[Mpi3-ft] Broken MPI_Request_free

[Erez Haba]

This is a good list, however the proposal is not to deprecate MPI_Request_free but rather deprecate its usage for active requests (and return an error in MPI 3.0). This interface is still required to free persistent requests.

I'll change the MPI 2.2 proposal to remove the text that encourages the usage of MPI_Request_free upon a reply. I'll put a proposal for 3.0 to return an error for active requests.


-----Original Message-----
From: mpi3-ft-bounces at lists.mpi-forum.org [mailto:mpi3-ft-bounces at lists.mpi-forum.org] On Behalf Of Greg Bronevetsky
Sent: Monday, December 01, 2008 8:53 AM
To: mpi3-ft at lists.mpi-forum.org
Subject: [Mpi3-ft] Broken MPI_Request_free

A while ago we discussed how MPI_Request_free is hopelessly broken
and should be removed from the MPI spec. The issue is coming back up
in the MPI collectives group and I'd like to put together a short
list of reasons for why we want to remove it. I recall the following:
- If there is an error in the original non-blocking operation, no
clear place to return the error back to the application.
- On single-threaded architectures (BG/L CNK, Cray Catamount) it is
completely possible for the non-blocking operation to not complete
until MPI_Finalize because the application is not required to issue
any more MPI calls during which progress may be made on the communication.
- Complicates compiler analysis of MPI applications because it is not
clear from the source code when MPI is done with the application buffer.
- Unsafe since the fact that a message arrives at the destination
processor does not guarantee that the OS on the sender processor has
let go of the send buffer. As such, we need to either remove
MPI_Request_free or be clearer about what message arrival implies
about how MPI and the OS manage memory. (Erez, does this sound right?)

Can anybody think of other reasons?

Greg Bronevetsky
Post-Doctoral Researcher
1028 Building 451
Lawrence Livermore National Lab
(925) 424-5756
bronevetsky1 at llnl.gov

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