On Fri, 2011-09-23 at 12:56 -0500, Pavan Balaji wrote:

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. I see your point but would like to postpone to the telecon.

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. I think we can just define that concurrent accesses are undefined.

(or)

2. Leave it as undefined.

Attempting a half-definition is making it worse than leaving it undefined. It's not a half-definition. It's very clear in that it's not supporting some architectures (that may not expose shared memory windows then).

All the Best, Torsten