MPI usage patterns are changing as applications move towards fully-multithreaded runtimes. However, the impact of these patterns on MPI message matching is not well-studied. In particular, MPFs mechanic for receiver-side data placement, message matching, can be impacted by increased message volume and nondeterminism incurred by multithreading. While there has been significant developer interest and work to provide an efficient MPI interface for multithreaded access, there has not been a study showing how these patterns affect messaging patterns and matching behavior. In this paper, we present a framework for studying the effects of multithreading on MPI message matching. This framework allows us to explore the implications of different common communication patterns and thread-level decompositions. We present a study of these impacts on the architecture of two of the Top 10 supercomputers (NERSC's Cori and LANL's Trinity). This data provides a baseline to evaluate reasonable matching engine queue lengths, search depths, and queue drain times under the multithreaded model. Furthermore, the study highlights surprising results on the challenge posed by message matching for multithreaded application performance.
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