[Mpi3-rma] Strict vs. relaxed RMA in MPI

[Pavan Balaji]

And I just looked at the time-stamp of your email, and it says May 28th. 
Maybe we are chasing an old email chain that got buffered and resent :-).

  -- Pavan

On 08/02/2010 10:09 AM, Pavan Balaji wrote:
> Brad,
>
> The working group is planning to provide both relaxed (as in MPI-2.2),
> as well as some form of strictness (as in ordering from the same source
> to the same memory location on the same target) for MPI-3. For other
> forms of strictness, the user will need to take care of that by waiting
> for the operations to finish before calling other operations.
>
> Based on our discussion at the SC PC meeting, I thought this would be
> sufficient for Chapel. And based on my discussion with Jeff, I thought
> this would be sufficient for GA as well.
>
> Were you looking for something additional?
>
>    -- Pavan
>
> On 05/28/2010 06:44 PM, Brad Chamberlain wrote:
>>
>> Hi MPI-3 RMA team --
>>
>> I ran into Jeff Hammond at a workshop a few weeks back and we had a brief
>> chat about whether, as a potential client of MPI-3 RMA, I would prefer its
>> semantics to err more on the strict or relaxed side.  He requested that I
>> consider sending a brief note to this group with my thoughts, so this is
>> that note.  I hope that this opinion will be considered useful and not
>> out-of-turn given how little time I've had to invest in following the work
>> of the MPI-3 team.
>>
>> I should start with the disclaimer that I'm not an expert on memory
>> consistency models -- I probably know more than the average programmer,
>> but have typically been insulated from worrying about it in a great amount
>> of detail, either by relying on other software layers or languages to take
>> care of it for me or by having the fortune to work with codes and idioms
>> that don't fall afoul of the differences.
>>
>> My gut response to the question is that I'd prefer things to be on the
>> more relaxed side.  I think one of the key benefits of single-sided
>> communication is its separation of data transfer from synchronization.
>> I'd worry that by trying to enforce too much strictness in the RMA
>> interface, it would work break down this separation and result in
>> performance overheads that couldn't be recouped.
>>
>> On the other hand, if MPI-3 exported a model that was more relaxed than a
>> particular programmer/programming model wanted, my assumption is that they
>> could increase the strictness by doing more manual synchronization/memory
>> fences/etc. themselves.  That is, a relaxed model would not seem to
>> exclude strictness while a strict model may impact performance negatively
>> without any recourse.  If that's a correct interpretation, the relaxed
>> approach seems like the one to take to me.
>>
>> I'm reluctant to speak for others, but wanted to note (if he hasn't
>> already done so) that Dave Grove from IBM's X10 team was with us and
>> seemed to agree with this point-of-view (though perhaps we were both
>> simply falling prey to Jeff's subliminal hypnosis? :).  All that said,
>> owing to my lack of depth in this area, I would say that if the GASNet
>> team and/or the UPC/Titanium teams who built on top of GASNet felt that
>> this was clearly the wrong approach, I would tend to cast my vote with
>> them since I think they've studied this issue in far more detail than most
>> parallel language groups, ours included.  (I do think that Kathy Yelick
>> voiced a compatible opinion in another context at this same workshop,
>> which gave me some reassurance that relaxed was the way to go, but again,
>> these were fairly high-level conversations.  More generally, I would
>> encourage you to get input from the GASNet team as you consider this issue
>> and others related to 1-sided communication if you haven't).
>>
>> If you think it would be useful for me to hear the other side of the
>> debate and/or consider some specific case examples in more detail, I'd be
>> happy to do so as time permits.
>>
>> Have a good weekend,
>> -Brad
>>
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>

-- 
Pavan Balaji
http://www.mcs.anl.gov/~balaji