Simplifying Communication Overlap in OpenSHMEM Through Integrated User-Level Thread Scheduling.

Overlap of communication with computation is a key optimization for high performance computing (HPC) applications. In this paper, we explore the usage of user-level threading to enable productive and efficient communication overlap and pipelining. We extend OpenSHMEM with integrated user-level thread scheduling, enabling applications to leverage fine-grain threading as an alternative to non-blocking communication. Our solution introduces communication-aware thread scheduling that utilizes the communication state of threads to minimize context switching overheads. We identify several patterns common to multi-threaded OpenSHMEM applications, leverage user-level threads to increase overlap of communication and computation, and explore the impact of different thread scheduling policies. Results indicate that user-level threading can enable blocking communication to meet the performance of highly-optimized, non-blocking, single-threaded codes with significantly lower application-level complexity. In one case, we observe a 28.7% performance improvement for the Smith-Waterman DNA sequence alignment benchmark.