[Mpi3-rma] MPI-3.1 consideration slides

[Jim Dinan]

On 12/3/12 9:44 AM, Jeff Hammond wrote:
> On Mon, Dec 3, 2012 at 7:39 AM, Jim Dinan <dinan at mcs.anl.gov> wrote:
>> On 12/2/12 7:44 AM, Jeff Hammond wrote:
>>>>>
>>>>> (1) Define semantics of shared memory windows in separate model.
>>>>>
>>>>> In Section 11.2.3 it says "MPI does not define semantics for accessing
>>>>> shared memory windows in the separate memory model."
>>>>>
>>>>> I would like to try to define these semantics in MPI 3.1.  Shouldn't
>>>>> it at least be possible to use MPI_WIN_(UN)LOCK with
>>>>> MPI_LOCK_EXCLUSIVE here?
>>>>>
>>>>> I believe that supporting RMA for weakly- or non- coherent memory
>>>>> architectures is vital for future systems.
>>>>
>>>> I agree. We had this discussion and decided to postpone it because we
>>>> had other more pressing issues. Now may be the time. A discussion would
>>>> be good.
>>>
>>>
>>> Yes, there was no urgency.  I'm glad the standard left this issue wide
>>> open instead of defining something restrictive.
>>
>>
>> IMHO, defining shared memory windows in the separate model is not a worthy
>> use of the Forum's time.  This will be hard to get right, and AFAIK no
>> (reasonable) system requires it.  As we've currently left it, if this
>> functionality is needed, one can define the semantics as an MPI extension.
>> I think this is a better option that defining a new model in the absence of
>> a platform that uses it.
>
> Intel SCC is non-coherent but supports POSIX shared memory.  It's
> really quite simple to use there, but one has to have synchronization
> primitives.  I just want the Forum to define those in the same manner
> as for MPI-2 RMA, which supports non-coherent platforms.
>
> What's the harm in saying that MPI_WIN_UNLOCK approach causes global
> visibility of the window to all processes?

I think that what you suggested would be a useful and reasonable 
extension to an MPI implementation.  However, the SCC is still a 
research prototype; I don't think it has reached the level of maturity 
needed to define semantics in a standards document.

  ~Jim.