Reuse MPI contexts across different mpi calls #552
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Shillaker
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Dec 23, 2021
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At every MPI call we were creating a new
ContextWrapperinstance. This, albeit not really necessary, wasn't harming performance much. However, iniside the constructor, we were callingMpiWorldRegistry::getOrInitialiseWorld(int worldId)which acquires a lock to access the MPI world.The mentioned method in the world registry is meant to be called upon world creation time from each different rank, not every time. There is a lock-free method,
MpiWorldRegistry::getWorld(int worldId)that can be used when we know the world has been created (i.e. after we have calledMPI_Init). Given that all MPI calls, in every thread, must happen afterMPI_Init, it is safe to assume that the world will already exist.In this PR I change the way we query for a world in the
ContextWrapperand, while at it, re-use context wrappers per MPI rank. Preliminary results in high-contention environments show a 50% execution time reduction.