At 09:35 AM 3/27/2009, Darius Buntinas wrote:

I've just created a ticket: https:// svn.mpi-forum.org/trac/mpi-forum-web/ticket/144 to suggest that we strengthen the ordering guarantees between sends (and receives) from different threads.

I think that the real question here is whether the potential benefit to the user outweighs the potential for optimization in a multithreaded implementation.

Please have a look at it and feel free to comment.

I like the spirit of this proposal but in practice it will be hard to specify. In particular, consider the sentence "two sends do not overlap if the first MPI_Send() returns before the second MPI_Send() is called". The word "before" here is only properly defined on a sequentially consistent memory. On any other memory model "before" is only defined by synchronization primitives supplied by the shared memory system. As such, I don't know how to be precise about something like "before" without talking about the shared memory system. This will be hard because we'll need to talk about concepts that all these systems share but for which they all provide different primitives and semantics. I don't know how the wider forum will react to such a specification. However, I agree that something like this is necessary. Greg Bronevetsky Post-Doctoral Researcher 1028 Building 451 Lawrence Livermore National Lab (925) 424-5756 bronevetsky1_at_[hidden] http://greg.bronevetsky.com