Potential and Achievable Parallelism in Unsymmetric-Pattern
Multifrontal LU Factorization *
Steven M. Hadfield
Department of Mathematical Sciences
United States Air Force Academy
Colorado Springs, Colorado, USA
phone: (719) 472-4470
Timothy A. Davis
Computer and Information Sciences Department
University of Florida
Gainesville, Florida, USA
phone: (904) 392-1481
email: dav;y- .- ll edu
Technical Report TR-94-027,
Computer and Information Sciences Department,
University of Florida
April 19, 1994
*This project is supported the National Science Foundation (ASC-9111263, DMS-9223088)
The unsymmetric-pattern multifrontal method of Davis and Duff  generalizes earlier mul-
tifrontal approaches to LU factorization by removing the assumption of a symmetric-pattern of
nonzeros in the sparse matrix. As a result, the underlying computational structure becomes a
directed acyclic graph (DAG) instead of a tree. This research explores the potential parallelism
available in the unsymmetric-pattern multifrontal method using both unbounded and bounded
parallelism models based on this DAG. The bounded parallelism model is extended to reflect the
performance characteristics of the nCUBE 2 distributed memory multiprocessor to investigate
the achievable parallelism. Finally, a factorization-only version of the method is implemented
on the nCUBE 2 and its achieved parallelism evaluated.
List of Symbols
rj Lower case r with subscripted lowercase j
cj Lower case c with subscripted lowercase j
pj Lower case p with subscripted lowercase j
aj Lower case a with subscripted lowercase j
sj Lower case s with subscripted lowercase j
I Lower case 1 in italics
d Lower case d in italics
p Lowercase greek letter Mu
Large sparse systems of linear equations frequently arise in many application areas such as circuit
design, power systems, oil reservoir modeling, chemical engineering, structural engineering, and
computational fluid dynamics . Direct solution methods, as opposed to iterative approximations,
are often required for these systems. When the coefficient matrix is symmetric and positive-definite,
Cholesky factorization can be efficiently and effectively employed . In the more general case,
LU factorization is necessary .
While sparsity in the coefficient matrix allows a significant reduction of required computations, it
also provides opportunities for the exploitation of parallelism. However, much of the investigation of
parallelism has focused on Cholesky factorization . The multifrontal method originally proposed
by Duff and Reid [6, 7] has application to both Cholesky and LU factorization  and significant
parallel potential .
With the multifrontal approach, the sparse matrix is decomposed into a set of partially overlap-
ping dense submatrices and a structure is defined for the computations by the relationships between
these dense submatrices. While parallelism is available between independent submatrices, there is
also parallelism within these dense submatrices and means of exploiting this parallelism have been
extensively investigated .
Duff and Johnsson  explored the available parallelism in the multifrontal method applied to
symmetric-pattern matrices using analytical models based on unbounded parallelism. In this work,
we explore the both the available and achievable parallelism of a new multifrontal method for the
LU factorization of unsymmetric-pattern matrices developed by Davis and Duff .
First some of the key concepts of the unsymmetric-pattern multifrontal method are outlined.
Then the available parallelism is determined using unbounded parallelism models similar to those
of Duff and Johnsson . These models are refined into bounded parallelism models to determine
how much parallelism is available on various finite processor sets. Achievable parallelism is then
explored using a simulation based on the performance characteristics of the nCUBE 2 distributed
memory multiprocessor. Finally, the unsymmetric-pattern multifrontal method is implemented on
the nCUBE 2 and its performance assessed.
1 Unsymmetric-Pattern Multifrontal Method
Multifrontal methods for the factorization of sparse matrices [2, 6, 7, 12] decompose the sparse
matrix into a set of overlapping dense submatrices called frontal matrices. Each frontal matrix
contains one or more pivots and is partially factorized according to these pivots. The updated
entries in the unfactorized portion of a frontal matrix (called the contribution block) are passed
and assembled (added) into subsequent frontal matrices. Each contribution block entry must be
passed and assembled into one and only one subsequent frontal matrix. The assembly directed
acyclic graph (DAG) represents frontal matrices as nodes and the passing of contributions blocks
as directed edges. With previous multifrontal methods, the assumption of a symmetric-pattern
matrix caused the assembly DAG to be a tree (or a forest). The tree structure results since the
contribution block of a frontal matrix can be completely absorbed within a single subsequent frontal
matrix. When the assumption of symmetry in the pattern is removed, contribution blocks need to
be fragmented with the various pieces passed to different frontal matrices.
The assembly DAG also serves as a task graph with the nodes representing the factorization of
individual frontal matrices and the edges as inter-task data dependencies. Large grain parallelism
is available among independent nodes in the assembly DAG. Furthermore, the factorization of
the dense frontal matrices provides a second level of parallelism because of the independence and
regularity in the factorization for each pivot. Earlier efforts have shown that exploitation of both
levels of parallelism is necessary for the best performance [3, 5].
2 Unbounded Parallelism Models
The unbounded parallelism models (UPMs) explore the amount of theoretical parallelism available
using analytical techniques and assume an unbounded number of available processors. The un-
derlying model of computation is a multiple instruction, multiple data (:.1.. 11)) parallel random
access machine pramM). The memory model is assumed to allow concurrent reads but only ex-
clusive writes (CREW). The models are based on the assembly DAG. Nodes (representing frontal
matrices) are weighted according to their predicted parallel factorization time. Three models are
presented. Model 1 assumes no parallelism within frontal matrices with only a single processor
assigned to each frontal matrix. Model 2 uses a medium grain parallelism within each frontal ma-
trix with a processor assigned to each column of the frontal matrix. Model 3 allows a fine grain
parallelism with a processor assigned to each entry of a frontal matrix. The assembly of contribu-
tion blocks is represented by edge weights that estimate the time required for assembly. Potential
parallelism in the three models is determined by taking the ratio of all node and edge weights over
the node and edge weights for the heaviest weighted path through the assembly DAG.
The node weights for a frontal matrix j under these three models are shown in equations (1),
(2), and (3). In these models frontal matrix j has rj rows, cj columns, pj pivots, aj incoming
entries to assemble, and sj frontal matrices from which contributions are received. The four terms
in each node weight equation stand for assembly of incoming contributions, numerical checking
of each pivot, computation of the multipliers (column of the L factor), and update of the active
Model 1: aj + E (r I 1) + E (r, i) + 2E (rj i)(cj i) (1)
Model 2: aj/cj + C (r i+ 1) + E (r, i) + 2E 1(rj i) (2)
Model 3: [log2 s] + Ej flog2(r i + 1)] + pj + 2pj (3)
In the case of a dense matrix, Model 1 corresponds to using a single processor with O(n3) execution
time, where n is the order of the matrix. Model 2 would use O(n) processors and require O(n2)
time. Model 3 would use O(n2) processors and require O(nlog n) time.
The four test matrices analyzed are GRE_1107, GEMAT11, SHERMAN5, and RDIST1. They
are of orders 1107, 4929, 3312, and 4134, respectively with the number of nonzero entries being
5664, 33108, 20793, and ', III- SHERMAN5 is the only matrix with a near-symmetric pattern.
With Model 1, the speedups ranged from only 1.2 to 4.3 indicating little available parallelism
within the assembly DAGs themselves. The Model 2 & 3 results are seen as the maximum achieved
speedups in Figures 1 and 2, respectively. These results reveal significant potential benefits when
parallelism is exploited both within and across frontal matrices.
3 Bounded Parallelism Models
The bounded parallelism models use simulation to determine how much speedup can be achieved
on a fixed number of available processors. Only Models 2 and 3 have corresponding bounded par-
allelism models. Node and edge weights are assigned as they were in the UPM models. Processors
sets are defined as powers of 2 from 20 to 216 (1 to i, "..;i,). Frontal matrix tasks (corresponding
to the nodes) are placed in a ready queue when all of their predecessors have completed execution.
Tasks are scheduled from this ready queue based on a heaviest node first priority scheme. Allo-
cation of processors to tasks is nominally one per frontal matrix column in Model 2 and one per
frontal matrix entry in Model 3. If a sufficient number of processors are not available, the smaller
available set is allocated, unless waiting for the next task to complete and free its processors will
result in a sooner completion time. The task's parallel execution time is .,.1I1-i. .1 per the number
of processors allocated.
The speedup results of the bounded parallelism model corresponding to Model 2 and Model 3
are shown in Figures 1 and 2, respectively. These results indicate that the theoretically available
parallelism of the unbounded parallelism models can be achieved on reasonably sized processor sets.
With the medium grain parallelism of Model 2, almost all of the potential parallelism is achieved
with only 512 processors (29). The fine grain parallelism of Model 3 was achieved with i.". "..:,
4 Distributed Memory Performance Model
The distributed memory performance model extends the medium grain bounded parallelism model
(Model 2) to reflect the performance characteristics of the nCUBE 2 multiprocessor. Within this
model, individual frontal matrices are broken up by columns and distributed to the processors
of an assigned subcube in a scattered fashion. The factorization process has the processor that
owns the current pivot column compute the multipliers (column of L) and broadcast them to the
other processors. All of the processors in the frontal matrix's subcube then update their active
columns using these multipliers. This is commonly referred to as a fan-out algorithm . Node
weights correspond to a predicted parallel execution time. This predicted time is based on an
analytical model of the fan-out algorithm with specific parameters set according to results from an
implementation and evaluation of this algorithm on the nCUBE 2. Equation (4) describes the time
(in psecs) required for the multipliers' broadcast with I the length of the message in bytes and d
the dimension of the subcube.
broadcasttime = 195.22 + 0.1221 + 50.02d + 0.451d (4)
Assembly of incoming contributions is assumed to be evenly distributed across the subcube pro-
cessors. A distinct message is assumed for each edge. The edge weights are based on the size
of the contribution block passed on the edge. Point-to-point messages are assumed for passing
contributions with the time required for a particular message (in psecs) determined by Equation
directil.,tqltm,, = 170.32 + 4.54d + 0.5731 (5)
The scheduling of tasks is done based on a critical path priority scheme. The critical path priority
for a particular node (task) is the heaviest weighted path from that node to an exit node (a node
with no out-going edges). Processor allocations are done as subcubes with subcube size based on
the size of the frontal matrix. Specific subcube assignments are not tracked and fragmentation of
the hypercube is neglected.
The results of this distributed memory performance model are shown in Figure 3. These results
indicate that about 25 percent of the theoretically available parallelism should be achieved on the
nCUBE 2. This is consistent with similar results for Cholesky factorization .
5 Implementation Results
Within the implementation, factorization of the frontal matrices is done using a pipelined, fan-out
algorithm similar to that used in the distributed memory model . With pipelining, however, the
processor owning the next pivot column will update only that column and compute and send the
next pivot's multipliers before doing the rest of the updates for the current pivot. This allows the
communication to be overlapped with computation. Experiments revealed that about 35 percent
of the communication is effectively overlapped by using pipelining. The Basic Linear Algebra
Subroutines Level 1 (BLAS-1) are used for the bulk of the computations required for factorization.
These routines are about four times faster than the C-language code upon which the distributed
memory model is based. While the inclusion of pipelining improved parallelism, use of the BLAS-1
routines reduced the ratio of computation to communication and the combined effect was to reduce
the parallelism achieved by frontal matrix factorization. (Of course overall execution times were
Task scheduling for the implementation was done using critical path priorities. Processor allo-
cation was done using a proportional scheme that favored inter-frontal matrix parallelism (between
nodes) over intra-frontal matrix parallelism (within nodes). Specific subcubes were assigned to
maximize overlapping of communicating frontal matrix tasks. Subcube management was done
with a variant of the binary buddy system. Frontal matrix columns were assigned to processors
in either a scattered or blocked format depending upon which produced the best overall execution
The speedups achieved by this implementation are shown in Figure 4. Typically, they are less
than that predicted by the distributed memory model but this is mainly attributed to the effects
of using the faster BLAS-1 routines.
The primary conclusion of this research is that the unsymmetric-pattern multifrontal method has
significant potential for parallelism. The unbounded parallelism models quantified this potential
parallelism and the bounded parallelism models indicated that the parallelism was fully achievable
on reasonably sized processor sets. The distributed memory model revealed that about 2,.', of
the available parallelism is achievable on contemporary architectures. Finally, the results of these
models were validated by an actual implementation of the method on the nCUBE 2.
Importantly, the assembly DAGS used in this effort were produced using a sequential formula-
tion of the unsymmetric-pattern multifrontal method . Parallelism was not a specific objective
in their derivation. Alternative formulations of these assembly DAGs are possible and could reveal
significant additional parallelism. Such alternatives are currently under investigation.
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0 2 4 6 8 10
Log2 of Processor Set Size
12 14 16
Figure 1: Model 2 Results (.1. .1,,iiii Grain Parallelism)
2 4 6 8 10 12
Log2 of Processor Set Size
Figure 2: Model 3 Results (Fine Grain Parallelism)
0 1 2 3 4 5 6 7 8 9 10
Log2 of Processor Set Size
Figure 3: Distributed Memory Model Results
2 3 4
Log2 of Processor Set Size
Figure 4: Implementation Results