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This paper details the implementation and usage of software-based performance counters to understand the performance of a particular implementation of the MPI standard, Open MPI. Such counters can expose intrinsic features of the software stack that are not available otherwise in a generic and portable way. The PMPI-interface is useful for instrumenting MPI applications at a user level, however it is insufficient for providing meaningful internal MPI performance details. While the Peruse interface provides more detailed information on state changes within Open MPI, it has not seen widespread adoption. We introduce a simple low-level approach that instruments the Open MPI code at key locations to provide fine-grained MPI performance metrics. We evaluate the overhead associated with adding these counters to Open MPI as well as their use in determining bottlenecks and areas for improvement both in user code and the MPI implementation itself.