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From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2007-10-31 19:36:24


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