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

[Rajeev Thakur]

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
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