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

[Jeff Hammond]

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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