On many of the shared-memory implementations (device ch_shmem, mpich reserves some shared
memory in which messages are transferred back and forth. By default, mpich reserves 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.