Unfortunately even the MPI_Barrier doesn't guarantee a synchronous
exit on all processes. There is no such thing in the MPI and there is
no way to implement such a synchronization primitive in general (if
one take in account metrics such as performance or scalability).
In this particular context the MPI_Allreduce offers you exactly the
same kind of synchronization as the MPI_Barrier. Moreover, all non
rooted collectives (with the exception of the v versions) imply a
synchronous behavior simply because all processes involved in the
collective have to participate with some data.
george.
On Mar 23, 2009, at 17:11 , 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.
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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