[Mpi3-hybridpm] Shared memory segments proposal

[James Dinan]

Hi All,

I wanted to follow up on my questions during today's hybrid telecon. 
Thanks, Ron, for taking the time to explain the shared memory segments 
proposal and sorry if I took us off on a tangent.  I just joined Argonne 
in September so I wasn't able to arrange travel in time to join you all 
in Stuttgart.

I took a look at the slides and overall I think the shared memory 
segments proposal is a good idea.  It will provide a good alternative 
for programmers who want to create node-level shared structures but want 
to use processes instead of threads.  I think that in order for this to 
be more generally useful, we should consider adding something like 
semaphores, mutexes, or some other data-driven (as opposed to send-recv 
which will require an MPI rank) synchronization mechanism.

Another comment: Given that we are doing allocation collectively, could 
we actually guarantee that the shared memory is mapped at the same base 
pointer on every process?  We do have MPI at our disposal so we could, 
in principle, do some communication up front to coordinate a base 
address that works for everyone before we map the shared segment.

Best,
  ~Jim.