Tim,

Since MPI_Comm_create sets the created communicator to MPI_COMM_NULL for the processes that are not in the group , may be preceding your collectives by a:
if (MPI_COMM_NULL != new_comm) {
   <your collective>
}
could be enough.

But may be I'm wrong: I'll let the specialists answer.

Regards,
Nadia
 
--
Nadia Derbey
 

users-bounces@open-mpi.org wrote on 03/06/2012 02:32:03 PM:

> De : Timothy Stitt <Timothy.Stitt.9@nd.edu>
> A : Open MPI Users <users@open-mpi.org>
> Date : 03/06/2012 02:32 PM
> Objet : Re: [OMPI users] Scatter+Group Communicator Issue
> Envoyé par : users-bounces@open-mpi.org
>
> Hi Nadia,
>
> Thanks for the reply. This is were my confusion with the scatter
> command comes in. I was really hoping that MPI_Scatter would
> automagically ignore the ranks that are not part of the group
> communicator, since this test code is part of something more
> complicated were many sub-communicators are created over various
> combinations of ranks, and used in various collective routines. Do I
> really have to filter out manually the non-communicator ranks before
> I call the scatter...it would be really nice if the scatter command
> was 'smart' enough to do this for me by looking at the communicator
> that is passed to the routine.
>
> Thanks again,
>
> Tim.
>
> On Mar 6, 2012, at 8:28 AM, <nadia.derbey@bull.net> wrote:
>
> Isn't it because you're calling MPI_Scatter() even on rank 2 which
> is not part of your new_comm?
>
> Regards,
> Nadia
>
> users-bounces@open-mpi.org wrote on 03/06/2012 01:52:06 PM:
>
> > De : Timothy Stitt <Timothy.Stitt.9@nd.edu>
> > A : "users@open-mpi.org" <users@open-mpi.org>
> > Date : 03/06/2012 01:52 PM
> > Objet : [OMPI users] Scatter+Group Communicator Issue
> > Envoyé par : users-bounces@open-mpi.org
> >
> > Hi all,
> >
> > I am scratching my head over what I think should be a relatively
> > simple group communicator operation. I am hoping some kind person
> > can put me out of my misery and figure out what I'm doing wrong.
> >
> > Basically, I am trying to scatter a set of values to a subset of
> > process ranks (hence the need for a group communicator). When I run
> > the sample code over 4 processes (and scattering to 3 processes), I
> > am getting a group-communicator related error in the scatter operation:
> >
> > > [stats.crc.nd.edu:29285] *** An error occurred in MPI_Scatter
> > > [stats.crc.nd.edu:29285] *** on communicator MPI_COMM_WORLD
> > > [stats.crc.nd.edu:29285] *** MPI_ERR_COMM: invalid communicator
> > > [stats.crc.nd.edu:29285] *** MPI_ERRORS_ARE_FATAL (your MPI job
> > will now abort)
> > >  Complete - Rank           1
> > >  Complete - Rank           0
> > >  Complete - Rank           3
> >
> > The actual test code is below:
> >
> > program scatter_bug
> >
> >   use mpi
> >
> >   implicit none
> >
> >   integer :: ierr,my_rank,procValues(3),procRanks(3)
> >   integer :: in_cnt,orig_group,new_group,new_comm,out
> >
> >   call MPI_INIT(ierr)
> >   call MPI_COMM_RANK(MPI_COMM_WORLD,my_rank,ierr)
> >
> >   procRanks=(/0,1,3/)
> >   procValues=(/0,434,268/)
> >   in_cnt=3
> >      
> >   ! Create sub-communicator
> >   call MPI_COMM_GROUP(MPI_COMM_WORLD, orig_group, ierr)
> >   call MPI_Group_incl(orig_group, in_cnt, procRanks, new_group, ierr)
> >   call MPI_COMM_CREATE(MPI_COMM_WORLD, new_group, new_comm, ierr)
> >      
> >   call MPI_SCATTER(procValues, 1, MPI_INTEGER, out, 1, MPI_INTEGER,
> > 0, new_comm, ierr);
> >
> >   print *,"Complete - Rank", my_rank
> >
> > end program scatter_bug
> >  
> > Thanks in advance for any advice you can give.
> >
> > Regards.
> >
> > Tim.
> > _______________________________________________
> > users mailing list
> > users@open-mpi.org
> > http://www.open-mpi.org/mailman/listinfo.cgi/users
> <ATT00001..txt>
>
> Tim Stitt PhD (User Support Manager).
> Center for Research Computing | University of Notre Dame |
> P.O. Box 539, Notre Dame, IN 46556 | Phone:  574-631-5287 | Email:
> tstitt@nd.edu
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