[mpiwg-rma] FW: [Mpi3-rma] [EXTERNAL] Re: MPI-3 UNIFIED model updates

[Pavan Balaji]

Keith,

On 08/26/2013 08:00 PM, Underwood, Keith D wrote:
>> Well, after asking somebody what happens with SHMEM on such
>> architectures, I believe they basically tell the user "program this like you
>> would a multithreaded program - all such rules apply".  I don't see why we
>> don't take that approach.  It seems to me to be the closest to the objective
>> of unified...

I think your comment is really talking about the previous thread of 
emails on this topic.  Let me try to clarify this here, but that was not 
the intent of this email thread.

There are really three memory models: SEPARATE, 
UNIFIED_REQUIRING_WIN_SYNC, and UNIFIED_NOT_REQUIRING_WIN_SYNC.

SEPARATE is what we already have in the standard. 
UNIFIED_REQUIRING_WIN_SYNC is where the target has to call WIN_SYNC to 
synchronize the public/private windows for overlapping data, but 
nonoverlapping data can be accessed without synchronization. 
UNIFIED_NOT_REQUIRING_WIN_SYNC is where the target data is always 
synchronized between the public/private windows.

Some folks on the WG thought that "UNIFIED" in the MPI-3 standard meant 
UNIFIED_REQUIRING_WIN_SYNC, while others thought it meant 
UNIFIED_NOT_REQUIRING_WIN_SYNC.  So the chapter is in an inconsistent 
state right now.  Most of the previous email thread was to come to a 
consensus on these different memory models and we agreed that these 
three exist, irrespective of whether we want to support them in the MPI 
standard or not.

In this email thread, we are trying to pick one of two options:

1. Change the wording in the MPI standard so "UNIFIED" means 
UNIFIED_REQUIRING_WIN_SYNC.  This seems to be the option that got most 
votes so far.  In fact, folks feel that this can be done as an errata 
item, though whether the Forum will accept that as an errata remains to 
be seen.  It is important to note that this option would mean that we 
will not have a way to represent the UNIFIED_NOT_REQUIRING_WIN_SYNC model.

2. Change the wording in the MPI standard so "UNIFIED" means 
UNIFIED_NOT_REQUIRING_WIN_SYNC, but add another memory model for 
UNIFIED_REQUIRING_WIN_SYNC.  This option only got my vote so far :-).

  -- Pavan

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