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Subject: Re: [OMPI users] Asynchronous behaviour of MPI Collectives
From: Eugene Loh (Eugene.Loh_at_[hidden])
Date: 2009-01-23 11:24:32

Jeff Squyres wrote:

> As you have notes, MPI_Barrier is the *only* collective operation that
> MPI guarantees to have any synchronization properties (and it's a
> fairly weak guarantee at that; no process will exit the barrier until
> every process has entered the barrier -- but there's no guarantee
> that all processes leave the barrier at the same time).

Actually, many collectives have that property due to data-causality
conditions. E.g., MPI_Allreduce cannot exit from any process until
every process has finished.

As Jeff mentions, however, exit times can be "ragged" (and unfortunately
often are).