 16-processor CMP simulation has twice as large a working set as an 8-processor CMP and

therefore much poorer data locality. It is important to note that even the 32-processor CMP

simulation fits in the 256 MB main memory of one of the testbed nodes, so swapping to disk is

not an issue. Such a performance limitation would be relatively easy to fix by increasing the

memory capacity, but cache sizes are much more difficult and expensive to increase.

5.2 Parallel Performance

The SPLASH-2 components from Table 4 were simulated on the parallel CMP simulator for a

variety of target CMP sizes and processor thread counts. Figure 11 shows the speedup obtained

over the sequential version of the simulator. As before, the horizontal axis refers to the number of

processor threads and does not include the L2 thread.

Several trends are apparent from the results in Figure 11. First, for CMPs of 8 processors

and less, the speedup is relatively modest, reaching almost 4 for 8 threads. There is a slightly

increased speedup for an 8-processor CMP over a 4-processor CMP even when both systems are

simulated with the same number of host threads. This increase is due to the fact that the

synchronization overhead is a lower percentage of the total communication time when each host

thread simulates more than one target processor. In effect, twice the number of instructions and

twice the number of L2 cache requests can be simulated per barrier synchronization.

For 16-processor CMPs, the speedup shows a somewhat larger improvement. At 32-

processors, the performance is markedly improved. This effect can be explained by the improved

cache locality in the parallel simulation. As illustrated in Figure 10, cache locality becomes

problematic in a uniprocessor simulation of CMPs of this size. In the parallel approach, however,







2002, HCS Research Lab, U of Florida All Rights Reserved


a 16-thread simulation requires only one or two target processors per host thread for a 16- or 32-

processor CMP, respectively. The parallelization effectively scales the size of the host L2 cache

with the number of threads, improving the performance beyond the simple computational overlap.

16 16

12- 12

a8 / 8

4 4

0 0
1 2 4 8 16 1 2 4 8 16
LU # of processor threads OCEAN # of processor threads
16 16 -




a 8 8


0 0




--p 2 ----p = 4 -*- 8 --p =16 ----p 32

Figure 11. Speedup for selected components of SPLASH-2 for target CMPs of varying size.


Figure 12 better illustrates this effect by showing the parallel efficiency (relative to the total

number of threads, including the L2 thread). For system sizes of between 2 and 8 threads, parallel

simulation of the 32-processor CMP shows a superlinear speedup due to the increased cache hit

rate. At 16 threads, the reduced efficiency predicted by the microbenchmarks and illustrated in

Figure 8 reduces the performance to sublinear levels.

Interestingly, the improved cache locality shows benefit even in the simplest case of a single

processor thread plus the centralized cache node. The theoretical maximum efficiency is 0.5 in

such a system, but the 32-processor CMP simulation exceeds this efficiency for all benchmarks

while the 16-processor CMP simulation does so for the LU and radix benchmarks.







2002, HCS Research Lab, U of Florida All Rights Reserved


200% 200%

u 150% 150%

S100% 100%

S50% 550%

0% 0%
1 2 4 8 16 1 2 4 8 16
LU # of processor threads OCEAN # of processor threads
200% 200%

o 150% 150%

100% 100%

5o0% 50%

0% 0%
1 2 4 8 16 1 2 4 8 16
RADIX # of processor threads RAYTRACE # of processor threads

--p = 2 ----p = 4 -- p 8 ---p = 16 -Up 32

Figure 12. Parallel efficiency for selected components of SPLASH-2 for CMPs of varying size.


5.3 Comparison to Microbenchmarks

In order to assess the accuracy of the microbenchmarks in predicting the parallel simulator

performance, microbenchmarks were run for eachfc and A value in Table 4 corresponding to one

of the SPLASH-2 benchmarks and the results averaged. Due to the cache effects with CMP

configurations of greater than 8 target processors for which the microbenchmarks do not account,

comparisons are made for an 8-processor CMP with host thread counts from N= 1 to N = 8. The

average result for the CNB-based microbenchmarks and the measured performance on the fully

implemented simulator are shown in Figure 13.

The CNB microbenchmark does not track particularly well with the actual parallel simulator

performance. At N = 1, the difference is only 9.8%, but the disparity increases to 25% for N = 8.

The disparity was found to be largely due to a neglected effect that has a non-negligible impact on

the overall performance. Namely, target cache requests from the L1 caches to the L2 cache can







2002, HCS Research Lab, U of Florida All Rights Reserved


cause the L2 to require a writeback of dirty data from a second L1 in order to satisfy the first

request. In the implementation used in the parallel simulator, the L2 blocks on such a request

until the dirty line is returned by the second L1. Though the writeback transaction is included in

the A parameter, the L2 cache blocking effect was not accounted for in the CNB microbenchmark.


45
40
35
30
I 25
S' 20-
15 -
10
05
00
1 2 4 8
Average # of processor threads
CNB -- modified CNB -u Measured

Figure 13. Comparison of microbenchmark prediction and actual parallel simulator performance.


The CNB microbenchmark was modified to accept another parameter: the probability that an

L1 request will require a writeback request and concomitantly block in the L2. For the

benchmarks studied, this probability ranged from 0.3% to 14% with an average of 9.3%. Using

the measured probabilities for each SPLASH-2 component, the modified microbenchmarks were

executed and the results plotted in Figure 13.

As illustrated in the figure, the modified CNB microbenchmark predicts the performance of

the parallel simulator with much greater accuracy: within 10% of the measured results for the

system sizes shown. The remaining difference can be attributed to the fact that the actual

execution of the benchmarks frequently results in bursty L2 accesses, often synchronized with

barriers or locks in the parallel code, reducing the performance as contention increases the latency

for each transaction. By contrast, the microbenchmarks yield a more uniform distribution of

cache accesses. Including a variance term in the microbenchmarks could account for this effect,

but the results are already within acceptable limits.






2002, HCS Research Lab, U of Florida All Rights Reserved


6 RELATED RESEARCH

Much existing research exists in the field of parallel simulation of multiprocessors. The

Wisconsin Wind Tunnel simulates a parallel, CC-NUMA system on various parallel systems

including a Myrinet-connected cluster of workstations [29],[24]. Synchronized Active Messages

provide the messaging layer rather than MPI as in our study. Also, WWT makes use of direct-

execution simulation with each parallel process running on a separate CPU, capturing only the

memory accesses and sending them through a sequential timing simulation of the memory

hierarchy. This approach does not offer flexibility in the processor model and is unable to model

effects such as issue widths, speculative memory accesses, and out-of-order execution. Analytical

modeling has been used to approximate the performance of WWT for a variety of system sizes

[12]. Interestingly, [12] shows an optimal cost/performance ratio around 16 to 32 processors in

the target system due largely to memory costs, mirroring the cache effects noted in our study.

The Integrated Simulation Environment (ISE) takes a similar approach for simulation of

MPI-based parallel programs [16]. In ISE, MPI code executes natively on distributed nodes in a

cluster with the MPI function calls being intercepted and sent to a centralized, sequential

simulation of the interconnect. This "software-in-the-loop" approach allows flexibility in

selecting the interconnect type and topology but is limited in performance by the relatively slow

interconnect simulation.

In [15], parallel simulation is applied to a parallel digital signal processor (DSP) system. In

this study, the parallel programming language Linda is applied in a clustered environment to

achieve speedup on a multi-DSP simulation despite the use of low-performance, 10 Mbps

Ethernet as the interconnect.

The RSIM [26] simulation environment provides great flexibility in the configuration of the

individual processors in a simulated multiprocessor, but the simulation itself runs only on a






2002, HCS Research Lab, U of Florida All Rights Reserved


uniprocessor. Rather than parallelizing the simulator, DirectRSIM [11] provides a performance

improvement by adapting RSIM to be trace-driven instead of execution-driven. Using an

approach similar to RSIM, the MINT [33] multiprocessor simulator has even been modified to

simulate a CMP [22], but neither approach involves parallel simulation.


7 CONCLUSIONS

Simulation of parallel systems is an important tool in evaluating design alternatives. As the

complexity of such systems increases down to the chip level with the advent of CMP-based

systems, the simulation overhead becomes increasingly prohibitive. In this paper, an MPI-based

parallel simulation environment was presented to reduce the execution time of performance-level

CMP simulations running on a cluster of workstations connected by a high-speed interconnect.

The parallel simulation is a natural extension of conventional, sequential simulation of a

uniprocessor combined with traditional, event-driven parallel simulation techniques. The

proposed shared-L2 design of future target CMP architectures requires a tighter coupling of

shared communications in the host platform than traditional DSM-based target systems, making

the selection of parallel algorithm and a high-performance network very important for the host.

Several design alternatives were considered, including centralized vs. distributed simulation

of the target L2 cache, blocking vs. non-blocking L2 accesses, and null-message vs. barrier clock

synchronization. Through MPI-based microbenchmarks, the optimal combination was found to

be a CNB scheme combining centralized L2, non-blocking accesses, and barrier synchronization

with a predicted speedup of around 5 for 16 threads. The microbenchmarks also demonstrate that

the low-latency communication afforded by SCI makes it a more suitable interconnect for the host

platform than Myrinet.






2002, HCS Research Lab, U of Florida All Rights Reserved


Performance results for a fully implemented parallel simulator were presented for a variety of

workloads. Due to increased cache capacity of the parallel platform, the results show higher-than-

predicted speedups of between 12 and 16 when simulating a large CMP architecture with 16

processor threads on 9 dual-CPU cluster nodes. The parallel simulation results are shown to

differ somewhat from the microbenchmark predictions even without the cache effect, but the

difference is largely accounted for with a slight modification to the microbenchmarks.

Future work in this area could pursue several avenues. Larger system sizes, in terms of the

numbers of simulated CMP processors in the target or processor threads and cluster nodes in the

host, could be studied. CMP processor counts were restricted to 32 in this study due to the fact

that limitations in VLSI technology will likely yield CMPs of this size or smaller in the near

future, but further advances or simpler processor designs may allow chip-level architectures with

hundreds of processors. The cluster node counts and, by extension, processor threads in this

study were limited by the hardware available.

Further study could also be directed at more efficient parallel simulation algorithms. For

example, a more aggressive clock synchronization scheme might speculatively execute

instructions past the null-message or barrier clock cycle, rolling back the computation if an

earlier-stamped message is received. The speculative nature of the processor pipeline being

simulated would facilitate such an approach with little extra overhead.

Parallel simulation should increase in importance as parallel systems become larger and more

complex. Fortunately, the existence of such systems will also enable parallel simulators to study

the next-generation design alternatives with greater speed and efficiency. As the simulation

application itself becomes more complex, techniques such as microbenchmark-based evaluation

of design alternatives will become increasingly valuable in assessing the most effective

parallelization technique for a given system.






2002, HCS Research Lab, U of Florida All Rights Reserved


8 ACKNOWLEDGMENTS

This work was funded in part by the Department of Defense and by an NSF Graduate

Fellowship (Chidester). Support was also provided through equipment donations from Nortel

Networks, Intel Corporation, Dolphin Interconnect LLC, and Scali AS. We also wish to thank the

anonymous reviewers for their valuable feedback.


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