[mpich-discuss] MPI_Accumulate with MAXLOC does not error for incompatible types

[Jeff Hammond]

MPI_Accumulate permits the use of pre-defined reduction operations
with user-defined datatypes, provided the user-defined datatypes are
homogeneous.  Your use of MPI_MAXLOC with MY_2INT for MPI_Accumulate
is legal in my reading of the standard.  It may not be legal for
MPI_Reduce because of the stupid asymmetry (that I believe you have
pointed out in the past) wherein MPI_Reduce (and MPI_Allreduce) cannot
do things that MPI_Accumulate can do w.r.t. built-in MPI_Op with
user-defined MPI_Datatype.

Perhaps MPI_Accumulate is correct in not generating but providing the
wrong answer for MPI_MAXLOC, which would be a bug of a different
nature.

Jeff

On Tue, Jan 1, 2013 at 1:09 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
> With MPI_Reduce, we get a nice helpful error message:
>
> Fatal error in PMPI_Reduce: Invalid MPI_Op, error stack:
> PMPI_Reduce(1217)...........: MPI_Reduce(sbuf=0x7fff3e39fa50,
> rbuf=0x7fff3e39fa40, count=1, dtype=USER<contig>, MPI_MAXLOC, root=0,
> MPI_COMM_WORLD) failed
> MPIR_MAXLOC_check_dtype(151): MPI_Op MPI_MAXLOC operation not defined for
> this datatype
>
>
> With MPI_Accumulate, we just get wrong results. See attached code using a
> user-defined contiguous results.
>
> I care about this because I'm stuck [1] dispatching into alternative
> mechanisms for type combinations that are not directly supported by MPI
> (e.g., struct {int u; long long k;} and __float128). I missed a spot and had
> a confusing bug that I think MPICH should be catching.
>
>
> [1] As usual, the crud would evaporate if only MPI_Accumulate were
> extensible over datatypes.
>
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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