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Subject: Re: [OMPI users] MPI_Comm_create with unequal group arguments
From: Jens Jørgen Mortensen (jensj_at_[hidden])
Date: 2012-01-20 09:50:22


On 20-01-2012 15:26, Josh Hursey wrote:
> That behavior is permitted by the MPI 2.2 standard. It seems that our
> documentation is incorrect in this regard. I'll file a bug to fix it.
>
> Just to clarify, in the MPI 2.2 standard in Section 6.4.2
> (Communicator Constructors) under MPI_Comm_create it states:
> "Each process must call with a group argument that is a subgroup of
> the group associated with comm; this could be MPI_GROUP_EMPTY. The
> processes may specify different values for the group argument. If a
> process calls with a non-empty group then all processes in that group
> must call the function with the same group as argument, that is the
> same processes in the same order. Otherwise the call is erroneous."
>
> Thanks for reporting the man page bug.

Thanks for the quick reply.

Is it also allowed to call MPI_Comm_create only on those processes that
are in the sub-group? This seems to work also. Or must one always call
MPI_Comm_create on all processes in comm - as the description says.

Jens Jørgen

>
> -- Josh
>
> 2012/1/20 Jens Jørgen Mortensen <jensj_at_[hidden]
> <mailto:jensj_at_[hidden]>>
>
> Hi!
>
> For a long time, I have been calling MPI_Comm_create(comm, group,
> newcomm) with different values for group on the different
> processes of comm. In pseudo-code, I would create two
> sub-communicators from a world with 4 ranks like this:
>
> if world.rank < 2:
> comm = world.create([0, 1])
> else:
> comm = world.create([2, 3])
>
> Now I read from the MPI_Comm_create description that this way of
> calling MPI_Comm_create is erroneous:
>
> "The call is erroneous if not all group arguments have the same
> value"
>
> http://www.open-mpi.org/doc/v1.4/man3/MPI_Comm_create.3.php#toc7
>
> So, I guess I have just been lucky that it has worked for me? Or
> is it OK to do what I do?
>
> Jens Jørgen
>
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>
>
>
> --
> Joshua Hursey
> Postdoctoral Research Associate
> Oak Ridge National Laboratory
> http://users.nccs.gov/~jjhursey <http://users.nccs.gov/%7Ejjhursey>