Reducing communication in parallel graph search algorithms with software caches

In many scientific and computational domains, graphs are used to represent and analyze data. Such graphs often exhibit the characteristics of small-world networks: few high-degree vertexes connect many low-degree vertexes. Despite the randomness in a graph search, it is possible to capitalize on the characteristics of small-world networks and cache relevant information of high-degree vertexes. We applied this idea by caching remote vertex ids in a parallel breadth-first search benchmark. Our experiment with different implementations demonstrated significant performance improvements over the reference implementation in several configurations, using 64 to 1024 cores. We proposed a system design in which resources are dedicated exclusively to caching and shared among a set of nodes. Our evaluation demonstrates that this design reduces communication and has the potential to improve performance on large-scale systems in which the communication cost increases significantly with the distance between nodes. We also tested a memcached system as the cache server finding that its generic protocol, which does not match our usage semantics, hinders significantly the potential performance improvements and suggested that a generic system should also support a basic and lightweight communication protocol to meet the needs of high-performance computing applications. Finally, we explored different configurations to find efficient ways to utilize the resources allocated to solve a given problem size; to this extent, we found utilizing half of the compute cores per allocated node improves performance, and even in this case, caching variants always outperform the reference implementation.