Nonblocking Epochs in MPI One-Sided Communication

The synchronization model of the MPI one-sided communication paradigm can lead to serialization and latency propagation. For instance, a process can propagate non-RMA communication-related latencies to remote peers waiting in their respective epoch-closing routines in matching epochs. In this work, we discuss six latency issues that were documented for MPI-2.0 and show how they evolved in MPI-3.0. Then, we propose entirely nonblocking RMA synchronizations that allow processes to avoid waiting even in epoch-closing routines. The proposal provides contention avoidance in communication patterns that require back to back RMA epochs. It also fixes the latency propagation issues. Moreover, it allows the MPI progress engine to orchestrate aggressive schedulings to cut down the overall completion time of sets of epochs without introducing memory consistency hazards. Our test results show noticeable performance improvements for a lower-upper matrix decomposition as well as an application pattern that performs massive atomic updates.