OpenCoarrays: open-source transport layers supporting coarray Fortran compilers

Coarray Fortran is a set of features of the Fortran 2008 standard that make Fortran a PGAS parallel programming language. Two commercial compilers currently support coarrays: Cray and Intel. Here we present two coarray transport layers provided by the new OpenCoarrays project: one library based on MPI and the other on GASNet. We link the GNU Fortran (GFortran) compiler to either of the two OpenCoarrays implementations and present performance comparisons between executables produced by GFortran and the Cray and Intel compilers. The comparison includes synthetic benchmarks, application prototypes, and an application kernel. In our tests, Intel outperforms GFortran only on intra-node small transfers (in particular, scalars). GFortran outperforms Intel on intra-node array transfers and in all settings that require inter-node transfers. The Cray comparisons are mixed, with either GFortran or Cray being faster depending on the chosen hardware platform, network, and transport layer.