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

[Jed Brown]

On Mon, Apr 23, 2012 at 20:36, 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.
>

Right, but why is it this way? What was the rationale for not having the
same extended capability of predefined operations (a la MPI_Accumulate)
apply to reductions? It's obnoxious when exposing a common API that may be
implemented in terms of one-sided or collectives.
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