Mitigating Mpi Message Matching Misery

To satisfy MPI ordering semantics in the presence of wildcards, current implementations store posted receive operations and unexpected messages in linked lists. As applications scale up, communication patterns that scale with the number of processes or the number of threads per process can cause those linked lists to grow and become a performance problem. We propose new structures and matching algorithms to address these performance challenges. Our scheme utilizes a hash map that is extended with message ordering annotations to significantly reduce time spent searching for matches in the posted receive and the unexpected message structures. At the same time, we maintain the required MPI ordering semantics, even in the presence of wildcards. We evaluate our approach on several benchmarks and demonstrate a significant reduction in the number of unsuccessful match attempts in the MPI message processing engine, while at the same time incurring low space and time overheads.