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Subject: Re: [OMPI users] Collective operations and synchronization
From: Ashley Pittman (ashley_at_[hidden])
Date: 2009-03-23 17:49:11

On 23 Mar 2009, at 21:11, Ralph Castain wrote:
> Just one point to emphasize - Eugene said it, but many times people
> don't fully grasp the implication.
> On an MPI_Allreduce, the algorithm requires that all processes -enter-
> the call before anyone can exit.
> It does -not- require that they all exit at the same time.
> So if you want to synchronize on the -exit-, as your question
> indicated, then you must add the MPI_Barrier as you describe.

All MPI_Barrier requires is that all processes enter the call before
anyone can exit, I'm not sure that "synchronising on exit" has any
particular meaning at all.

Putting a MPI_Barrier call immediatly after a MPI_Allreduce call would
be superfluous.