[mpiwg-rma] same_op_no_op and SHMEM

Pavan Balaji balaji at mcs.anl.gov
Thu Oct 24 12:32:46 CDT 2013

The motivation was that it’s hard to maintain atomicity when different operations are done.  For example, if the hardware only supports some atomic operations, but not all, this might result in some operations happening in hardware and some in software making atomicity hard.  In such cases, the MPI implementation might need to fall back to software-only implementations.

  -— Pavan

On Oct 24, 2013, at 12:23 PM, Jeff Hammond <jeff.science at gmail.com> wrote:

> I recall that Brian and/or Keith wanted same_op_no_op because of
> SHMEM.  However, SHMEM requires the use of MPI_NO_OP (for atomic Get
> via Get_accumulate), MPI_REPLACE (for atomic Put via Accumulate) and
> MPI_SUM (for add, fadd, inc and finc).  So what is the benefit of
> same_op_no_op to SHMEM?  Perhaps I remember completely wrong and the
> motivation was something that does not use the latter atomics.  Or
> perhaps it is common for SHMEM codes to not use these and thus the
> assumption is MPI_SUM can be ignored.
> Jeff
> -- 
> Jeff Hammond
> jeff.science at gmail.com
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Pavan Balaji

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