[mpich-discuss] Why do predefined MPI_Ops function elementwise in MPI_Accumulate, but not in MPI-1 routines?

[Jeff Hammond]

Jed-

If you implement your user-defined reduction using MPI_Accumulate
locally, does that not solve the problem?  In this case, whatever you
can do for MPI_Accumulate is valid for MPI_Reduce, right?

my_reduction(..)
{
MPI_Win_create(MPI_COMM_SELF,..);
MPI_Accumulate(rank=me,..);
MPI_Win_free(MPI_COMM_SELF,..);
}

Jeff

On Mon, Apr 23, 2012 at 8:40 PM, Jeff Hammond <jhammond at alcf.anl.gov> wrote:
> Well that's just silly.  It would seem that everyone who attends the
> MPI Forum regularly is a nincompoop :-)
>
> The standard should be amended since clearly one can implement
> MPI_Reduce(MPI_SUM, DerivedDatatype) using
> MPI_Win_fence+MPI_Accumulate(MPI_SUM, DerivedDatatype)+MPI_Win_fence.
>
> Jeff
>
> On Mon, Apr 23, 2012 at 8:36 PM, Rajeev Thakur <thakur at mcs.anl.gov> wrote:
>> MPI_Accumulate allows you to do MPI_SUM on a derived datatype. MPI_Reduce does not. It requires you to create your own reduction operation for derived datatypes. MPI_Accumulate, on the other hand, does not allow user-defined reduction operations at all.
>>
>> Rajeev
>>
>>
>>
>> On Apr 23, 2012, at 8:29 PM, Jeff Hammond wrote:
>>
>>> I was under the impression that the use for MPI_Accumulate was a
>>> proper subset of MPIReduce, etc.  What part of the standard leads you
>>> to believe that restrictions pertaining to MPI_Reduce, etc. are
>>> stricter than for MPI_Accumulate?
>>>
>>> Jeff
>>>
>>> On Mon, Apr 23, 2012 at 7:53 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>>>> The standard contains the following statement about MPI_Accumulate, which
>>>> can only use predefined operations.
>>>>
>>>> Each datatype argument must be a predefined datatype or a derived datatype,
>>>> where all basic components are of the same predefined datatype. Both
>>>> datatype arguments must be constructed from the same predefined datatype.
>>>>
>>>> Under this restriction, MPI_SUM can be applied to derived datatypes, but it
>>>> cannot be used in the same context with MPI_Allreduce and similar. This
>>>> causes an ugly interface in which we
>>>>
>>>> 1. MUST create a new MPI_Op to be used when calling MPI_Allreduce with this
>>>> data type.
>>>>
>>>> 2. Must NOT create a new MPI_Op when used with MPI_Accumulate.
>>>>
>>>> This causes awkward translation if we have an API that may be implemented in
>>>> terms of one-sided operations or may be implemented in terms of collective
>>>> reductions (e.g. if we do communication graph analysis and choose an
>>>> implementation based on sparsity or the network).
>>>>
>>>> Is there any way to circumvent this annoyance?
>>>>
>>>> What was the rationale for not having MPI_SUM work with MPI_Allreduce and
>>>> company when the same conditions as specified for MPI_Accumulate are met?
>>>>
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>>>
>>>
>>>
>>> --
>>> Jeff Hammond
>>> Argonne Leadership Computing Facility
>>> University of Chicago Computation Institute
>>> jhammond at alcf.anl.gov / (630) 252-5381
>>> http://www.linkedin.com/in/jeffhammond
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>>
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>
>
>
> --
> Jeff Hammond
> Argonne Leadership Computing Facility
> University of Chicago Computation Institute
> jhammond at alcf.anl.gov / (630) 252-5381
> http://www.linkedin.com/in/jeffhammond
> https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond (in-progress)
> https://wiki.alcf.anl.gov/old/index.php/User:Jhammond (deprecated)
> https://wiki-old.alcf.anl.gov/index.php/User:Jhammond(deprecated)



-- 
Jeff Hammond
Argonne Leadership Computing Facility
University of Chicago Computation Institute
jhammond at alcf.anl.gov / (630) 252-5381
http://www.linkedin.com/in/jeffhammond
https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond (in-progress)
https://wiki.alcf.anl.gov/old/index.php/User:Jhammond (deprecated)
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