[mpiwg-p2p] cancel in standard

[Balaji, Pavan]

If you interpret received at the destination as "matched", then what I mentioned below holds -- if the user never posts a receive, the send cancel should always succeed.

  -- Pavan




On 6/3/15, 7:59 PM, "Sur, Sayantan" <sayantan.sur at intel.com> wrote:

>+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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>>>			
>>>		
>>>	
>>>
>