[Mpi3-rma] Updated Proposal 1

Jeff Hammond jhammond at mcs.anl.gov
Fri Nov 26 16:37:42 CST 2010


> 2) Keith and Brian, could you please elaborate on the arguments against
> allowing multiple elements (count>1) in MPI_Get_accumulate? I remember
> there was some discussion about buffering and failures if one wanted to
> support it in hardware but I don't remember what the issues were. It
> seems like one could simply pipeline the hardware operations or just
> fall back to a software implementation if count is bigger than a certain
> threshold.

With pipelining, I assume atomicity is only per element?  I do not see
how you could realize anything else in hardware.  I don't see any
value in multi-element RMW if it is element-wise atomic.  One could
just as easily send multiple messages.

What is the use case multi-element RMW anyways?  I'm definitely going
to need message-atomic put, but I'll do the portable i.e. MPI version
with RMW-lock + put + RMW-unlock.


Jeff Hammond
Argonne Leadership Computing Facility
jhammond at alcf.anl.gov / (630) 252-5381

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