Designing Multithreaded Algorithms for Breadth-First Search and st-connectivity on the Cray MTA-2

Graph abstractions are extensively used to understand and solve challenging computational problems in various scientific and engineering domains. They have particularly gained prominence in recent years for applications involving large-scale networks. In this paper, we present fast parallel implementations of three fundamental graph theory problems, Breadth-First Search, st-connectivity and shortest paths for unweighted graphs, on multithreaded architectures such as the Cray MTA-2. The architectural features of the MTA-2 aid the design of simple, scalable and high-performance graph algorithms. We test our implementations on large scale-free and sparse random graph instances, and report impressive results, both for algorithm execution time and parallel performance. For instance, Breadth-First Search on a scale-free graph of 400 million vertices and 2 billion edges takes less than 5 seconds on a 40-processor MTA-2 system with an absolute speedup of close to 30. This is a significant result in parallel computing, as prior implementations of parallel graph algorithms report very limited or no speedup on irregular and sparse graphs, when compared to the best sequential implementation.