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

[Ebert Maximilian]

The reason I am asking is because I want to use two GPUs and 8 CPUs. So for now I have 2 MPI ranks and 4 OpenMP threads. Is there a way to have 8 MPI ranks but only use 2 GPUs? I also tried 8 MPI ranks with -gpu_id 00001111 but it was about the same as 2 MPI ranks with 4 OpenMP.

Max

-----Ursprüngliche Nachricht-----
Von: gromacs.org_gmx-users-bounces at maillist.sys.kth.se [mailto:gromacs.org_gmx-users-bounces at maillist.sys.kth.se] Im Auftrag von Carsten Kutzner
Gesendet: Donnerstag, 8. Januar 2015 15:48
An: gmx-users at gromacs.org
Betreff: Re: [gmx-users] Performance difference between MPI ranks and OpenMP


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
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