[Mpi3-rma] MPI-3 UNIFIED model clarification

[Pavan Balaji]

On 07/29/2013 05:30 PM, Jed Brown wrote:
> "Sur, Sayantan" <sayantan.sur at intel.com> writes:
>> If we require apps working with UNIFIED model to call MPI_Win_sync, an
>> one-sided PGAS language might be forced to call sync for any buffer
>> that may have been touched remotely. Thereby potentially causing some
>> performance degradation.
>
> They need a memory fence anyway on most architectures.
>
> What is the problem with creating a fast path for MPI_Win_sync with
> UNIFIED?  Are we crying about 10 cycles (function call/fast path) along
> with an explicit statement that (architecture-specific) memory fences
> could be used directly if the caller wants that responsibility?

If a PGAS application is polling on a strict global variable, I'm not 
sure it would mind that overhead.

But the part that concerns me is not the overhead, but rather that 
requiring a WIN_SYNC now will force a nonbackward compatible change to 
the MPI-3 standard.

Unlike Sayantan, I don't think UNIFIED is useful for applications.  But 
I also don't think something specified in the standard should be changed 
in such a subtle non-backward-compatible way.  If we really hate UNIFIED 
so much, it's better to deprecate it, rather than change its definition.

  -- Pavan

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