MPI within SMP machines question

[Daniel Kidger]

>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 (!).


Yours,
Daniel.

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