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Subject: Re: [OMPI users] open-mpi behaviour on Fedora, Ubuntu, Debian and CentOS
From: Gus Correa (gus_at_[hidden])
Date: 2010-04-27 12:59:38


Hi Dave

Dave Love wrote:
> Gus Correa <gus_at_[hidden]> writes:
>
>> Or run a serial version on the same set of machines,
>> compiled in similar ways (compiler version, opt flags, etc)
>> to the parallel versions, and compare results.
>> If the results don't differ, then you can start blaming MPI.
>
> That wouldn't show that there's actually any OpenMPI-specific problem,
> though -- the parallelism potentially introduces indeterminacy. [I
> don't mean to imply Guy thinks otherwise, or that anyone has enough
> information to guess what's actually happening.] General discussion of
> numerical issues and scientific computing war stories must be way
> off-topic here...
>

You are right.
Actually, on a case of an ocean model that I tested this way,
exactly the opposite happened.
The optimization flags were the main cause of changes in the results,
as expected.
Whether the code was compiled and run serial or parallel with MPI
made little difference (although it might).

Well, IMHO, this is not totally off topic.
I don't think it hurts the list protocol to have a relatively
lax criterion for what is on or off.
After all, the original question and the following
discussion was about whether MPI (OpenMPI in particular)
can affect or not numerical accuracy.

Regards,
Gus Correa
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