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.

Ashley,

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