On Oct 31, 2007, at 11:47 AM, Karsten Bolding wrote:
Does OpenMPI detect if procceses share memory and hence do not
communicate via sockets.
Yes.
But if you lie to Open MPI and tell it that there are more processors
than there really are, we may not recognize that the machine is
oversubscribed and therefore not call yield(). Hence, performance
will *really* go down the drain.
and if I don't give any hints - just start my 13 jobs on 4 cores where
the load balancing is done based on CPU-requirement (this could
also be
on 4 single-core processors where jobs can't be swapped) - that is in
principle OK?
Yes, it should be; Open MPI should detect that you're oversubscribed
and set itself to yield() during polling in the MPI library.
--
Jeff Squyres
Cisco Systems
