[Mpi3-rma] Updated Proposal 1

[Barrett, Brian W]

On Nov 26, 2010, at 3:47 PM, Torsten Hoefler wrote:

> Jeff,
>>> 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.
> Correct, the current RMA doesn't offer more than element-wise atomicity
> and we don't plan to extend this. You are right, the only benefit would
> lie in reduced overheads which isn't all that much. I'm not sure if
> full-message atomicity would be easy/fast to implement.


I think we could do multi-element with the very loose element-wise guarantee.  At one point, there was discussion about strengthening that guarantee, which would make the multi-element Get_accumulate painful.

Brian

-- 
  Brian W. Barrett
  Dept. 1423: Scalable System Software
  Sandia National Laboratories