OpenSHMEM as a Portable Communication Layer for PGAS Models: A Case Study with Coarray Fortran

Languages and libraries based on the Partitioned Global Address Space (PGAS) programming model have emerged in recent years with a focus on addressing the programming challenges for scalable parallel systems. Among these, Coarray Fortran (CAF) is unique in that as it has been incorporated into an existing standard (Fortran 2008), and therefore it is of particular importance that implementations supporting it are both portable and deliver sufficient levels of performance. OpenSHMEM is a library which is the culmination of a standardization effort among many implementers and users of SHMEM, and it provides a means to develop light-weight, portable, scalable applications based on the PGAS programming model. As such, we propose here that OpenSHMEM is well situated to serve as a runtime substrate for CAF implementations. In this paper, we demonstrate how OpenSHMEM can be exploited as a runtime layer upon which CAF may be implemented. Specifically, we re-targeted the CAF implementation provided in the OpenUH compiler to OpenSHMEM, and show how parallel language features provided by CAF may be directly mapped to OpenSHMEM, including allocation of remotely accessible objects, one-sided communication, and various types of synchronization. Moreover, we present and evaluate various algorithms we developed for implementing remote access of non-contiguous array sections and acquisition and release of remote locks using the OpenSHMEM interface.