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This paper presents a performance model of many-to-one collective communications for MPI platforms on a switched Ethernet network. The model is based on empirical findings from observation of many-to-one operations over a wide range of message sizes. The model reflects a significant increase in the execution time for medium-sized messages, persistently observed for different parallel platforms and MPI implementations and not reflected in traditional communication performance models. We also demonstrate that the use of the model can significantly improve the performance of parallel applications, intensively using many-to-one communications.