[Mpi3-rma] Fix for win_allocate_shared

[Barrett, Brian W]

I disagree. It provides a rational target for future architectures and provides the ability of all cache coherent systems to support shared memory, something I'm not convinced the unified model does.

Brian

----- Original Message -----
From: Pavan Balaji [mailto:balaji at mcs.anl.gov]
Sent: Friday, September 23, 2011 11:56 AM
To: mpi3-rma at lists.mpi-forum.org <mpi3-rma at lists.mpi-forum.org>
Cc: Barrett, Brian W; Torsten Hoefler <htor at illinois.edu>; mpi3-hybridpm at lists.mpi-forum.org <mpi3-hybridpm at lists.mpi-forum.org>
Subject: Re: [Mpi3-rma] Fix for win_allocate_shared


On 09/23/2011 12:47 PM, Barrett, Brian W wrote:
> It might not be able to. If non-coherent load/store is byte access
> (or word access), it can provide a separate public and private window
> in separate and meet the semantics.  If it provides cacheline access
> to non-coherent space, it will not be able to provide shared memory
> across uncached space; such is the cost of standardization.

Do we know any non-cache-coherent system that provides byte access (note 
that word access is not sufficient)?

What I'm arguing is that we are not giving any additional flexibility 
for non-cache-coherent systems with this new "clarified semantics". In 
fact, we are making it more difficult for future MPI versions to 
actually define a semantics by saying that "multiple remote stores to 
the same window have to work" -- we'll be stuck with this if we say that 
in the standard now.

Instead, if we leave it undefined, in MPI-3.1 (or 4.0), we might be able 
to separate out local load/store and remote load/store operations and 
say that multiple remote load/store operations is undefined (or 
erroneous or whatever).

So the options I'm proposing are:

1. Either define remote load/stores in SEPARATE.

(or)

2. Leave it as undefined.

Attempting a half-definition is making it worse than leaving it undefined.

  -- Pavan

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