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MPI derived datatypes are a powerful method to define arbitrary collections of non-contiguous data in memory and to enable non-contiguous data communication in a single MPI function call. In this paper, we employ MPI datatypes in four NAS benchmarks (MG, LU, BT, and SP) to transfer non-contiguous data. Comprehensive performance evaluation was carried out on two clusters: an Itanium-2 Myrinet cluster and a Xeon InfiniBand cluster. Performance results show that using datatypes can achieve performance comparable to manual packing/unpacking in the original benchmarks, though the MPI implementations that were studied also perform internal packing and unpacking on noncontiguous datatype communication. In some cases, better performance can be achieved because of the reduced costs to transfer non-contiguous data. This is because some optimizations in the MPI packing/unpacking implementations can be easily overlooked in manual packing and unpacking by users. Our case study demonstrates that MPI datatypes simplify the implementation of non-contiguous communication and lead to application code with portable performance. We expect that with further improvement of datatype processing and datatype communication such as [10, 24], datatypes can outperform the conventional methods of noncontiguous data communication. Our modified NAS benchmarks can be used to evaluate datatype processing and datatype communication in MPI implementations.
Date of Conference: 18-18 Aug. 2004