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Prospects for truly asynchronous communication with pure MPI and hybrid MPI/OpenMP on current supercomputing platforms

We investigate the ability of MPI implementations to perform truly asynchronous communication with nonblocking point-to-point calls on current highly parallel systems, including the Cray XT and XE series. For cases where no automatic overlap of communication with computation is available, we demonstrate several different ways of establishing explicitly asynchronous communication by variants of functional decomposition using OpenMP threads or tasks, implement these methods in a parallel sparse matrix-vector multiplication code, and show the resulting performance benefits. The impact of node topology and the possible use of simultaneous multithreading (SMT) is studied in detail.