[Mpi3-rma] Fix for win_allocate_shared

[Torsten Hoefler]

On Fri, 2011-09-23 at 12:25 -0500, Pavan Balaji wrote:
> On 09/23/2011 02:39 AM, Torsten Hoefler wrote:
> > "The semantic rules for shared memory windows in the MPI_RMA_SEPARATE
> > model allow concurrent load/store accesses from two distinct processes
> > to the same target window that is remote to both processes. An
> > implementation cannot expose shared memory windows on hardware that does
> > not support this access mode."
> 
> This seems to suggest that it won't work for non-cache-coherent systems. 
> Didn't we decide against that?
Brian probably wants to comment on this :-). 

Here's my recap: The issue was that many architectures may not be able
to support UNIFIED. So we don't want to limit the SEPARATE model too
much in order to still make it useful. If we later, based on our
experience, find that we need this limitation to support existing
architectures, then we can add the restriction. However, if we add it
now, it may limiting the programming model unnecessarily.

All the Best,
	Torsten