On many of the shared-memory implementations (device ch_shmem, mpichreserves some shared
memory in which messages are transferred back and forth. By default, mpichreserves roughly four MBytes of shared memory. You can change this with the
environment variable MPI_GLOBMEMSIZE.
For example, to make it 8 MB, enter
setenv MPI_GLOBMEMSIZE 8388608Large messages are transfered in pieces, so MPI_GLOBMEMSIZE does not limit the maximum message size but increasing it may improve performance.