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From: Karsten Bolding (karsten_at_[hidden])
Date: 2007-10-31 14:47:35

On Wed, Oct 31, 2007 at 11:13:46 -0700, Jeff Squyres wrote:
> On Oct 31, 2007, at 10:45 AM, Karsten Bolding wrote:
> > In a different thread I read about a performance penalty in OpenMPI if
> > more than one MPI-process is running on one processor/core - is that
> > correct? I mean having max-slots>4 on a quad-core machine.
> Open MPI polls for message passing progress (to get the absolute
> minimum latency -- it can be faster than blocking/waking up). If you
> overload a machine, Open MPI will usually detect that and know to
> call yield() in the middle of its polling so that other processes can
> get swapped in and make progress.

Does OpenMPI detect if procceses share memory and hence do not
communicate via sockets.

> 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?

> --
> Jeff Squyres
> Cisco Systems


Karsten Bolding                    Bolding & Burchard Hydrodynamics
Strandgyden 25                     Phone: +45 64422058
DK-5466 Asperup                    Fax:   +45 64422068
Denmark                            Email: karsten_at_[hidden],5466,11,3