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

[Pavan Balaji]

On 08/27/2013 08:56 AM, Underwood, Keith D wrote:
>> On 08/27/2013 07:15 AM, Underwood, Keith D wrote:
>>>>> 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.
>>>
>>> My statement:  this seems to be what SHMEM does - except they don't
>>> provide the equivalent of Win_sync.  If we are going to hack on the
>>> text, I would prefer that we be *very* careful in our wording.
>>> Specifically, we need to make it very clear that the SHMEM approach is
>>> *legal*, but the burden is on the user to do potentially
>>> implementation specific things to make it work right.
>>
>> Hmm.  OK.  Just so there's no confusion, by "SHMEM approach", you are
>> saying that the application can use its own platform-specific memory
>> consistency calls and avoid using WIN_SYNC, correct?
>
> Yes.  WIN_SYNC will do that for them, but they can do everything
> without calling WIN_SYNC as long as they call the right
> platform-specific things and the platform-specific approach might be
> faster/lower overhead.

OK.  I'm not disagreeing with this option, but I just want to point out 
that this will essentially discard MPI implementations that would 
implement UNIFIED in software (e.g., by trapping store operations). 
That's probably not a big deal, but from a pure CS perspective, it's not 
symmetric.

Thoughts from others?

  -- Pavan

-- 
Pavan Balaji
http://www.mcs.anl.gov/~balaji