[Mpi-forum] MPI_Comm_split_type question

[Guillaume Mercier]

George,

Thanks, I didn't think about that but it seems to
solve my problem.

Thanks.

Guillaume

On 15/12/2016 20:41, George Bosilca wrote:
> 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
>> <mailto:jeff.science at gmail.com>> wrote:
>>
>>
>>
>> On Thu, Dec 15, 2016 at 10:29 AM, Guillaume
>> Mercier <guillaume.mercier at u-bordeaux.fr
>> <mailto: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
>>
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>>
>>
>>
>> --
>> Jeff Hammond
>> jeff.science at gmail.com <mailto:jeff.science at gmail.com>
>> http://jeffhammond.github.io/
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>
>
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