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For MPI jobs, the system provided openmpi will not bind processes to cores by default, as would be the normal default for openmpi. This is set this way to avoid inadvertently oversubscribing processes on cores. In addition, the system openmpi settings will map processes by socket. This should give a good process distribution in all cases. However, if you wish to use a number of processes less than 28 processes half of the slots per node in an MPI job then you may want to map by node to get the most even distribution of processes across nodes. You can do that with the --map-by node option flag to mpirun.
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