Extending openMP metadirective semantics for runtime adaptation

Abstract

OpenMP 5.0 introduces the metadirective to support selection from a set of directive variants based on the OpenMP context, which is composed of traits from active OpenMP constructs, devices, implementations or user-defined conditions. OpenMP 5.0 restricts the selection to be determined at compile time, which requires that all traits must be compile-time constants. Our analysis of real applications indicates that this restriction has its limitation, and we explore extension of user-defined contexts to support variant selection at runtime. We use the Smith-Waterman algorithm as an example to show the need for adaptive selection of parallelism and devices at runtime, and present a prototype implemented in the ROSE compiler. Given a large range of input sizes, our experiments demonstrate that one of the adaptive versions of Smith-Waterman always chooses the parallelism and device that delivers the best performance, with improvements between 20% and 200% compared to non-adaptive versions that use the other approaches.