Vectorized Barrier and Reduction in LLVM OpenMP Runtime

Paper i proceeding, 2021

Barrier synchronization is a well known operation in parallel processing that can be an obstacle for getting performance in parallel programs, particularly for high thread counts. Similarly, reduction is a collective communication pattern frequently used in parallel applications and needs to be optimized for applications to achieve their best performance. With the introduction of multi-core and many-core processors several new barrier and reduction implementations have been proposed. As the number of cores per node continues to grow, implementation of these primitives need to be revisited and adapted for upcoming architectures. We see an opportunity to improve synchronization by exploiting vector units present in modern and future CPU designs based on vector ISAs such as ARM’s Scalable Vector Extension and the RISC-V Vector extension. In this work we propose vectorized barriers and reductions using the vector length agnostic paradigm and implement them in the LLVM OpenMP runtime. Our barrier implementation achieves up to 2.2 × and 1.4 × speedup over the default LLVM OpenMP implementation on Intel KNL and Fujitsu A64FX, respectively.