2013 IEEE 27th International Symposium on Parallel & Distributed Processing Minimizing communication in all-pairs shortest paths Edgar Solomonik Aydın Buluç James Demmel Univ. of California, Berkeley Lawrence Berkeley Nat. Lab. Univ. of California, Berkeley Department of EECS Computational Research Division Department of EECS
[email protected] [email protected] [email protected] Abstract— over the (min,+) semiring. Several theoretical improvements We consider distributed memory algorithms for the all-pairs have been made, resulting in subcubic algorithms for the shortest paths (APSP) problem. Scaling the APSP problem APSD problem. However, in practice, these algorithms are to high concurrencies requires both minimizing inter-processor communication as well as maximizing temporal data locality. typically not competitive with simpler cubic algorithms. The 2.5D APSP algorithm, which is based on the divide-and- Variants of the Floyd-Warshall algorithm are most suitable conquer paradigm, satisfies both of these requirements: it can for dense graphs. Johnson’s algorithm [26], which is based utilize any extra available memory to perform asymptotically less on repeated application of Dijkstra’s single-source shortest communication, and it is rich in semiring matrix multiplications, path algorithm (SSSP), is theoretically faster than the Floyd- which have high temporal locality. We start by introducing a block-cyclic 2D (minimal memory) APSP algorithm. With Warshall variants on sufficiently sparse graphs. However, the a careful choice of block-size, this algorithm achieves known data dependency structure of this algorithm (and Dijkstra’s communication lower-bounds for latency and bandwidth. We algorithm in general) make scalable parallelization difficult. extend this 2D block-cyclic algorithm to a 2.5D algorithm, which SSSP algorithms based on Δ-stepping [32] scale better in can use c extra copies of data to reduce the bandwidth cost by a c1/2 practice but their performance is input dependent and scales factor of , compared to its 2D counterpart.