[Mpi3-rma] MPI-3.1 consideration slides

[Jeff Hammond]

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?

Jeff

-- 
Jeff Hammond
Argonne Leadership Computing Facility
University of Chicago Computation Institute
jhammond at alcf.anl.gov / (630) 252-5381
http://www.linkedin.com/in/jeffhammond
https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond