[Mpi-forum] MPI_Comm_split_type question

[Martin Schulz]

Hi Guillaume,

Just to add to George¹s suggestion and following the same idea ­ instead of
the Allgather, which could cause scalability problems, you could just do a
Reduce or Allreduce with all processes with rank 0 in the new communicator
contributing 1 and the rest 0 and then use an add as the reduction operator.

Martin


________________________________________________________________________
Martin Schulz, schulzm at llnl.gov, http://scalability.llnl.gov/
CASC @ Lawrence Livermore National Laboratory, Livermore, USA


From:  <mpi-forum-bounces at lists.mpi-forum.org> on behalf of George Bosilca
<bosilca at icl.utk.edu>
Reply-To:  Main mailing list <mpi-forum at lists.mpi-forum.org>
Date:  Thursday, December 15, 2016 at 11:41 AM
To:  Main mailing list <mpi-forum at lists.mpi-forum.org>
Subject:  Re: [Mpi-forum] MPI_Comm_split_type question

Guillaume, 

If I understand you correctly you are trying to figure out how many unique
groups (and therefore communicators) were globally created as a result of an
MPI_Comm_split_type operation. Unfortunately, depending on the type passed
to this function (there is a single one defined by MPI, but in Open MPI we
support several extensions), the resulting communicator can be created
without the need for global knowledge, and thus it is impossible to know how
many have communicators have been create in total.

To extend on what JeffH proposed, you can count the resulting number of
communicators by doing an MPI_Allgather on the initial communicator on a
data where each participant provide their rank in the newly created
communicator (then returned by the MPI_Comm_split* operation), and then
counting the number of 0.

  George.



> On Dec 15, 2016, at 14:05 , Jeff Hammond <jeff.science at gmail.com> wrote:
> 
> 
> 
> On Thu, Dec 15, 2016 at 10:29 AM, Guillaume Mercier
> <guillaume.mercier at u-bordeaux.fr> wrote:
>> 
>> Hi Jeff,
>> 
>> On 15/12/2016 18:48, Jeff Hammond wrote:
>>> The number of output communicators from MPI_Comm_split(_type) is always
>>> one.
>> 
>> Yes, obviously.
>> 
>>> Different ranks may get different outputs, but one cannot transfer
>>> a communicator object from one rank to another.
>> 
>> That's my issue actually.
>> 
>>> If you want to do how many total output communicators there are, you can
>>> perform an MPI_Allgather on the color arguments and see how many unique
>>> values there are.
>> 
>> I'm not sure about that: tae the case of MPI_Comm_split_type with
>> MPI_COMM_TYPE_SHARED. According to tha standards :"this type splits
>> the communicator into subcommunicators each of which can create  a shared
>> memory region".
>> So, there is only one color, but several subcommunicators.
>> Or am I understanding this the wrong way?
>> 
> 
> Each rank gets its own subcommunicator object that captures the ranks of the
> shared memory domain, as supported by MPI_Win_allocate_shared.
> 
> I'm still not sure I understand what the issue is.  What are you trying to do?
> 
> Jeff
>  
>> Regards
>> Guillaume
>> 
>> _______________________________________________
>> mpi-forum mailing list
>> mpi-forum at lists.mpi-forum.org
>> https://lists.mpi-forum.org/mailman/listinfo/mpi-forum
> 
> 
> 
> -- 
> Jeff Hammond
> jeff.science at gmail.com
> http://jeffhammond.github.io/
> _______________________________________________
> mpi-forum mailing list
> mpi-forum at lists.mpi-forum.org
> https://lists.mpi-forum.org/mailman/listinfo/mpi-forum



-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mpi-forum.org/pipermail/mpi-forum/attachments/20161215/cdd651d1/attachment.html>