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LANS Publications

"Optimizing the Synchronization Operations in MPI One-Sided Communication"

R. Thakur, W. Gropp, B. Toonen

Preprint ANL/MCS-P1232-0205

Preprint Version: [pdf]

One-sided communication in MPI requires the use of one of three different synchronization mechanisms, which indicate when the one-sided operation can be started and when the operation is completed. Efficient implementation of the synchronization mechanisms is critical to achieving good performance with one-sided communication. Our performance measurements, however, indicate that in many MPI implementations, the synchronization functions add significant overhead, resulting in one-sided communication performing much worse than point-to-point communication for short- and medium-sized messages. In this paper, we describe our efforts to minimize the overhead of synchronization in our implementation of one-si9ded communication in MPICH2. We describe our optimizations for all three synchronization mechanisms defined in MPI: fence, post-start-complete-wait, and lock-unlock. Our performance results demonstrate that, for short messages, MPICH2 performs six times faster than LAM for fence synchronization and 50% faster for post-start-complete-wait synchronization, and it performs more than twice as fast as Sun MPI for all three synchronization methods.