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