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Communication balancing in parallel sparse matrix-vector multiplication.

Abstract. Given a partitioning of a sparse matrix for parallel matrix-vector multiplication, which determines the total communication volume, we try to find a suitable vector partitioning that balances the communication load among the processors. We present a new lower bound for the maximum communication cost per processor, an optimal algorithm that attains this bound for the special case where each matrix column is owned by at most two processors, and a new heuristic algorithm for the general case that often attains the lower bound. This heuristic algorithm tries to avoid raising the current lower bound when assigning vector components to processors. Experimental results show that the new algorithm often improves upon the heuristic algorithm that is currently implemented in the sparse matrix partitioning package Mondriaan. Trying both heuristics combined with a greedy improvement procedure solves the problem optimally in most practical cases. The vector partitioning problem is proven to be NP-complete.

Key words. vector partitioning, matrix-vector multiplication, parallel computing, sparse matrix, bulk synchronous parallel

AMS subject classifications. 05C65, 65F10, 65F50, 65Y05

1. Introduction. Sparse matrices from emerging applications such as information retrieval, linear programming, and Markov modelling of polymers may have a highly irregular structure and no underlying three-dimensional physical structure, in contrast to many finite element matrices, and this irregularity poses a challenge to parallel computation. Often, the sparse matrix must be repeatedly multiplied by a vector, for instance in iterative linear system solvers and eigensystem solvers. In recent years, hypergraph partitioning has become the tool of choice for partitioning the sparse matrix, delivering good balance of the computation load and a minimal communication volume during parallel sparse matrix-vector multiplication. The problem of partitioning the input and output vectors is just as important as the matrix partitioning problem, since it affects the balance of the communication load, but it has received much less attention.

Assume that we have an m x n sparse matrix A, which must be multiplied by an input vector v of length n, to give an output vector u of length m, using p processors of a parallel computer with distributed memory. The natural parallel algorithm for sparse matrix-vector multiplication with an arbitrary distribution of matrix and vectors consists of the following four phases:

1. Each processor sends its components [v.sub.j] to those processors that possess a nonzero [a.sub.ij] in column j.

2. Each processor computes the products [a.subij] [v.sub.j] for its nonzeros [a.sub.ij], and adds the results for the same row index i. This yields a set of contributions [u.sub.is], where s is the processor identifier, 0 [less than or equal to] s < p.

3. Each processor sends its nonzero contributions [u.sub.is] to the processor that possesses [u.sub.i].

4. Each processor adds the contributions received for its components [u.sub.i], giving [u.subi] = [[summation]].sup.p-1.subt=0] [u.sub.it].

Processors are assumed to synchronise globally between the phases. In the language of the bulk synchronous parallel (BSP) model [21], the phases are called supersteps. This model motivated the present work, because it encourages balancing communication loads, besides balancing computation loads. Phases 1 and 3 are communication supersteps. Their costs include a fixed overhead l representing the global latency, which lumps together the time of the global synchronisation and the startup times of an all-to-all message exchange. The superstep approach allows combining data words destined for the same processor into one packet and also reordering of packets, both for the purpose of communication optimisation. Sometimes, we call l just the synchronisation cost. The BSP model also assumes a cost of g time units per data word sent or received by a processor. The processor with the maximum number h of data sent or received determines the overall communication cost. The cost of a communication superstep can thus be expressed as

(1.1) [T.sub.superstep] = [h.sub.g] + l.

A natural time unit for g and l in scientific computation is the time of a floating-point operation. The synchronisation cost l does not grow with the problem size and it has to be paid only twice for the matrix-vector multiplication algorithm. The communication cost, however, grows with the problem size, and depends very much on the data partitioning chosen. We are mainly interested in large, highly irregular problems, and for this reason we will exclusively be concerned with the communication cost [h.sub.g], aiming to minimise it.

We assume that the matrix A has already been partitioned for p processors and that we have to find a suitable vector partitioning. Thus, each nonzero all has been assigned to a processor and we need to assign vector components [v.sub.j] and [u.sub.i] to processors. We assume that the vectors can be partitioned independently, which is usually the case for rectangular matrices, but also for square matrices if the output vector is used as the input for multiplication by [A.sup.T]. We only treat the partitioning problem for the input vector, because the partitioning of the output vector is similar and can be done by running the same algorithm applied to [A.sup.T]. If the matrix A is symmetric and it has been partitioned symmetrically, i.e., with nonzero elements [a.sub.ij] and [a.sub.ji] assigned to the same processor, then we can partition the input vector and use the same solution for the output vector as well.

The preceding matrix partitioning can be done by using any of the currently available matrix partitioners, either based on hypergraph partitioning or graph partitioning. The result can be: one-dimensional [5], e.g. a row distribution or column distribution; two-dimensional Cartesian [8], each processor obtaining a submatrix defined by a partitioning of the matrix rows and columns; two-dimensional non-Cartesian with Mondriaan structure [23], defined by recursively bipartitioning the matrix in either the row or column direction; or completely arbitrary [7], each nonzero having been assigned individually to a processor. In all these cases, it may be beneficial to partition the vector by the methods presented here.

Let [P.sub.j] be the set of processors that own nonzeros in matrix column j and [[lambda].sub.j] = [absolute value of [P.sub.j]] be the number of such processors, for 0 [less than or equal to] j < n. We make a number of assumptions, all without loss of generality, to facilitate the exposition. We assume that [[lambda].sub.j] [greater than or equal to] 2, because columns with [[lambda].sub.j] = 0,1 do not cause communication so that they can be removed from the problem. We assume that all processors are involved in communication, i.e., occur in at least one matrix column; it is easy to remove the other processors. Furthermore, we assume that duplicate nonzeros have been removed, i.e., nonzeros [a.sub.i'j] that are in the same column and are owned by the same processor as a nonzero [a.sub.ij], where V > i; a duplicate nonzero does not cause extra communication because [v.sub.j] has to be sent only once to the owner of [a.sub.ij]. Since the processor that owns a nonzero [a.sub.ij] in matrix column j is important, but the row number i is irrelevant, we can transform the matrix into a p x n sparse matrix A', which contains a nonzero in position (s, j), 0 [less than or equal to] s < p, 0 [less than or equal to] j < n, if and only if processor s has a nonzero in column j of A. (The matrix A' is introduced in [20] as the communication matrix corresponding to a partitioned matrix A.) We assume without loss of generality that A has already been transformed into A', so that we can drop the prime. In Section 4, however, where we present our experimental results, we will distinguish between the original matrix A and its communication matrix A'. Our aim is to assign each input vector component [v.sub.i] to a processor [phi](j) [member of] [P.sub.j], such that we minimise the communication cost [C.sub.[phi]] of the partitioning [phi]. This cost is defined as

(1.2) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where

(1.3) [N.send](s) = [summation over (j: [phi](j)=s)] [[lambda].sub.j] -1],

and

(1.4) [N.sub.recv](s)= [[absolute value of] {j : 0 [less than or equal to] j < n [conjunction] s [member of] [P.sub.j] [conjunction][phi] (j) [not equal to]s}]

Sometimes we refer to the communication cost of a processor s, which is [C.sub.[phi]](s) = max{[N.sub.send] (8), [N.sub.recv] (s) 1. Note that the number of sends expressed by [N.sub.send] (8) is the number of data words sent, and not the number of messages in which they are packaged. We will use the terms 'send' or 'receive' to denote the communication of a single data word, irrespective of the way this is done (the data word is most likely sent as part of a larger packet). The total communication volume is

(1.5) [[V.sub.[phi]] = [p-1.summation over (s=0)] [N.sub.send](s) = [p-1.summation over (s=0)] [N.sub.recv](s).

We will drop the subscript '[phi]' from [C.sub.[phi]], [C.sub.[phi]] (s), and [V.sub.[phi]] if the partitioning involved is clear from the context.

2. Related work. Catalyurek and Aykanat [5] present a one-dimensional matrix partitioning method for square matrices based on hypergraphs. The method is also applicable to rectangular matrices, see [4]. They partition the input and output vectors conformally with the matrix partitioning; for a rowwise matrix partitioning, this means that matrix row i, [v.sub.i], and [u.sub.i] are all assigned to the same processor. Thus, no specific effort is made to balance the communication by finding a better vector partitioning. This may be beneficial for the output vector u, but for the input vector v this leaves no choice in the partitioning, and it might increase the communication volume V. Catalyurek and Aykanat [5] do report results, however, on their vector partitioning. For instance, experiments with rowwise partitioning by the HCM variant of their partitioner PaToH [6] on a set of square, structurally nonsymmetric matrices show for p = 64, on average, a scaled volume of 0.92 and a maximum scaled volume per processor of 0.025 (only counting the sends, not the receives), which is not too far from the theoretical optimum of 0.0143. The scaling is by matrix size, i.e., the scaled volume is V/n. The maximum scaled volume per processor would probably be somewhat higher if receives were included in the metric as well.

Vastenhouw and Bisseling [23] pose the problem of minimising the cost in the metric we use here. They present a vector partitioning algorithm which is the default in version 1.0 of the Mondriaan package. The algorithm works as follows. First, it handles the components with [[lambda].sub.j] > 2 in random order, trying to minimise [max.sub.0 [less than or equal to] s [less than or equal to] p([N.sub.send](s) + [N.sub.rev],(s)). The algorithm assigns [v.sub.j] to the processor with the current lowest sum. The sum of processor s is initialised at ncols(s) = [absolute value of J(s)], where J(s) = {j : 0 < j < n [conjunction] s [member of] P;}, in an attempt to give the greedy algorithm at least a partial view of the future. The initial sum can be seen as the number of inevitable communication operations: a processor must either send or receive at least one data word if it occurs in a column. If [v.sub.j] is assigned to processor s, this increases the sum of s by [[lambda].sub.j] - 2. Second, the algorithm handles the components with [lambda].sub.j] = 2 in an arbitrary order, trying to balance the number of sends with the number of receives. The components with [lambda].sub.j] = 2 do not increase the sums any more. Let s and s' be the two processors in a column j. The algorithm chooses s as owner of column j if [N.sub.send] (s) + [N.sub.recv](s') [less than or equal to] [N.sub.send](s') + [N.sub.recv](s); otherwise, it chooses s'. This gives rise to one data word being sent in the least busy direction. The numerical experiments in [23] for the five rectangular matrices show reasonable communication balance, with the largest problem instance (matrix tbdl inux, p = 64) showing a normalised cost of 3.06 (relative to the average volume V/p). A disadvantage of this algorithm is that matrix partitionings with few two-processor columns have little opportunity to optimise the send/receive balance; in the worst case, this may double the communication cost. In the present work, we investigate the performance of this algorithm further and try to improve it; in the remainder of the paper, we denote the original Mondriaan vector partitioning algorithm by 'Mon'.

Ucar and Aykanat [20] treat the problem of vector partitioning given a certain matrix partitioning, with the objective of minimising the total number of messages and hence the sum of the message latencies while satisfying a balancing constraint on the maximum communication volume per processor, measured in sends. This enables a trade-off between latency and maximum volume. To find a solution, they formulate the problem in terms of a hypergraph with p hyperedges and n vertices, and then try to partition the vertices into p sets using a multilevel hypergraph partitioner, in this case PaToH. Each vertex gets a weight of [lambda].sub.j] - 1, representing the number of sends for the corresponding vector component [v.sub.j]. A cut hyper-edge s, 0 [less than or equal to] s < p, with vertices on different processors, means that processor s has to receive a message from these processors, except from itself. For comparison, Ucar and Aykanat also implemented a method which they call the naive method, which assigns components in order of decreasing [lambda].sub.j], trying to balance the sends. The hypergraph method significantly reduces the total number of messages, by about a factor of two compared to the naive method, but it doubles the maximum number of sends per processor and it also increases the total volume by up to 41 %. (An increase in communication volume may occur if a component [v.sub.j] is moved to a processor outside [P.sub.j] to reduce the number of messages.) The time of the vector partitioning by the hypergraph method is usually less than that of the matrix partitioning, but it is much more than that of the naive method. The difference between the present work and the work of Ucar and Aykanat is that we try to achieve the utmost in communication balance, ignoring latency, and that we also try to balance the receives of the processors.

A different area, related to the present work, is that of computation load balancing. Viewing communication as just another type of work that has to be balanced among the processors, we may be able to benefit from methods developed to balance computation work. Pinar and Hendrickson [17] propose a general framework for balancing work in complex situations, such as overlapping subdomains in domain decomposition, or computation and communication without synchronisation in between (where work represents the sum of computation time and communication time). They diffuse data and associated work from overloaded processors to others, starting from an initial partitioning, and improving the balance until it is satisfactory. In different work, Pinar and Hendrickson [18] present a method for assigning computation tasks to processors in situations where this can be done flexibly, i.e., without consequences for the communication in a parallel computation. Each task is of unit size and can be assigned to one processor from a set of processors. An optimal task assignment is found by solving a maximum network-flow problem. In principle, such a method can also be applied to assign communication tasks. The restrictions of the problem formulation mean that the method can be applied to balance the sends or the receives, but not both simultaneously, in the case that [[lambda].sub.j] = 2 for all j .

The best sparse matrix partitioning methods for parallel sparse matrix-vector multiplication are those based on hypergraph partitioners, since these reflect the communication volume accurately in their objective function; graph partitioners only approximate the volume. Cur rently, a variety of hypergraph partitioners are available: PaToH [4, 5, 6], which was the first hypergraph partitioner to be used for sparse matrix partitioning; hMetis [16], a hypergraph version of the Metis graph partitioner; Mondriaan [23], a two-dimensional hypergraph-based sparse matrix partitioner, which can also be used to solve hypergraph partitioning problems by running it in one-dimensional mode; MONET [15], a hypergraph-based matrix ordering package, which permutes the rows and columns of a matrix to obtain a bordered block-diagonal form; Zoltan [10], a dynamic load balancing library for a wide range of parallel applications, which has recently been extended by a serial hypergraph partitioner [2]; MI-part [3], a multilevel hypergraph partitioner developed for circ[u.sub.i]t design, a traditional application area of hypergraph partitioning. Parkway [19] is a parallel k-way hypergraph partitioner that has been developed for very large Markov transition matrices (n = O([10.sup.7])) from voting models.

3. Algorithms for vector partitioning.

3.1. Special case: two processors per matrix column. A special case arises if [[lambda].sub.j] = 2 for all j. This will happen if p = 2, but it can also happen for larger p. For instance, if the matrix partitioning first splits the columns into two sets of columns, and then splits each set independently in the row direction, the resulting matrix partitioning over four processors satisfies the condition both for rows and columns.

In the special case, every assignment of a vector component [v.sub.j] to one of the two processors in matrix column j causes one processor to send a data word and the other to receive one. Let ncols(s) be the number of columns in which processor s occurs. Therefore, processor s will have to perform a total of ncols(s) send and receive operations. A lower bound on the communication cost of processor s is thus [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. This bound is attained if s is assigned (nearly) half of the vector components corresponding to the columns it shares. A lower bound on the overall communication cost is

(3.1) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

We now present an algorithm, Opt2, that assigns the vector components corresponding to the matrix columns in three phases. For brevity, we say that the algorithm assigns matrix columns. Let [B.sub.st] be the number of columns shared by processors s and t, where 0 [less than or equal to] s, t < p; note that [B.sub.ss]= 0. In Phase 1, the algorithm assigns columns shared by s and t in pairs, assigning one column to s and the other to t, until no such pair of columns is left. After these assignments, the remaining number of columns shared by processors s and t is [B.sub.st] = [B.sub.st] mod 2. We can view B' as the adjacency matrix of the graph G = (V, E), where V = {0' ..., p-1} and E [subset] V x V is defined by (s, t) [member of] E if and only if [B'.sub.st] = 1. Fig. 3.1 (b) presents this graph; Fig. 3.1 (a) presents a weighted graph corresponding to B.

In Phase 2, the algorithm walks paths in the graph G, each time starting at a vertex with odd degree. After an edge (s, t) is traversed in the direction from s to t, it is removed from E, and the corresponding column is assigned to s. A path is finished when the walk reaches a dead end, i.e., when the vertex reached has no more edges left. After the degree of a vertex becomes zero, the vertex is removed. This phase ends when all remaining vertices have even degree. Phase 2 is illustrated by Fig. 3.1(c). In Phase 3, the algorithm walks paths in the remaining graph until the graph is empty. Phase 3 is illustrated by Fig. 3.1(d). We will prove optimality of the algorithm by using the following lemma.

[FIGURE 3.1 OMITTED]

LEMMA 3.1. Let G = (V, E) be a graph and ([s.sub.0], [s.sub.1], ..., [s.sub.r]) be a path in G with all edges distinct. Let deg([s.sub.0]), the degree of [s.sub.0], be odd. Assume that the path cannot be extended any more by adding an edge from E that is distinct from the edges already contained in the path. Then: [s.sub.0] [not equal to] [s.sub.r] and a traversal of the path with edge removal changes deg([s.sub.0]) from odd to even and deg([s.sub.r]) from odd to zero, and it does not change the parity of deg(s) for s 54 [s.sub.0], s,.

Proof. The removal of the first edge ([s.sub.0], [s.sub.1]) makes deg([s.sub.0]) even. If deg([s.sub.0]) becomes zero, the walk cannot reach [s.sub.0] any more; otherwise, if the walk enters [s.sub.0], it will leave. Thus, the end vertex [s.sub.r] must differ from the start vertex [s.sub.0] and deg([s.sub.0]) remains even. Furthermore, deg([s.sub.r]) becomes zero since no edges remain. The parity of deg([s.sub.i]), i = 1, ..., r - 1, does not change when the path reaches an intermediate vertex [s.sub.1], because one edge is removed on entry and one on exit. Note that vertices may occur more than once on a path; our statements still hold then. Just before the last edge is removed, we have deg([s.sub.r]) = 1; at the start of the walk, deg([s.sub.r]) must have been odd. Vertices not on the path are not affected by the traversal. []

THEOREM 3.2. Let p, n [member of] N, with p [greater than or equal to] 2, n [greater than or equal to] 1. Let [P.sub.j] [subset] {0, ..., p-1} with [absolute value of [P.sub.j]] = 2 represent the pair of processors that share column j, for j = 0, ..., n - 1. Then algorithm Opt2 produces a vector partitioning [phi] : {0, ..., n - 1} [right arrow] {0, ..., p - 1} with minimal cost [C.sub.[phi]].

Proof. The algorithm terminates because the number of columns and edges is finite and because each walk in Phases 2 and 3 removes at least one edge. Each column is assigned to a processor at some time during the algorithm, either in Phase 1 or when its corresponding edge is removed in Phase 2 or 3. At the end of the algorithm, a complete vector partitioning has been obtained.

After Phase 1, [N.sub.send](8) - [N.sub.recv](s) = 0 for all vertices s. The value [N.sub.send](8) - [N.sub.recv](s) does not change for an intermediate vertex s in a walk in Phase 2. The value can change, however, if s is a start or end vertex in Phase 2. This can happen only once for every s, 0 [less than or equal to] s < p, because a vertex can only once be a start or end vertex of a walk. This is because a start vertex [s.sub.0] has an odd degree, which becomes even after the walk, see Lemma 3.1. Therefore, [s.sub.0] cannot become a start vertex again in this phase. Furthermore, it cannot become an end vertex of a walk, because of its even degree. An end vertex has no edges left, is removed, and hence does not occur again in further walks.

In Phase 3, the first walk starts at a vertex [s.sub.0] with even degree. After traversal and removal of the first edge ([s.sub.0], [s.sub.1]), the two vertices [s.sub.0] and [s.sub.1] have odd degree, but all other vertices have even degree. We can apply Lemma 3.1 to the path ([s.sub.1], ..., [s.sub.r]) representing the remainder of the first walk. The lemma says that [s.sub.r] [not equal to] [s.sub.1] and that deg([s.sub.r]) is odd at the start of the remaining walk, so [s.sub.r] = [s.sub.0] must hold. Thus, the path ([s.sub.0], ..., [s.sub.r]) must be cycle. After walking a complete path, all vertices have even degree again, and further walks can be carried out in the same way. Because each path is a cycle, the value [N.sub.send](s) - [N.sub.recv](s) does not change for the vertices s on the paths.

As a result of the algorithm, [absolute value of [N.sub.send] (s) - [N.sub.recv] (s)] [less than or equal to] 1 for all s, and equality [absolute value of [N.sub.send] (s) - [N.sub.recv](s)] = 1 1 only happens if deg(s) is odd at the start of Phase 2. Because deg(s) [[summation].sub.t [no equal to] s [B.sub.st], this is equivalent to ncols(s) = [[summation].sub.t [no equal to] s [B.sub.st] being odd. This proves that the resulting assignment is optimal.

3.2. General lower bounds on the communication cost. A simple lower bound on the communication cost based on the communication volume is

(3.2) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].

This bound occurs because not all processors can have a communication cost below the average V/p, [s.sub.0] that in every vector partitioning there must exist a processor s with C(s) [greater than or equal to] V/p. Because costs are integers, we even have [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. which shows that the cost C must be at least [L.sub.vol].

A different lower bound can be obtained by considering the vector components that a processor would like to possess in order to minimise its cost. Assume that a processor s can freely choose its components. Every vector component v j that the processor obtains will decrease its number of receives by one, irrespective of the size of the corresponding matrix column. At the same time, this will increase the number of sends by [[lambda].sub.j] - 1. Thus, a processor would prefer to obtain vector components corresponding to small columns, i.e., with low [[lambda].sub.j]. If given a free choice, a processor would take components in order of increasing column size, stopping when adding another component would give a larger number of sends than receives. This leads to the lower bound expressed by the following theorem.

THEOREM 3.3. Let p, n, s [member of] N, with p [greater than or equal to] 2, n [greater than or equal to] 1, and 0 [less than or equal to] s < p. Let [P.sub.j] [subset] {0, ..., p - 1} and [[lambda].sub.j] = [absolute value of [P.sub.j]], for j = 0, ..., n - 1. Assume that [[lambda].sub.j] [greater than or equal to] 2 for all j, and [[lambda].sub.j] [less than or equal to] [[lambda].sub.j]', if j < j'. Define

[J.sub.k] = {j:0 [less than or equal to] < j < k [conjunction] s [member of] [P.sub.j]},

for k = 0, ..., n. Let k be the largest integer with k < n for which

[summation over (j [member of][J.sub.k])] ([[lambda].sub.j] - 1) [less than or equal to] [absolute value of [J.sub.n]\[J.sub.k]].

Define L(s) = [absolute value of [J.sub.n]\[J.sub.k]]. Then for every vector assignment [phi] with [phi](j) [member of] [P.sub.j] for all j, we have

[C.sub.[phi]] [greater than or equal to] L(s).

Proof. Let [J.sub.k] be the set defined in the theorem and let J = [J.sub.n] be the set of all indices j with s [member of] [P.sub.j]. Let [phi] be a vector assignment with [phi](j) E [P.sub.j] for all j, and J' [subset] J be the set of indices j with [phi](j) = s. We will transform J' into [J.sub.k] by changes that do not increase the cost for processor s. This will prove that the cost of [J.sub.k] is less than or equal to that of Y. First, we transform J' into a [J.sub.r] by adding [j.sub.min], the smallest j [member of] J\J' to Y, and removing [j.sub.max], the largest j from J', and repeating this procedure as long as [j.sub.min] < [j.sub.max]. Each change reduces the cost, or keeps it the same, because it replaces an index j with a larger size [[lambda].sub.j] by one with a smaller size, or a lower numbered one of equal size. The changes fill the holes in J' until it contains all the indices from J up to a certain size, i.e., J' = [J.sub.r] for a certain r. If A [??] [J.sub.r], we have [[summation].sub.j[member of][J.sub.r] ([[lambda].sub.j] - 1 > [absolute value of J\[J.sub.r]] by the definition of k. Thus, we can remove the largest j from [J.sub.r] while not increasing the cost. (This is because the number of sends decreases, and the number of receives increases by one, but it will not exceed the number of sends before the change.) We can repeat this until r = k. If [J.sub.k] C [J.sub.r] does not hold, we have [J.sub.k] = [J.sub.r], or [J.sub.r] [??] [J.sub.k]. If [J.sub.k] = [J.sub.r], we are done, but otherwise we can add the smallest element of J\[J.sub.r] to [J.sub.r]. This does not increase the cost, because the number of receives decreases and this number determines the cost. We have proven that the cost for processor s in an assignment [phi] is at least the cost incurred by assigning all components from [J.sub.k] to s, which is L(s) = [absolute value of J\[J.sub.k]].

By using Theorem 3.3 we can determine a local lower bound L(s) for each processor s. The resulting lower bound for the overall communication cost is

(3.3) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].

We call this bound the local lower bound because it is based on a local assignment of vector components, not taking other processors into account. In contrast, the volume-based bound [L.sub.vol] represents a global view, but it does not take local details into account. Combining the two lower bounds, we have max{[L.sub.vol], L} as the best lower bound.

The bound L(s) for processor s can be generalised to the situation where a processor does not start from scratch, but instead already has some communication obligations (for instance for vector components that have already been assigned). Let [N.sub.send] be the number of data words the processor has to send already, and [N.sub.recv] the number it has to receive. Choose k as large as possible, while ensuring that

(3.4) [N.sub.send] + [summation over (j[member of][j.sub.k])] ([[lambda].sub.j] - 1) < [N.sub.recv] + [absolute value of J/[J.sub.k]].

The lower bound for processor s then equals

(3.5) L(s) = [N.sub.recv] + [absolute value of J/[J.sub.k]].

The number of data words the processor has to send to attain the lower bound is

(3.6) [L.sub.send](s) = [N.sub.send] + [summation over (j[member of][j.sub.k])] ([[lambda].sub.j] - 1).

The number of data words it has to receive to attain the lower bound equals L(s).

3.3. Vector partitioning by the local-bound algorithm. We can use the local lower bound as the basis for a heuristic algorithm. The heuristic is to choose the processor [s.sub.max] that has the highest local bound L(s) for its current index set J(s), and let this processor choose a vector component [v.sub.j]. It chooses a component with minimal [[lambda].sub.j], and thus will not increase its lower bound. This heuristic tries to avoid increasing the highest lower bound L(s) and hence raising L. The algorithm then updates the number of sends and receives incurred, removes j from all the index sets J(s), and updates the lower bounds L(s) and the associated number of sends [L.sub.send] (s) needed to attain the lower bound.

A processor stops accepting new components when its number of sends [N.sub.send] (s) equals the current optimal number of sends [L.sub.send](s)- (By definition, [N.sub.send] (s) [less than or equal to] [L.sub.send] (s).) Accepting more components would only increase L(s), so that instead it is better to stop and let other processors accept components. The algorithm terminates when no processor is willing any more to accept new components. It may be possible that some (large) components are left over at the end of the algorithm; these can be handled by greedy assignment, see the next subsection. Algorithm 3.1 presents the details of the local-bound based partitioning. The notation 'argmax' used in Algorithm 3.1 means an index for which the maximum is obtained, and similarly for 'argmin'. An efficient implementation of the algorithm would use the compressed column storage (CCS) data structure (see e.g. [1]) for the communication matrix A' to facilitate access to the processors in each column, and compressed row storage (CRS) with the nonzeros of each row stored in increasing order to enable direct access for processor s to the next local component [v.sub.j] with minimal [[lambda].sub.j].

3.4. Greedy assignment. Vector partitioning by greedy assignment (GA) is done by handling vector components in an arbitrary order, each time assigning a component [v.sub.j] to a processor [lambda](j) that would have the minimum current cost if it were to obtain the component. This assignment is determined by first incrementing [N.sub.recv] (s) for all s [member of] [P.sub.j], and then finding a processor s from [P.sub.j] with minimal value max {[N.sub.send] (s) + [[lambda].sub.j] - 1, [N.sub.recv](S) - 1}. The current send and receive counts are then updated by decrementing [N.sub.recv]([lambda](j)) and adding [[lambda].sub.j] - 1 to [N.sub.send]([phi](j)). GA can also be applied when part of the components have already been assigned by a different method.
ALGORITHM 3.1
Local-bound based vector partitioning.

input: A is a p x n sparse matrix,
 [P.sub.j] = {i:0 [less than or equal to] i < p [conjunction]
 [a.sub.ij] [not equal to] 0} [less than or equal to] j < n.

output: [phi] = distr(v): vector distribution over p processors,
 such that [phi](j) [member of] [P.sub.j], for 0
 [less than or equal to] j < n.

for all s:0 [less than or equal to] s < p do
 [N.sub.send](s) := 0;
 [N.sub.recv](s) := 0;
 J(s) := {j: 0 [less than or equal to] j < n
 [conjunction] s [member of] [P.sub.j]};
 [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII],
 where [J.sub.k] (s) = {j : j [member of] J(s)
 [conjunction] 0 [less than or equal to] j < k};
 L(s) := [absolute value of J(s)/[J.sub.k](s)];
 [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII];
 if [L.sub.send](8) = 0 then active(s) := false;
 else active(s) := true;
while ([there exists]s : 0 [less than or equal to] s < p
 [conjunction] active(s)) do
 [s.sub.max] := argmax{L(s) : 0 [less than or equal to]
 s < p [member of] active(s)j;
 j := argmin{[[lambda].sub.j] : j [member of] J([s.sub.max])};
 [phi](j) := [s.sub.max];
 [N.sub.send](smax) := [N.sub.send]([s.sub.max]) +
 [[lambda].sub.j] - 1;
 for all s : s [member of] [P.sub.j] [conjunction] s
 [no equal to] [s.sub.max] do
 [N.sub.recv](s) := [N.sub.recv](s) + 1;
 for all s : s [member of] [P.sub.j] do
 J(s) := J(s)\W;
 for all s : s [member of] [P.sub.j] [conjunction] s
 [not equal to] [s.sub.max] do
 [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
 where [J.sub.k](s) = {j : j [member of] J(s)
 [conjunction] 0 [less than or equal to] j < k};
 L(s) := [N.sub.recv](s) +
 [absolute value of J(s)\[J.sub.k](s)];
 [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII];
 for all s : s [member of] [P.sub.j]
 [conjunction] [N.sub.send](S) = [L.sub.send](s) do
 active(s) := false;


3.5. Greedy improvement of a given partitioning. A given partitioning [phi] can be improved by a simple greedy improvement (GI) procedure, as follows. A vector component [v.sub.j] is chosen at random, and for each processor s [member of] [P.sub.j], s [not equal to] [theta](j), the change in cost caused by reassigning [v.sub.j] to s is computed. If there is a change that reduces the cost, it is carried out and the vector component is reassigned. In case of several possible changes, the best one is taken by using a secondary criterion. Often, the size of the cost reduction is the same for all cost-reducing reassignments, so the size is less suitable for breaking ties. (It could be used together with a ternary criterion.) We break ties by reassigning the component to the processor in [P.sub.j] with the current least number of sends. The reason is that the number of sends of a processor is affected most by a reassignment, more than the number of receives. Any remaining ties are resolved arbitrarily. After that, another component j is randomly chosen, and the same procedure is executed. This is repeated until no more improvement can be obtained. Note that GI is based on moves, like the well-known Fiduccia-Mattheyses algorithm [13] which is at the heart of today's successful hypergraph partitioners, but GI is less sophisticated (and cheaper) since it does not accept cost increases and does not select a move with largest possible gain.

This procedure can be implemented efficiently by organising it in passes. A new pass starts every time a reduction has been obtained, or at the start of the GI algorithm. The indices j are stored in an array, initially in the natural order. The first part of the array stores the indices of the columns that have not been tried yet in the current pass and the second part the indices of the remaining columns. The number [N.sub.tried] is used to keep track of the number of components tried in the current pass. If reassigning a vector component [v.sub.j] cannot reduce the cost, the index j is swapped with the last untried index, and [N.sub.tried] is incremented. If reassignment succeeds, [N.sub.tried] is reset to 0. Thus, a random index can always be chosen from a contiguous subarray of untried indices.

4. Results. The algorithms presented in the previous section have been implemented within the sparse matrix partitioning package Mondriaani (1). First, the matrices were partitioned on a 375 MHz Sun Enterprise 420 computer at Sandia National Laboratories in Albuquerque, N M, which has four Sparc-2 processors, 4 Gbyte RAM, 64-bit hardware arithmetic, and which runs the Solaris 8 operating system. Since the Mondriaan program itself is serial, only one processor is used per program run. Second, numerical experiments with different vector partitioning methods were performed for the partitioned matrices on an 867 MHz Apple PowerBook G4 computer with 768 Mbyte RAM, 32-bit hardware arithmetic, and a PowerPC G4 processor, which runs the Mac OS 10.2 operating system.

We have checked the quality of the vector partitioning by using a test set of sparse matrices from publicly available collections, supplemented with a few matrices from Sandia applications and a few matrices from our own applications. Table 4.1 presents the matrices; the matrix be s s t k 3 2 was obtained from the Rutherford-Boeing collection [11, 12]; the matrices 1hr34 and nug30 were obtained from the University of Florida collection [9]. The matrix rhpent ium_new provided by Robert Hoekstra represents a circuit simulation by the Sandia package Xyce for part of a Pentium processor. The matrix polyDFT originates in a polymer self-assembly simulation by the Density Functional Theory package Tramanto from Sandia. The matrix tbdl inux is a term-by-document matrix describing the documentation of the SuSE Linux 7.1 operating system. The matrix cage 13 [22] (available through [9]) is a stochastic matrix describing transition probabilities in the cage model of a DNA polymer of length 13 moving in a gel under the influence of an electric field. Each matrix has been partitioned using the Mondriaan matrix partitioner with default parameters, allowing a computational load imbalance of 3%.

Table 4.2 presents the partitioned sparse matrices. We can view the resulting vector partitioning problem (for a vector v) as a sparse m' x n' matrix A', where the m' = [p.sub.comm] rows represent the processors that are active in communication and the n, columns represent the columns from the original matrix with [[lambda].sub.j] [greater than or equal to] 2. (For a vector u, we can transpose the partitioned matrix A and proceed in the same way.) From now on, we distinguish between A and A', and between [p.sub.comm] and p. Table 4.2 presents several properties of A' such as the communication volume, two lower bounds on the communication cost, the optimal solution for the special case [[lambda].sub.j] [less than or equal to] 2, if applicable, and a balance ratio, for the seven matrices from the test set and for p = 4, 16, 64. Note that

(4.1) V [greater than or equal to] n',

because every column in A' has two or more processors and hence causes at least one communication. If V = n', this means that all columns have two nonzeros, so that our optimal algorithm Opt2 from Subsection 3.1 is applicable. Furthermore, we have

(4.2) V + n' = nz(A'),

because the number of communications caused by a column in A' equals its number of nonzeros minus 1. It is easily verified that Eqns (4.1) and (4.2) are indeed satisfied for the data given in Table 4.2. The bounds given in the table are the volume-based bound [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and the local lower bound L, see Eqn (3.3). The ratio max{[L.sub.vol], L}/(V/p) gives the balance of the communication load in case we manage to attain the best lower bound. Perfect balance corresponds to a ratio of 1.

For each matrix, both the input vector v and the output vector u are partitioned. A problem instance is thus a triple matrix/p/vector. The instances nug30/p/v and be s stk3 2/4/v are omitted because the preceding matrix partitioning produces a one-dimensional column partitioning, so that no communication is needed for v. The number of communicating processors [p.sub.comm] can be less than the available number of processors p: for instance, [p.sub.comm] = 38 for rhpentium new/64/v and 1hr34/64/u. A processor does not communicate if all the components j that it owns satisfy [[lambda].sub.j] = 0 or [[lambda].sub.j] = 1; for [[lambda].sub.j] = 0, the corresponding matrix column j must be empty, but for [[lambda].sub.j] = 1 it may have many nonzeros. Table 4.2 shows that the lower bound L is the best (highest) bound for 34 out of 38 instances, that L = [L.sub.vol] for three instances, and that the bound [L.sub.vol] is best for only one instance, namely nug30/4/u. The table also shows that applicability of Opt2 is not restricted to the case [p.sub.comm] [less than or equal to] 2; e.g., lhr3 4/16/v with [p.sub.comm] = 10 can be solved by the optimal algorithm. The ratios given in the last column of Table 4.2 show that even if the balancing problem can be solved optimally, this does not mean that the resulting balance is perfect. For 17 problem instances out of 38, the ratio exceeds 2, meaning that one processor has to communicate at least twice the average amount based on V and p; the maximum ratio observed is 4.02 for lhr3 4/64/u. This imbalance is inevitable and is caused by the preceding matrix partitioning.

Table 4.3 presents results of Mon, the algorithm from [23] implemented in the original Mondriaan package, version 1.0, which has been explained in Section 2, and of LB, the local-bound algorithm presented in Subsection 3.3, with and without the greedy improvement procedure presented in Subsection 3.5. The matrix partitioning was fixed by using Mondriaan once with a fixed random number seed, giving the partitioned matrices presented in Table 4.2. The vector partitioning was performed a hundred times, each time with a different seed. Like the Mondriaan matrix partitioning, the Mondriaan vector partitioning is a randomised algorithm. Still, there may be some dependence on the input ordering. The LB vector partitioning is deterministic, since it uses the natural vector ordering. To remove any possible dependence on the input ordering, the columns of A' were randomly permuted before each run, both for Mon and LB. This makes the average results more meaningful, but it also enhances the chances of encountering a good solution in one of the runs. The greedy improvement procedure is terminated if a pass through all the vector components shows that no improvement can be found any more, or if a preset maximum number of tries is reached, 10n for partitioning of v and 10m for u. Thus, the maximum number of tries is of the order of the size of the original matrix A.

The average communication costs given in Table 4.3 show that LB is better than Mon for 18 out of 38 problem instances, equal to Mon for 12 instances, and worse for 8 instances. With greedy improvement (GI) switched on, LB is better for 11 instances, equal for 24, and worse for 3. All instances with equal performance (with or without GI) correspond to cases where both algorithms solve the problem to optimality in every run of the algorithm. LB+GI solves the problem to optimality in every run for 34 instances; Mon+GI does this for 25 instances. We may conclude that LB is superior to Mon. Furthermore, it is clear that GI is useful, since it improves the average result in most cases for Mon, and in 9 cases for LB. Although LB is better than Mon, we need not discard the latter: Mon+GI performs better on average than LB+GI for nug3 0/4/u, cage I 3/16/u, and cage I 3/64/v. This means that it is worthwhile to try both LB+GI and Mon+GI, instead of just using the better method LB+GI. For nug3 0/4/u, with ratio 1.00 in Table 4.2, Mon+GI managed to achieve an optimal solution and hence a perfect balance in at least one run.

The best communication costs given in Table 4.3 show that all problems, except nug30/ 16/u, can be solved to optimality by one of the methods in at least one of the runs. The cost 8935 of the best solution for nug3 0/16/u is 1.3% higher than the best bound L = 8823. We tried to improve the solution in 10,000 additional runs, but only obtained a marginal reduction of the cost to 8931.

In an actual application, it is possible to perform several runs of the vector partitioner and keep the best solution, because vector partitioning by our methods is cheap and takes much less time than the preceding matrix partitioning. Our largest problem cage13/64/v took about 9 s for both Mon+GI and LB+GI on the PowerBook computer, whereas the corresponding matrix partitioning took 1123 s on the same machine.

5. Conclusion and future work. We have presented two new algorithms and a new lower bound for solving the vector partitioning problem for parallel sparse matrix-vector multiplication. We have concentrated on the input vector, since the partitioning of the output vector is a similar problem. The first algorithm, Opt2, is optimal and can be applied in the special case that every matrix column is owned by at most two processors. This situation occurs in particular for small p, but also if the preceding matrix partitioner has been highly successful, which often limits the number of processors that own a column. The second algorithm, LB, is a heuristic that in practice often finds the optimal solution, in particular if it is post-processed by greedy improvement. The lower bound L helps to steer the use of the different vector partitioners; in particular, it tells us when we can stop attempts at further improvement. The lower bound seems sharp, since it could be attained in all practical cases we tested, except one. Still, we have to realise that practical communication matrices A' are very special, since they typically contain many columns with a small number of nonzeros, and in most cases two nonzeros. For other types of matrices, other heuristics may be needed. The general vector partitioning problem presented in this paper is NP-complete, which is proven by Ali Pinar in Appendix A.

A good vector partitioning must have a good balance of the communication load, for instance in iterative linear system solvers. The balance is measured by the maximum amount of communication per processor, which is an important metric, perhaps second only to the metric of total communication volume. As communication will become more important when applications scale up to thousands of processors, balancing communication will become crucial. This gives rise to a class of new and interesting optimisation problems. A promising avenue of research may be trying to generalise computation balancing methods, such as by Pinar and Hendrickson [18], so that they can be applied to communication balancing as well. Here, it is crucial that both sends and receives are balanced; to complicate matters, these two objectives depend on each other. Another issue is that communication tasks can vary in size; for the algorithm in [18], this would mean that the assumption of unit task size must be dropped.

Our problem originated in parallel sparse matrix--vector multiplication, but it has a wider range of applicability, within the field of parallel computing and beyond. One way of viewing the problem is as follows. We have to distribute items among a group of people. For each item, a specific subgroup is interested in owning it. One member of the subgroup wins and becomes the owner. This winner has to compensate all the losers, each with the same amount of money, say 1 dollar or euro. The problem is to distribute the items such that the maximum amount of money any person has to pay or receive is minimised. (Note that this problem is not about minimising the net amount of money paid or received.)

The problem for the special case that could be solved optimally by algorithm Opt2 may also have wider application, for instance in parallel molecular dynamics simulations. Because of Newton's Third Law of Motion, each 2-atom force at the boundary between two processors can be computed by either of the processors involved. The processor that computes the force must send the result to the other processor, which must receive it. Algorithm Opt2 can be used to balance such communication obligations.

Several open research questions remain, such as: can the general vector partitioning problem be solved optimally under less restrictive assumptions? Can the methods be generalised to the situation where the vector components have communication weights, so that sending component [v.sub.j] to a processor costs [w.sub.j] time units; this occurs sometimes in the context of hypergraph partitioning for circuit simulation. Can we extend the methods to the situation, not uncommon for square matrices, where we require the same partitioning for the input and output vectors? If the matrix is symmetric and has been partitioned symmetrically, this is easy, but otherwise it is a two-objective optimisation problem, which is much harder to solve. (The original Mondriaan package contains an option distr(u) = distr(v), but often the communication load is out of balance for this option, because there is not much choice in the assignment of vector components to processors.)

The existence of a lower bound L > V/p points to unavoidable imbalance in the communication for a given matrix partitioning. An optimal solution to the vector partitioning problem may still have imperfect balance. Another indicator of such imbalance is [p.sub.comm] < p, meaning that not all processors participate in the communication. This leads to a bound [L.sub.vol] = [V/[p.sub.comm] > V/p. To reduce this imbalance, we must modify the preceding matrix partitioning. The challenge is to do this without increasing the communication volume. We are currently investigating lower-bound based tie-breaking in the matrix partitioner as one means of achieving perfect communication balance.

Appendix. Vector partitioning is NP-complete. Proof provided by Ali Pinar Lawrence Berkeley National Laboratory.

We can formulate the vector partitioning problem as the following decision problem, which we call the Column Assignment problem.

INSTANCE: Positive integers p, n, a p x n matrix A with elements [a.sub.ij] E {0,1} and with [[lambda].sub.j] = [absolute value of {i s 0 [less than or equal to] i < p [conjunction] [a.sub.ij] = 1] [greater than or equal to] 2 for j = 0, ..., n - 1, and a positive integer maximum communication load C.

[FIGURE A.1 OMITTED]

QUESTION: Does there exist an assignment : [phi] ..., {0, ..., n-1} [right arrow] {0, ..., p-1} of the columns of the matrix such that: (i) [a.sub.[phi]](j),j = 1, for j = 0, ..., n - 1; (ii) [N.sub.send](i) = [[summation].sub.j: [phi](j)=i] ([[lambda].sub.j] - 1) [less than or equal to] C, for i = 0, ..., p - 1; and (iii) [N.sub.recv](i) = [absolute value of {j : 0 [less than or equal to] j < n [conjunction] [a.sub.ij] = 1 [conjunction] [phi](j) [not equal to] i}] [less than or equal to] < C?

The column assignment problem can be related to the Bin Packing problem, defined as follows by Garey and Johnson [14].

INSTANCE: A finite set U of items, a positive integer size s(u) for each item u [member of] U, a positive integer bin capacity C, and a positive integer P.

QUESTION: Does there exist a partition of U into disjoint sets [U.sub.0], [U.sub.1], ..., [U.sub.P-1], such that the sum of the sizes of the items in each [U.sub.i] is at most C?

Bin packing is NP-complete in the strong sense, which means that there is no pseudopolynomial time solution unless P=NP [14].

THEOREM A.1. The column assignment problem is NP-complete.

Proof. We will prove the NP-completeness of the column assignment problem by reduction from the bin packing problem. Our reduction is not polynomial, but pseudo-polynomial. The reduction still proves NP-completeness, since the bin packing problem is NP-complete in the strong sense. Observe that for any instance of the bin packing problem, multiplying the size of each item and the bound C by a constant will not change the original problem. Let (3.3) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], where [u.sub.min] is the minimum size among items in U. Multiply each size s(u), [u.sub.min], and C by c. In the rest of the proof, we assume that s(u), [u.sub.min], and C denote the numbers after this multiplication, so that [u.sub.min] > max {P, [absolute value of]}. Without loss of generality, we assume that C > [u.sub.min] and [u.sub.min] [greater than or equal to] 3.

Given an instance of the bin packing problem, the column assignment problem will have p = P + Q processors and n = [absolute value of U] + Q columns, where Q = [[summation].sub.u [member of] U] [sup.s(u)] - [absolute value of U] (P - 1). The first P processors correspond to the bins of the bin packing problem, and the remaining Q are auxiliary processors. Similarly, the first [absolute value of U] columns correspond to items in U and the remaining Q are auxiliary columns.

Let A = [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] be the p x n matrix of the column assignment problem after reduction, where the P x [absolute value of U] submatrix [A.sub.11] is defined by the bin processors and item columns, and the Q x Q submatrix [A.sub.22] is defined by the auxiliary processors and auxiliary items. The matrix A is defined as follows and is illustrated by Fig. A.1.

* All elements of [A.sub.11] are 1, which means that each column representing an item can be assigned to any processor representing a bin.

* [A.sub.12] = 0, which means that auxiliary columns cannot be assigned to a processor representing a bin.

* We define [A.sub.22] such that each row and each column in [A.sub.22] has C nonzeros. This can be achieved by assigning nonzeros to the C positions on and below the main diagonal in each column (continuing with the first row, if the last row is reached). In other words, [a.sub.ij] = 1 in [A.sub.22] for i = j, (j + 1) mod Q, ..., (j + C - 1) mod Q. All other elements are 0. We assume without loss of generality that Q [greater than or equal to] C. (Otherwise, Q can be suitably enlarged.)

* We define [A.sub.21] such that each row has a single nonzero, and such that the ith column, which represents the ith item [u.sub.i], has s([u.sub.i]) - P + 1 nonzeros. This can be achieved by putting nonzeros in the first s([u.sub.o]) - P + 1 rows and the first column, the next s([u.sub.1]) - P + 1 rows and the second column, and so on. Altogether this requires [summation over ([u.sub.i] [member of] U)] s([u.sub.i]) - (P + 1) = [summation over ([u.sub.i] [member of] U)] s([u.sub.i]) - [absolute value of U] (P - 1) = Q rows, which are exactly the auxiliary rows.

We claim that there is a solution to the reduced column assignment problem with no processor loaded more than C, if and only if there is a solution to the bin packing problem. The proof is based on the following observations on the reduced column assignment problem.

1. Each auxiliary column has to be assigned to an auxiliary processor. This will load every auxiliary processor with C - 1 sends and C - 1 receives for an auxiliary column. No auxiliary processor can obtain two (or more) auxiliary columns, since the number of sends 2C - 2 would then exceed C. Since each of the remaining columns has at least [u.sub.min] + 1 [greater than or equal to] 4 nonzeros, representing at least three sends, they cannot be assigned to an auxiliary processor without exceeding C. Therefore, each column representing an item needs to be assigned to a processor representing a bin.

2. The receiving load of every processor that represents a bin is at most [absolute value of]. Recall that [absolute value of] [less than or equal to] C because of the initial scaling operation. Every auxiliary processor has a receiving load of C, including one receive operation for the columns representing an item. Whether the communication load of a processor satisfies the bound C is thus solely determined by its sending load.

3. Each column representing an item u has s(u) + 1 nonzeros and hence assigns s(u) send operations to its owner.

By the first observation, each item must be assigned to a bin for the communication load to remain below C. By the second observation, distribution of remaining receives does not affect the feasibility of the solution. Only the assignment of sends is important. By the final observation, each column representing an item assigns a number of sends equal to the size of the item to its processor (bin). By construction, any item-column can be assigned to any bin-processor. As a result, the problem reduces to the bin packing problem.

Based on these observations, there will be a solution to the column assignment problem, if and only if there is a solution to the bin packing problem.

Dedicated to Alan George on the occasion of his 60th birthday

Acknowledgements. We would like to thank the referees for their careful reading and useful comments on the initial version of this paper, which led to significant improvements. We thank Ali Pinar for providing the proof of NP-completeness of our problem, which is given in the appendix. The first author wishes to thank Erik Boman, Omit Catalyurek, Karen Devine, Bob Heaphy, Bruce Hendrickson, Alex Pothen, and Tony Skjellum for many fruitful discussions during his visit to the US, April-July 2004.

* Received October 5, 2004. Accepted for publication June 14, 2005. Recommended by B. Hendrickson.

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[18] --, Exploiting flexibly assignable work to improve load balance, in Proceedings 14th Annual ACM Symposium on Parallel Algorithms and Architectures, 2002, pp. 155-163.

[19] A. TRIFUNOVIC AND W. J. KNOTTENBELT, A parallel algorithm for multilevel k-way hypergraph partitioning, in Proceedings Third International Symposium on Parallel and Distributed Computing, Cork, Ireland, July 2004.

[20] B. UCAR AND C. AYKANAT, Encapsulating multiple communication-cost metrics in partitioning sparse rectangular matrices for parallel matrix-vector multiplies, SIAM J. Sci. Comput., 25 (2004), pp. 1837-1859.

[21] L. G. VALIANT, A bridging model for parallel computation, Communications of the ACM, 33(8) (1990), pp.103-111.

[22] A. VAN HEUKELUM, G. T. BARKEMA, AND R. H. BISSELING, DNA electrophoresis studied with the cage model, J. Comput. Phys., 180 (2002), pp. 313-326.

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(1) The new vector partitioning algorithms will be made available in version 2.0 of Mondriaan, see http://www.math.uu.nl/people/bisseling/Mondriaan, to be released soon.

ROB H. BISSELING, Mathematical institute, Utrecht University, PO Box 80010, 3508 TA Utrecht, The Netherlands (Rob. Bisselinq@math.uu.nl). Part of the work was done while this author was a visiting scientist at the Computer Science Research Institute, Sandia National Laboratories, PO Box 5800, Albuquerque, NM 87185-1110, USA. Part of the research has been funded by the Dutch BSIK/BRICKS project. Computer time has been partially funded by the Dutch National Computer Facilities foundation (NCF).

WOUTER MEESEN, Mathematical Institute, Utrecht University, PO Box 80010, 3508 TA Utrecht, The Netherlands (woutermeesen@gmail.com).
TABLE 4.1
Properties of the test matrices. For each matrix, we give: the number
of rows m, the number of columns n, the number of nonzeros nz(A),
and the origin.

Matrix m n nz(A) Application area

rhpentium new 25187 25187 258265 circuit simulation
lhr34 35152 35152 764014 chemical engineering
nug30 52260 379350 1567800 linear programming
bcsstk32 44609 44609 2014701 structural engineering
tbdlinux 112757 20167 2157675 information retrieval
polyDFT 46176 46176 3690048 polymer simulation
cage13 445315 445315 7479343 DNA electrophoresis

TABLE 4.2
Properties of the m' x n' communication matrix A' for various
processor numbers p and for both the input vector v and the output
vector u . For each problem instance, we give: the communication
volume V ; the number of active processors m' = [p.sub.comm]; the
number of columns n' that cause communication; the number of nonzeros
of A' ; the lower bounds [L.sub.vol], L on the communication cost; the
ratio max{[L.sub.vol,], L}/(V/p). The values in boldface in the column
of L show the cases where the optimal algorithm Opt2 is applicable; in
all these cases, Opt2 attained the lower bound L .

Matrix p Vector V m' n' nz(A')

RHPentium_new 4 v 13791 4 12656 26447
 u 5614 2 5614 11228
 16 v 9888 10 9215 19103
 u 18519 16 14469 32988
 64 v 10108 38 7930 18038
 u 26223 64 17317 43540

lhr34 4 v 1172 4 1171 2343
 u 253 2 253 506
 16 v 1069 10 1069 2138
 u 2694 15 2512 5206
 64 v 8372 64 7813 16185
 u 2008 38 1946 3954

nug30 4 u 56348 4 39413 95761
 16 u 136712 16 47237 183949
 64 u 247032 64 51413 298445

bcsstk32 4 u 1986 4 1968 3954
 16 v 4872 16 4750 9622
 u 3106 12 3094 6200
 64 v 11839 64 10759 22598
 u 8001 56 7474 15475

tbdlinux 4 v 27800 4 15740 43540
 u 3263 2 3263 6526
 16 v 44179 16 15871 60050
 u 30299 14 15194 45493
 64 v 76914 64 16261 93175
 u 66564 58 21652 88216

PolyDFT 4 v 5352 4 5352 10704
 u 3636 4 3636 7272
 16 v 19756 16 17571 37327
 u 16405 14 14820 31225
 64 v 41625 64 30083 71708
 u 37541 64 28604 66145

cage13 4 v 57054 4 57054 114108
 u 58804 4 58804 117608
 16 v 95032 16 86329 181361
 u 160761 16 139624 300385
 64 v 236759 64 194172 430931
 u 208314 64 172388 380702

Matrix p Vector [L.sub.vol] L Ratio

RHPentium_new 4 v 3448 5822 1.69
 u 2807 2807 2.00
 16 v 989 1722 2.79
 u 1158 1941 1.68
 64 v 266 517 3.27
 u 410 621 1.52

lhr34 4 v 293 381 1.30
 u 127 127 2.01
 16 v 107 234 3.50
 u 180 344 2.04
 64 v 131 270 2.06
 u 53 126 4.02

nug30 4 u 14087 12435 1.00
 16 u 8545 8823 1.03
 64 u 3860 5816 1.51

bcsstk32 4 u 497 718 1.45
 16 v 305 741 2.43
 u 259 456 2.35
 64 v 185 366 1.98
 u 143 378 3.02

tbdlinux 4 v 6950 9149 1.32
 u 1632 1632 2.00
 16 v 2762 5906 2.14
 u 2165 2900 1.53
 64 v 1202 3407 2.83
 u 1148 1832 1.76

PolyDFT 4 v 1338 1410 1.05
 u 909 1135 1.25
 16 v 1235 2274 1.84
 u 1172 2346 2.29
 64 v 651 1281 1.97
 u 587 1454 2.48

cage13 4 v 14264 19497 1.37
 u 14701 16466 1.12
 16 v 5940 12313 2.07
 u 10048 15152 1.51
 64 v 3700 5456 1.47
 u 3255 5457 1.68

TABLE 4.3
Communication cost for different vector partitioning methods. The
value given is the percentage by which the cost exceeds the best known
lower bound, max{[L.sub.vol], L}&. A value '0' means exactly 0, and
hence an optimal solution. For each problem, we give the average over
100 runs of the vector partitioning by: Mon, the algorithm implemented
in the original Mondriaan package, version 1.0; Mon+GI, which is Mon
followed by greedy improvement; LB, the local-bound algorithm; LB+GI.
Furthermore, we give the best result obtained in the runs by the
methods with GI.

Matrix p Vector Average (%)

 Mon Mon+GI LB LB+GI

RHPentium_new 4 v 0 0 0 0
 u 0 0 0 0
 16 v 0 0 0 0
 u 3.4 0.03 0 0
 64 v 1.5 0.2 0 0
 u 12.6 0.3 0 0

1hr34 4 v 0 0 4.0 0
 u 0 0 0 0
 16 v 0.004 0 7.9 0
 u 0.03 0 0.1 0
 64 v 0 0 0 0
 u 0.02 0 3.0 0

nug30 4 u 0.003 0.003 0.6 0.6
 16 u 15.7 7.6 8.4 1.4
 64 u 32.7 12.0 0 0

bcsstk32 4 u 0 0 0 0
 16 v 0.001 0 0 0
 u 0.02 0 0 0
 64 v 0.006 0 0.7 0
 u 0.07 0 0 0

tbdlinux 4 v 31.8 0 0 0
 u 0 0 0 0
 16 v 34.9 5.6 0 0
 u 6.0 1.7 0 0
 64 v 22.5 7.3 0 0
 u 30.0 9.8 0.09 0

PolyDFT 4 v 0 0 0 0
 u 0 0 0 0
 16 v 1.0 0 0 0
 u 0.003 0 0 0
 64 v 14.1 0.8 0 0
 u 3.7 0.4 0 0

cage13 4 v 0 0 0 0
 u 0 0 0 0
 16 v 0 0 0 0
 u 0.001 0 3.0 0.4
 64 v 0.07 0.02 3.7 0.9
 u 3.0 0 0 0

Matrix p Vector Best (%)

 Mon+GI LB+GI

RHPentium_new 4 v 0 0
 u 0 0
 16 v 0 0
 u 0 0
 64 v 0 0
 u 0 0

1hr34 4 v 0 0
 u 0 0
 16 v 0 0
 u 0 0
 64 v 0 0
 u 0 0

nug30 4 u 0 0.6
 16 u 7.2 1.3
 64 u 11.6 0

bcsstk32 4 u 0 0
 16 v 0 0
 u 0 0
 64 v 0 0
 u 0 0

tbdlinux 4 v 0 0
 u 0 0
 16 v 5.3 0
 u 1.5 0
 64 v 6.8 0
 u 9.0 0

PolyDFT 4 v 0 0
 u 0 0
 16 v 0 0
 u 0 0
 64 v 0.5 0
 u 0.1 0

cage13 4 v 0 0
 u 0 0
 16 v 0 0
 u 0 0.4
 64 v 0 0.8
 u 0 0
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Author:Bisseling, Rob H.; Meesen, Wouter
Publication:Electronic Transactions on Numerical Analysis
Date:Dec 1, 2005
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