[mpiwg-rma] FW: [Mpi3-rma] [EXTERNAL] Re: MPI-3 UNIFIED model updates

[Barrett, Brian W]

On 8/27/13 12:19 PM, "Pavan Balaji" <balaji at mcs.anl.gov> wrote:

>On 08/27/2013 12:57 PM, Jeff Hammond wrote:
>> The MPI standard will not define by what mechanism the SHMEM
>> implementation will establish the necessary consistency.  This is what I
>> mean by undefined = implementation-defined.  I do not think that Keith
>> is proposing to enumerate all processor memory models and the
>> appropriate fences required to achieve the affect of win_sync but at
>> lower cost.  Thus, it is the implementation - or perhaps more
>> accurately, the platform on which the implementation resides - that will
>> define these semantics.
>
>Here's what I understood from Keith --
>
>He wants a statement in the standard that says the application can do
>platform-specific memory consistency to synchronize the public and
>private windows in UNIFIED.
>
>I do agree that that's a very flimsy statement and doesn't really mean
>much.  But I'm calling it a definition, while I think you are saying
>that it's pretty much undefined.
>
>I like leaving it totally undefined better than the above "definition".

I'm having trouble with the exact phrasing, but my thought is that we
should have an advice to users that says 1) while the view of memory is
consistent in UNIFIED, other artifacts (such as processor or memory
controller reordering) can result in unexpected behavior 2) MPI_WIN_SYNC
may be used to provide stronger ordering guarantees and 3) an application
programmer may use other, implementation defined, semantics to provide
required ordering guarantees.

That is, we should say explicitly that the user can cause oddness, that
MPI has a mechanism to fix that oddness, and that the user may also seek
other mechanisms to solve the oddness, should she so desire.

Brian

--
  Brian W. Barrett
  Scalable System Software Group
  Sandia National Laboratories



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