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Subject: Re: [OMPI users] Odd MPI_Bcast behavior
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2008-09-17 11:55:58


Are you using IB, perchance?

We have an "early completion" optimization in the 1.2 series that can
cause this kind of behavior. For apps that dip into the MPI layer
frequently, it doesn't matter. But for those that do not dip into the
MPI layer frequently, it can cause delays like this. See http://www.open-mpi.org/faq/?category=openfabrics#v1.2-use-early-completion
  for a few more details.

If you're not using IB, let us know.

On Sep 17, 2008, at 10:34 AM, Gregory D Abram wrote:

> I have a little program which initializes, calls MPI_Bcast, prints a
> message, waits five seconds, and finalized. I sure thought that each
> participating process would print the message immediately, then all
> would wait and exit - thats what happens with mvapich 1.0.0. On
> OpenMPI 1.2.5, though, I get the message immediately from proc 0,
> then 5 seconds later, from proc 1, and then 5 seconds later, it
> exits- as if MPI_Finalize on proc 0 flushed the MPI_Bcast. If I add
> a MPI_Barrier after the MPI_Bcast, it works as I'd expect. Is this
> behavior correct? If so, I so I have a bunch of code to change in
> order to work correctly on OpenMPI.
>
> Greg
>
> Here's the code:
>
> #include <stdlib.h>
> #include <stdio.h>
> #include <mpi.h>
>
> main(int argc, char *argv[])
> {
> char hostname[256]; int r, s;
> MPI_Init(&argc, &argv);
>
> gethostname(hostname, sizeof(hostname));
>
> MPI_Comm_rank(MPI_COMM_WORLD, &r);
> MPI_Comm_size(MPI_COMM_WORLD, &s);
>
> fprintf(stderr, "%d of %d: %s\n", r, s, hostname);
>
> int i = 99999;
> MPI_Bcast(&i, sizeof(i), MPI_UNSIGNED_CHAR, 0, MPI_COMM_WORLD);
> // MPI_Barrier(MPI_COMM_WORLD);
>
> fprintf(stderr, "%d: got it\n", r);
>
> sleep(5);
>
> MPI_Finalize();
> }
>
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-- 
Jeff Squyres
Cisco Systems