A comparative characterization of communication patterns in applications using MPI and shared memory on an IBM SP2
In this paper we analyze the characteristics of communication in three different applications, FFT, Barnes and Water, on an IBM SP2. We contrast the communication using two different programming models: message-passing, MPI, and shared memory, represented by a state-of-the-art distributed virtual shared memory package, TreadMarks. We show that while communication time and busy times are comparable for small systems, the communication patterns are fundamentally different leading to poor performance for TreadMarks-based applications when the number of processors increase. This is due to the request/reply technique used in TreadMarks that results in a large fraction of very small messages. However, if the application can be tuned to reduce the impact of small message communication it is possible to achieve acceptable performance at least up to 32 nodes. Our measurements also show that TreadMarks programs tend to cause a more even network load compared to MPI programs.
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