[Mpi3-rma] request-based ops

Jeff Hammond jhammond at alcf.anl.gov
Wed Jun 12 17:07:16 CDT 2013

Sure, let's separate these issues.  I think Pirate RMA (RRPUT,
RRACCUMULATE and RRRGET_ACCUMULATE) is a more _local_ solution in the
MPI standard sense :-)

SEND is also a nonblocking operation because the implementation can
always buffer the data and return before the remote RECV is posted.
Only SSEND is blocking if one asserts that blocking is equivalent to
"requires remote agency to complete".

So basically, I completely agree with the Forum back-pushers that
IWIN_FLUSH etc. are as useful as ISEND.


On Wed, Jun 12, 2013 at 4:58 PM, Pavan Balaji <balaji at mcs.anl.gov> wrote:
> The push-back at the Forum was that WIN_FLUSH and WIN_UNLOCK are local
> operations (in MPI's definition of local), so they are already nonblocking.
> I was not at the Forum.  I pointed out to Wesley after he returned that they
> are only local in the simple case where no one else is holding the lock.
> Otherwise they are not.
> Further, the proposal is not only about WIN_FLUSH and WIN_UNLOCK.  It was
> for a long list of blocking functions: COMM_ACCEPT, COMM_CONNECT, WIN_FENCE,
> WIN_FLUSH_ALL, WIN_FLUSH_LOCAL_ALL, ...  The point was that MPI should deal
> with concepts, not specific function exceptions. Nonblocking communication
> is one such concept, and it should be valid for all operations (that are not
> local).
> FWIW, my proposal was only to add nonblocking variants to blocking calls.
> Adding extra per-op remote completion requests is a separate issue and
> should not be mixed with this.  I personally think it's useful (in fact, my
> original proposal for RPUT/RGET equivalents had it), but that's still a
> separate issue.
>  -- Pavan
> On 06/12/2013 09:41 AM, Jeff Hammond wrote:
>> So Wes was telling me about IWIN_FLUSH yesterday (this function is a
>> great idea, btw) and the alternative of adding double-request RPUT and
>> RACCUMULATE that provide a request for both local and remote
>> completion.  By induction, this would imply a triple-request
>> RGET_ACCUMULATE operation, which I also think is a good idea.  It's
>> really silly to have to wait for the result buffer to be ready to
>> reuse the origin buffer in RGET_ACCUMULATE.  It seems that many
>> implementations will be able to send the entire origin buffer a
>> nontrivial amount of time before the result comes back, particularly
>> if the remote side doesn't have hardware-based progress and the data
>> has to wait in a network buffer.
>> I'm sorry that I missed the last day of the Forum in San Jose.  I
>> didn't know these operations were on the table.  They sound like a
>> great idea to me.
>> Jeff
> --
> Pavan Balaji
> http://www.mcs.anl.gov/~balaji

Jeff Hammond
Argonne Leadership Computing Facility
University of Chicago Computation Institute
jhammond at alcf.anl.gov / (630) 252-5381
ALCF docs: http://www.alcf.anl.gov/user-guides

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