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

Pavan Balaji balaji at mcs.anl.gov
Thu Aug 15 11:26:05 CDT 2013

Before I go out and start writing up the text for this, are there other 
comments on this?

Keith/Torsten/Jeff/Bill/Rajeev: thoughts?

On 08/06/2013 10:09 AM, Barrett, Brian W wrote:
> On 8/6/13 7:30 AM, "Jim Dinan" <james.dinan at gmail.com> wrote:
>> My preference would be to have the fewer memory models in the standard.
>> I prefer option #1 -- that a window synchronization (e.g. Win_sync) can
>> be used to order load/store operations with respect to actions performed
>> by other processes in the target's
>> window.  If no ordering is enforced, the MPI standard does not define
>> what is seen by load operations at the target process.  As a rationale,
>> the local process' view of the window may not be consistent with the
>> window because of performance optimizations or
>> the consistency model of the underlying architecture.  This would allow
>> e.g. SHMEM implementations to still use MPI-3 RMA, but they would have to
>> rely on a behavior that is defined by the architecture/implementation, as
>> they currently do.
>> I think there's also a good chance that this could be an erratum, whereas
>> a new memory model would have to go into a new version of the spec.  If
>> we were to decide later that we want a stronger memory model that defines
>> the ordering
>> seen by the target in the absence of synchronizations, this option would
>> still allow us to add it later.
> I tend to agree.  I also think the change can be quite small.  Section
> 11.4 (pg 436, line 37-38) already says that updates will eventually be
> seen by a load in UNIFIED, meaning that the FLUSH/SYNC example that
> started this thread already requires a SYNC in order to be correct (or a
> while (!updated) loop).  So the primary thing we need to do is to clarify
> that the "identical" refers to their view from memory, and not their view
> from a reordering processor, so Sync (or other system-specific operations)
> are required to force ordering.  This really feels like an erratum to me.
> Brian
> --
>    Brian W. Barrett
>    Scalable System Software Group
>    Sandia National Laboratories
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Pavan Balaji

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