[Mpi3-rma] MPI-3 UNIFIED model updates

[Pavan Balaji]

All,

Based on the very long email thread on "MPI-3 UNIFIED model 
clarification", I think we now agree that something needs to change in 
the MPI standard to allow for cache-coherent architectures that don't 
quite give the level of hardware support that is required for UNIFIED.

We came down to one of two solutions for this --

1. We weaken the UNIFIED model to say that a WIN_SYNC is required for 
data to be used.  I liked Jim's wording best, so I'm listing it here:

Unified does not guarantee that "public win == private win".  It 
guarantees that they are *eventually* the same, not immediately and 
always the same.

I would add that the user can force the windows to be the same using 
WIN_SYNC, just like in SEPARATE.

2. We leave UNIFIED as-is, and create a new memory model.  I'm going to 
use KIND_OF_UNIFIED here for discussion, though someone else will need 
to come up with a better name eventually.

The KIND_OF_UNIFIED memory model would not have the above described 
semantics.

I, personally, am OK with either option.  But I look forward to hearing 
others thoughts on these models.

  -- Pavan

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