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

William Gropp wgropp at illinois.edu
Thu Aug 15 12:30:31 CDT 2013

I agree: I'd rather not have any more memory models, and we should make Unified do what we intended.  The change isn't going to affect any current users, so treating it as an errata makes sense.


William Gropp
Director, Parallel Computing Institute
Deputy Director for Research
Institute for Advanced Computing Applications and Technologies
Thomas M. Siebel Chair in Computer Science
University of Illinois Urbana-Champaign

On Aug 15, 2013, at 11:26 AM, Pavan Balaji wrote:

> 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
> http://www.mcs.anl.gov/~balaji
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