[Mpi-forum] MPI_Request_free restrictions

[Jeff Hammond]

We should fix the RMA chapter with an erratum. I care less about NBC but share your ignorance of why it was done that way. 

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> On Aug 8, 2020, at 6:51 AM, Balaji, Pavan via mpi-forum <mpi-forum at lists.mpi-forum.org> wrote:
> 
>  Folks,
> 
> 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?
> 
> Regards,
> 
>   — 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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