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Subject: Re: [OMPI users] Collective operations and synchronization
From: Ralph Castain (rhc_at_[hidden])
Date: 2009-03-23 17:11:05


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.

Ralph

On Mar 23, 2009, at 3:01 PM, Eugene Loh wrote:

> Shaun Jackman wrote:
>
>> I've just read in the Open MPI documentation [1]
>
> That's the MPI spec, actually.
>
>> that collective operations, such as MPI_Allreduce, may synchronize,
>> but do not necessarily synchronize. My algorithm requires a
>> collective operation and synchronization; is there a better (more
>> efficient?) method than simply calling MPI_Allreduce followed by
>> MPI_Barrier?
>
> MPI_Allreduce is a case that actually "requires" synchronization in
> that no participating process may exit before all processes have
> entered. So, there should be no need to add additional
> synchronization. A special case might be an MPI_Allreduce of a 0-
> length message, in which case I suppose an MPI implementation could
> simple "do nothing", and the synchronization side-effect would be
> lost.
>
> The MPI spec is mainly talking about a "typical" collective where
> one could imagine a process exiting before some processes have
> entered. E.g., in a broadcast or scatter, the root could exit
> before any other process has entered the operation. In a reduce or
> gather, the root could enter after all other processes have exited.
> For all-to-all, allreduce, or allgather, however, no process can
> exit before all processes have entered, which is the synchronization
> condition effected by a barrier. (Again, null message lengths can
> change things.)
>
>> [1] http://www.mpi-forum.org/docs/mpi21-report-bw/node85.htm
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