[gmx-users] Performance difference between MPI ranks and OpenMP

[Carsten Kutzner]

On 08 Jan 2015, at 15:38, Ebert Maximilian <m.ebert at umontreal.ca> wrote:

> Hi list,
> 
> I have another question regarding performance. Is there any performance difference if I start a process on a 8 CPU machine with 8 MPI ranks and 1 OpenMP or 4 MPI ranks an 2 OpenMP? Both should use the 8 CPUs right?
Right, but there is a performance difference (try it out! :)
Normally using just one layer of parallelization is fastest (less overhead), i.e.
using just OpenMP threads or just MPI ranks. However, more than 8 or so OpenMP 
threads per rank seldom yield optimal performance, so that a hybrid parallelization
approach makes sense in such situations.

Carsten


> 
> Thank you very much,
> 
> Max
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--
Dr. Carsten Kutzner
Max Planck Institute for Biophysical Chemistry
Theoretical and Computational Biophysics
Am Fassberg 11, 37077 Goettingen, Germany
Tel. +49-551-2012313, Fax: +49-551-2012302
http://www.mpibpc.mpg.de/grubmueller/kutzner
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