[Mpi3-rma] mpi3-rma post from bradc at cray.com requires approval

[Jeff Hammond]

If unordered is an optional assert, no one will optimize for it since
any implementor gets unordered for free with ordered.

If ordered is the default, let's forget about unordered altogether.
It will save those who wanted unordered the frustration of finding
this feature to be worthless down the road.

MPI RMA implementations have a history of failing to deliver
performance, so let us not add unordered to the list of features that
fail to live up to their potential.

Jeff

On Sat, Jun 5, 2010 at 9:42 AM, Pavan Balaji <balaji at mcs.anl.gov> wrote:
>
> On 05/30/2010 11:27 AM, Underwood, Keith D wrote:
>>
>> You are right, I should have chosen my words more carefully.  I believe we
>> tossed ordering on MPI_Put when we tossed atomicity.
>> I don't think we ever got to the discussion of what ordering would be for
>> the atomics.  And, this is where life gets weird.  I'm not positive that you
>> need ordering for atomics, except that they are the MPI_Puts you would
>> actually use when you needed ordering.  So...  I will go back to... all UPC
>> needs is ordering from one source to one target address.  I see three
>> options:
>>
>> 1) Just define accumulates/get_accumulates to have the ordering that UPC
>> needs
>> 2) Default to the UPC ordering and allow an assert to relax that
>> restriction.  This would be consistent with how the current one-sided
>> operations handle locks
>> 3) Add some other call to handle ordering, but we wouldn't want some call
>> that you had to call with EVERY put or anything...  After all, all we need
>> is relatively minimal ordering.
>
> There's an option (4) similar to (2):
>
> 4) Default to unordered, and allow an assert to add ordering.
>
> I believe we decided to use either (2) or (4) in the last meeting, but
> didn't finalize on which one.
>
> To me, (2) seems more logical since MPI's semantics have typically been: use
> a conservative approach by default (ordered in this case), and allow the
> application to assert that it can use a more relaxed mode as a performance
> optimization (unordered).
>
>  -- Pavan
>
> --
> Pavan Balaji
> http://www.mcs.anl.gov/~balaji
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>



-- 
Jeff Hammond
Argonne Leadership Computing Facility
jhammond at mcs.anl.gov / (630) 252-5381
http://www.linkedin.com/in/jeffhammond