Open MPI logo

Open MPI Development Mailing List Archives

  |   Home   |   Support   |   FAQ   |   all Development mailing list

Subject: Re: [OMPI devel] failure with zero-length Reduce() and bothsbuf=rbuf=NULL
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2010-02-10 07:31:40


Thanks for the reminder.

Note that from a standards perspective, note that MPI_REDUCE *does* require at least one element -- MPI-2.2 p163:34-35:

   "Each process can provide one element, or a sequence of elements..."

So I think that George's assertion is correct: your test code is incorrect.

But that's not what is causing your example to fail. Here's the issue in OMPI's MPI_Reduce:

        } else if ((ompi_comm_rank(comm) != root && MPI_IN_PLACE == sendbuf) ||
                   (ompi_comm_rank(comm) == root && ((MPI_IN_PLACE == recvbuf) || (sendbuf == recvbuf)))) {
            err = MPI_ERR_ARG;

The "sendbuf == recvbuf" check is what causes the MPI exception. I would say that we're not consistent about disallowing that (e.g., such checks are not in MPI_SCAN and the others you cited).

But FWIW, we do have logic in there because a popular benchmark (IMB) gets it wrong and calls MPI_REDUCE with a zero count (or at least, it used to -- I don't know if it has been checked). This is a case where we were backed into a corner because users kept complaining that OMPI was broken because it would fail to run IMB (although the opposite was actually true). So even though we didn't want to add the exception, we pretty much had to. :-\

Hence, we're not failing your example because of a 0 count -- we're failing your example because you didn't use MPI_IN_PLACE. The following works (because of the IMB exception), for example:

  ierr = MPI_Reduce(
                    (void*) 1, (void*) 2,
                    0,
                    MPI_INT,
                    MPI_SUM,
                    0,
                    MPI_COMM_WORLD);

On Feb 9, 2010, at 5:01 PM, Lisandro Dalcín wrote:

> BUMP. See http://code.google.com/p/mpi4py/issues/detail?id=14
>
>
> On 12 December 2009 00:31, Lisandro Dalcin <dalcinl_at_[hidden]> wrote:
> > On Thu, Dec 10, 2009 at 4:26 PM, George Bosilca <bosilca_at_[hidden]> wrote:
> >> Lisandro,
> >>
> >> This code is not correct from the MPI standard perspective. The reason is independent of the datatype or count, it is solely related to the fact that the MPI_Reduce cannot accept a sendbuf equal to the recvbuf (or one has to use MPI_IN_PLACE).
> >>
> >
> > George, I have to disagree. Zero-length buffers are a very special
> > case, and the MPI std is not very explicit about this limit case. Try
> > the code pasted at the end.
> >
> > 1) In Open MPI, the only one of these failing for sbuf=rbuf=NULL is MPI_Reduce()
> >
> > 2) As reference, all the calls succeed in MPICH2.
> >
> >
> >
> > #include <mpi.h>
> > #include <stdlib.h>
> >
> > int main( int argc, char ** argv ) {
> > int ierr;
> > MPI_Init(&argc, &argv);
> > ierr = MPI_Scan(
> > NULL, NULL,
> > 0,
> > MPI_INT,
> > MPI_SUM,
> > MPI_COMM_WORLD);
> > ierr = MPI_Exscan(
> > NULL, NULL,
> > 0,
> > MPI_INT,
> > MPI_SUM,
> > MPI_COMM_WORLD);
> > ierr = MPI_Allreduce(
> > NULL, NULL,
> > 0,
> > MPI_INT,
> > MPI_SUM,
> > MPI_COMM_WORLD);
> > #if 1
> > ierr = MPI_Reduce(
> > NULL, NULL,
> > 0,
> > MPI_INT,
> > MPI_SUM,
> > 0,
> > MPI_COMM_WORLD);
> > #endif
> > MPI_Finalize();
> > return 0;
> > }
> >
> >
> >
> > --
> > Lisandro Dalcín
> > ---------------
> > Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC)
> > Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
> > Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)
> > PTLC - Güemes 3450, (3000) Santa Fe, Argentina
> > Tel/Fax: +54-(0)342-451.1594
> >
>
>
>
> --
> Lisandro Dalcín
> ---------------
> Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC)
> Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
> Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)
> PTLC - Güemes 3450, (3000) Santa Fe, Argentina
> Tel/Fax: +54-(0)342-451.1594
>
> _______________________________________________
> devel mailing list
> devel_at_[hidden]
> http://www.open-mpi.org/mailman/listinfo.cgi/devel
>

-- 
Jeff Squyres
jsquyres_at_[hidden]
For corporate legal information go to:
http://www.cisco.com/web/about/doing_business/legal/cri/