MPI programs with more than one task on the same computing node may benefit from using shared memory to send messages between intranode tasks.
This support is controlled by the MP_SHARED_MEMORY environment variable. The default setting is no. In this case, no shared memory is used for message passing.
Setting this variable to yes directs MPI to use a shared-memory protocol for message passing whenever two or more tasks of a job are running on the same node.
For the 32-bit libraries, shared memory exploitation always allocates a 256MB virtual memory address segment that is not available for any other use. Thus, programs that are already using all available segments cannot use this option. For more information, see Available virtual memory segments.
Shared memory support is available for both IP and User Space MPI protocols. For programs on which all tasks are on the same node, shared memory is used exclusively for all MPI communication (if MP_SHARED_MEMORY=yes).