[Beowulf] programming multicore clusters

Toon Knapen toon.knapen at fft.be
Fri Jun 15 04:49:49 PDT 2007

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.


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