[Mpi3-rma] MPI_Fetch_and_op and MPI_Get_accumulate

[Jeff Hammond]

> Another way to look at this is that the user should expect a stronger
> performance "guarantee" from fetch_and_op.  If the combination of FOP
> and GACC results in decreased performance for a particular network, the
> MPI implementation could provide an info key or assertion to indicate
> that FOP and GACC will not be combined.  If such a hint is broadly
> useful, we could add it to the standard.

I assert that it is and we should try to do this, although there are
perhaps too many nuances here to capture in MPI proper.  For example,
some hardware can do atomic "i++" better than "i+=j".  I doubt that an
info key distinguishing these caess would ever make it into the
standard.

Jeff

-- 
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