[Beowulf] OpenMPI & Slurm: mpiexec/mpirun vs. srun

Christopher Samuel chris at csamuel.org
Mon Dec 18 16:00:32 PST 2017

On 19/12/17 09:20, Prentice Bisbal wrote:

> What are the pros/cons of using these two methods, other than the 
> portability issue I already mentioned? Does srun+pmi use a different 
> method to wire up the connections? Some things I read online seem to 
> indicate that. If slurm was built with PMI support, and OpenMPI was 
> built with Slurm support, does it really make any difference?

Benchmark.  In (much) older versions of Slurm we would find that NAMD 
built with OpenMPI would scale better with mpirun, but in more recent 
versions (from 15.x onwards I believe) we found srun scaled better instead.

Whether that's because of differences in wireup or other issues I'm not 
sure, but currently I recommend people use srun instead.  You will also 
get better accounting information.

With mpirun OMPI will start orted on the each node (via srun) and then 
that will launch the MPI ranks.  With srun slurmd will launch the MPI 
ranks itself.

Hope this helps!
  Chris Samuel  :  http://www.csamuel.org/  :  Melbourne, VIC

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