[Mpi3-hybridpm] Endpoints Proposal

[Jim Dinan]

Hi All,

I've attached the slides from the endpoints presentation yesterday.  I 
updated the slides with corrections, additions, and suggestions gathered 
during the presentation.

We received a lot of feedback, and a lot of support from the Forum.  A 
refinement to the interface that eliminates MPI_Comm_attach was suggested:

int MPI_Comm_create_endpoints(MPI_Comm parent_comm, int my_num_ep,
                               MPI_Info info, MPI_Comm output_comms[]);

This function would be collective over parent_comm, and produce an array 
of communicator handles, one per endpoint.  Threads pick up the endpoint 
they wish to use, and start using it; there would be no need for 
attach/detach.

This interface addresses two concerns about the interface originally 
presented that were raised by the Forum.  (1) The suggested interface 
does not require THREAD_MULTIPLE -- the original attach function could 
always require multiple.  (2) It places fewer dependencies on the 
threading model.  In particular, stashing all relevant state in the 
MPI_Comm object removes a dependence on thread-local storage.

Thanks to everyone for your help and feedback.  Let's have some 
discussion about the suggested interface online, and follow up in a 
couple weeks with a WG meeting.

Cheers,
  ~Jim.
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