[mpiwg-p2p] cancel in standard

[Sur, Sayantan]

+MPI P2P WG.

We should probably discuss it in the P2P WG as well.

I read the snippet of text and thought the “send” mentioned should
correspond to the MPI send operation (the one the user issued), and not
any internal buffering. “send completes normally” should mean the same
thing as sends completing as if cancel wasn’t called. Similarly, “received
at the destination” should mean the normal way messages are received at
the destination i.e. by posting an MPI receive. Of course, these are my
interpretations :)

On 6/3/15, 7:19 PM, "Balaji, Pavan" <balaji at anl.gov> wrote:

>
>[Cc'ed the Forum]
>
>This wording has been debated many times at the Forum.  Specifically, the
>"completes normally" and "received at the destination" pieces are
>ambiguous.  For instance, if the data is copied into a bounce buffer, the
>send would complete normally, but that does not mean that there is a
>receive posted for that data.  In this case, it would be the user's
>responsibility to post a corresponding receive.  If the MPI
>implementation does not have a bounce buffer (e.g., for large messages),
>"received at the destination" could mean into an unexpected buffer, not
>necessarily a user buffer.
>
>The consensus from MPI-1 veterans (Marc, Bill, etc.) during the last
>discussion was that if the application does not post a receive, then it
>is guaranteed that the send should be cancelable.  At least that was the
>intent.  The wording, of course, is too vague for such a definition.
>
>You could try to weaken that interpretation and say that if the MPI
>implementation is not able to cancel a send operation, then the user is
>responsible for posting a matching receive to receive that data.
>Irrespective of which interpretation we go with, I think it is important
>to clarify it in the standard.  Perhaps an MPI-3.1 errata or MPI-3.2
>change instead of waiting for MPI-4?  Do you want to create a ticket?
>
>  -- Pavan
>
>
>
>
>On 6/3/15, 7:05 PM, "Sur, Sayantan" <sayantan.sur at intel.com> wrote:
>
>>What lines in the standard support the interpretation that “a message
>>that
>>will not be successfully matched, must be cancelable”?
>>
>>This is what I found, and it doesn’t support the stricter interpretation.
>>
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>>"If a
>>send is marked for cancellation, then it must be the case that either the
>>send completes
>>normally, in which case the message sent was received at the destination
>>process, or that
>>the send is successfully cancelled, in which case no part of the message
>>was received at the
>>destination. Then, any matching receive has to be satisfied by another
>>send."
>>
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