Specification Guideline Violations by MPI_Dims_create

Abstract

In benchmarking a library providing alternative functionality for structured, so-called isomorphic, sparse collective communication [4], we found use for the MPI_Dims_create functionality of MPI [3] for suggesting a balanced factorization of a given number p (of MPI processes) into d factors that can be used as the dimension sizes in a d-dimensional Cartesian communicator. Much to our surprise, we observed that a) different MPI libraries can differ quite significantly in the factorization they suggest, and b) the produced factorizations can sometimes be quite far from balanced, indeed, for some composite numbers p some MPI libraries sometimes return trivial factorizations (p as factor). This renders the functionality, as implemented, useless. In this poster abstract, we elaborate on these findings.