[Mpi-comments] Use of MPI_DATATYPE_NULL

[George Bosilca]

Thanks Gilles for pointing this discussion out. It appears that the use of
the MPI_DATATYPE_NULL in the example 5.23 is correct, the standard clearly
mentions the receiver datatype is only significant at the root.

  George.


On Thu, Dec 8, 2022 at 7:03 PM Jim Edwards via mpi-comments <
mpi-comments at lists.mpi-forum.org> wrote:

> Right, so how do I formally request that the standard be updated to
> clarify this issue?
>
> On Thu, Dec 8, 2022 at 4:56 PM gilles--- via mpi-comments <
> mpi-comments at lists.mpi-forum.org> wrote:
>
>> I have a different recollection.
>>
>>
>>
>> The link is broken, here is one that works for me
>>
>> https://lists.mpi-forum.org/pipermail/mpi-forum/2016-January/003152.html
>>
>> TL;DR from Bill Gropp:
>>
>> "The standard says that the null handles are invalid for input unless explicitly permitted.  There’s no exemption for a datatype argument where the associated count is zero."
>>
>>
>>
>> IIRC there was no objection to updating the standard in order to allow
>> this, but unless this has been completed, I am afraid MPI_DATATYPE_NULL is
>> not a valid datatype here
>>
>>
>>
>> Gilles
>>
>> ----- Original Message -----
>>
>> You are correct. The MPI_DATATYPE_NULL is an acceptable datatype for
>> cases where the target processor sends or receives no data. The standard
>> uses MPI_DATATYPE_NULL in 2 instances (in example 5.23) to highlight no
>> data movement, but fails to make a clear reference to the fact that this is
>> an acceptable use.
>>
>>   George.
>>
>> On Thu, Dec 8, 2022 at 5:31 PM Jim Edwards via mpi-comments <
>> mpi-comments at lists.mpi-forum.org> wrote:
>>
>>> There seems to be some ambiguity in the standard with respect to usage
>>> of the MPI_DATATYPE_NULL variable.
>>> It would seem to me that if an MPI routine such as MPI_Send or
>>> MPI_ALLTOALLW has a data count argument of 0
>>> then MPI_DATATYPE_NULL should be an acceptable value for the datatype.
>>> Here is a discussion from several years
>>> ago with openmpi developers.  It references a discussion in the
>>> standards forum but I can no longer find it.
>>>
>>> http://lists.mpi-forum.org/mpi-forum/2016/01/3152.php
>>>
>>> Would it be possibly to clarify this case in the next iteration of the
>>> standard?
>>>
>>> --
>>> Jim Edwards
>>>
>>> CESM Software Engineer
>>> National Center for Atmospheric Research
>>> Boulder, CO
>>> _______________________________________________
>>> mpi-comments mailing list
>>> mpi-comments at lists.mpi-forum.org
>>> https://lists.mpi-forum.org/mailman/listinfo/mpi-comments
>>
>>
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>
>
> --
> Jim Edwards
>
> CESM Software Engineer
> National Center for Atmospheric Research
> Boulder, CO
> _______________________________________________
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> mpi-comments at lists.mpi-forum.org
> https://lists.mpi-forum.org/mailman/listinfo/mpi-comments
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