The Impact of Taskyield on the Design of Tasks Communicating Through MPI

摘要

The OpenMP tasking directives promise to help expose a higher degree of concurrency to the runtime than traditional worksharing constructs, which is especially useful for irregular applications. In combination with process-based parallelization such as MPI, the taskyield construct in OpenMP can become a crucial aspect as it helps to hide communication latencies by allowing a thread to execute other tasks while completion of the communication operation is pending. Unfortunately, the OpenMP standard only provides little guarantees on the characteristics of the taskyield operation. In this paper, we explore different potential implementations of taskyield and present a portable black-box tool for detecting the actual implementation used in existing OpenMP compilers/runtimes. Furthermore, we discuss the impact of the different taskyield implementations on the task design of the communication-heavy Blocked Cholesky Factorization and the difference in performance that can be observed, which we found to be over 20%.