Migrating a Scientific Application from MPI to Coarrays

MPI is a de facto standard for portable parallel programming using a message passing paradigm. Interest is growing in other paradigms, in particular Partitioned Global Address Space (PGAS) languages such as Coarray Fortran, UPC and Titanium. Most Computational Science and Engineering codes are written in Fortran, and the 2008 Fortran standard will include coarrays, a Cray initiated PGAS extension of the language. We report on the experience of taking a moderately large CFD program and migrating it to a Cray X1E using coarrays rather than MPI. The MPI and coarray versions are compared, both for ease of programming and legibility, and for performance. We find that coarrays are a useful and expressive addition to Fortran, and that their use does not impact on performance. We discuss various ways in which the use of coarrays can be optimized in a program.