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Subject: Re: [OMPI users] difference between single and double precision
From: Eugene Loh (eugene.loh_at_[hidden])
Date: 2010-12-06 10:31:40


Mathieu Gontier wrote:

> Nevertheless, one can observed some differences between MPICH and
> OpenMPI from 25% to 100% depending on the options we are using into
> our software. Tests are lead on a single SGI node on 6 or 12
> processes, and thus, I am focused on the sm option.

Is it possible to narrow our focus here a little? E.g., are there
particular MPI calls that are much more expensive with OMPI than MPICH?
Is the performance difference observable with simple ping-pong tests?

> So, I have two questions:
> 1/ does the option--mca mpool_sm_max_size=XXXX can change something (I
> am wondering if the value is not too small and, as consequence, a set
> of small messages is sent instead of a big one)

There was recent related discussion on this mail list.
http://www.open-mpi.org/community/lists/users/2010/11/14910.php

Check the OMPI FAQ for more info. E.g.,
http://www.open-mpi.org/faq/?category=sm

This particular parameter disappeared with OMPI 1.3.2.
http://www.open-mpi.org/faq/?category=sm#how-much-use

To move messages as bigger chunks, try btl_sm_eager_limit and
btl_sm_max_send_size:
http://www.open-mpi.org/faq/?category=sm#more-sm

> 2/ is there a difference between --mca btl tcp,sm,self and --mca btl
> self,sm,tcp (or not put any explicit mca option)?

I think tcp,sm,self and self,sm,tcp will be the same. Without an
explicit MCA btl choice, it depends on what BTL choices are available.