[Mpi3-ft] Stabilization Updated & MPI_Comm_size question

Joshua Hursey jjhursey at open-mpi.org
Fri Sep 17 03:27:23 CDT 2010

So the Run-Through Stabilization proposal has been updated per our discussion in the working group meeting at the MPI Forum. The changes are summarized below:
 - Add a Legacy Library Support example
 - Clarify new error classes
 - Update the MPI_Init and MPI_Finalize wording to be simpler and more direct.
 - Fix wording of group creation calls versus communicator creation calls.


One question that we discussed quite a bit during the meeting was the issue of the return value of MPI_Comm_size() when processes fail during launch. I attempted to capture the discussion in the room in the Open Question attached to the discussion of MPI_Init:

Open question:
If the user asks to start N processes on the command line, and only M processes were successfully launched (where M < N), then what should be returned from MPI_COMM_SIZE?

The return value must be consistent across all alive members of the group. The issue is if it should return N or M.

The feeling in the room was that since the MPI standard does not define the ability for the user to ask for a specific number of processes before initthen it is hard to define that this is the number it should be.

So it is left to the implementation whether it is M or N. If it is M, then the user has other techniques to find out what it originally asked for (e.g., by passing it as a command line argument to the application itself).

What do people think about the MPI_Comm_size issue?

-- Josh

Joshua Hursey
Postdoctoral Research Associate
Oak Ridge National Laboratory

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