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.
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.
-- Jeff Squyres Cisco Systems
