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
