[Mpi3-rma] Updated Proposal 1

Jeff Hammond jhammond at mcs.anl.gov
Fri Nov 26 16:52:26 CST 2010

Polling on CAS is lame, but unless MPI-3 is going to add a blocking
lock acquisition call, then that's the only general solution.


On Fri, Nov 26, 2010 at 4:47 PM, Torsten Hoefler <htor at illinois.edu> 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.
>> 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.
> That was the use-case. Manual lock/unlock (+flushes etc.) as you
> proposed is a valid workaround but seems to impose additional roundtrips
> (you'd have to poll on CAS).
> All the Best,
>  Torsten
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Jeff Hammond
Argonne Leadership Computing Facility
jhammond at alcf.anl.gov / (630) 252-5381

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