Greg Lindahl wrote:
Most MPI and OpenMP implementations lock processes to cores for this
very reason.
AFAICT this is not always the case. E.g. on systems with glibc, this
functionality (set_process_affinity and such) is only available starting
from libc-2.3.4.
In another mail in the same thread:
> One good example comes from codes which have both pure MPI and hybrid
> MPI/OpenMPI implementations. There's published data from John
> Michalakes MM5 is faster in pure MPI mode.
>
> In fact I've never seen a bid involving pure MPI and hybrid codes
> where hybrid was faster.
>
Mixing OpenMP and MPI in one and the same algorithm does indeed not
generally provide a big advantage. However MPI and OpenMP can be used on
different scales. E.g. you can obtain a big boost when running an
MPI-code where each process performs local dgemm's for instance by using
an OpenMP'd dgemm implementation. This is an example where running
mixed-mode makes a lot of sense.
toon
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