Load imbalance usually introduces wait states into the execution of parallel programs. Being able to identify and quantify wait states is therefore essential for the diagnosis and remediation of this phenomenon. An established method of detecting wait states is to generate event traces and compare relevant timestamps across process boundaries. However, large trace volumes usually prevent the analysis of longer execution periods. In this paper, we present an extremely lightweight wait-state profiler which does not rely on traces that can be used to estimate wait states in MPI codes with arbitrarily long runtimes. The profiler combines scalability with portability and low overhead.
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