[Mpi-forum] MPI_Request_free restrictions

Bangalore, Purushotham puri at uab.edu
Sat Aug 8 11:45:32 CDT 2020

I see discussion of this issue here:


From: mpi-forum <mpi-forum-bounces at lists.mpi-forum.org> on behalf of Balaji, Pavan via mpi-forum <mpi-forum at lists.mpi-forum.org>
Sent: Saturday, August 8, 2020 8:51 AM
To: mpi-forum at lists.mpi-forum.org <mpi-forum at lists.mpi-forum.org>
Cc: Balaji, Pavan <balaji at anl.gov>
Subject: [Mpi-forum] MPI_Request_free restrictions


Does someone remember why we disallowed users from calling MPI_Request_free on nonblocking collective requests?  I remember the reasoning for not allowing cancel (i.e., the operation might have completed on some processes, but not all), but not for Request_free.  AFAICT, allowing the users to free the request doesn’t make any difference to the MPI library.  The MPI library would simply maintain its own refcount to the request and continue forward till the operation completes.  One of our users would like to free NBC requests so they don’t have to wait for the operation to complete in some situations.

Unfortunately, when I added the Rput/Rget operations in the RMA chapter, I copy-pasted that text into RMA as well without thinking too hard about it.  My bad!  Either the RMA committee missed it too, or they thought of a reason that I can’t think of now.

Can someone clarify or remind me what the reason was?


  — Pavan

MPI-3.1 standard, page 197, lines 26-27:

“It is erroneous to call MPI_REQUEST_FREE or MPI_CANCEL for a request associated with a nonblocking collective operation.”

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