MPI within SMP machines question

Daniel Kidger Daniel.Kidger at
Mon Dec 3 09:10:27 PST 2001

>With MPI, messages are passing over network in typical beowulf
>environment. My question is, when src and dst are two processes
>running on the same machine, does it still communicate through
>network? or something smarter way like shared memory? or this
>is a feature I can explicitly enable? Thank you for your reply.

With MPICH over the Quadrics Interconnect, it short-circuits the NIC using a
direct memory copy when the source and destination are on the same node.

This behaviour can be switched on/off via an environment variable.

One oddity is that on at least one motheboard we have tested, it is *faster*
to go via the network card than get the processor to do the memory copy (!).


Dr. Dan Kidger, Quadrics Ltd.      daniel.kidger at
One Bridewell St., Bristol, BS1 2AA, UK         0117 915 5505
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