An Evaluation of Anticipated Extensions for Fortran Coarrays

A set of parallel features, broadly referred to as Fortran coarrays, was added to the Fortran 2008 standard. It is expected that several new parallel features, designed to complement or augment this feature set, will be added to the next revision of the standard. This includes statements for forming and changing between image teams, as well as statements for performing communication and synchronization with respect to image teams. In this paper, we describe an early implementation and evaluation of these anticipated features within the OpenUH compiler and its CAF runtime system. We demonstrate the utility of team-based barriers in comparison to the existing sync images statement for performing synchronization amongst a team of images. Techniques for hiding synchronization and incorporating locality-awareness with a collectives implementation based on 1-sided communication are described, and we present the impact of these optimizations for allreduce operations based on message length. Our results showed better performance for medium to large sized messages compared to the corresponding allreduce implementation using the Cray Fortran Coarrays implementation. Using the new teams and collectives features, we obtained 6.2% performance improvement compared to an original Fortran 2008 version of the CG benchmark from the NAS Parallel Benchmark Suite for class D, when using 1024 images.