[Mpi-forum] Question about the semantics of MPI_Comm_disconnect

[Jeff Squyres (jsquyres)]

Ok -- but how do you apply the phrase "and matched" to pending one-sided communication?  Or a pending comm_idup?  Or ...?

I guess I'm saying that "and matched" isn't correct here.



On Nov 12, 2013, at 6:25 PM, Rajeev Thakur <thakur at mcs.anl.gov> wrote:

> The matched is the sense of the matching mentioned in the paragraph I quoted from MPI_Finalize. (If the process is the target of a send, it must have called the matching receive, if it is part of a group doing a collective, it must have called its collective, etc.)
> 
> Rajeev
> 
> 
> On Nov 12, 2013, at 5:10 PM, Jeff Squyres (jsquyres) <jsquyres at cisco.com> wrote:
> 
>> Rajeev --
>> 
>> Any insight on why it says "...complete *and matched*" (emphasis is mine)?
>> 
>>> MPI_COMM_DISCONNECT may be called only if all communication is complete and
>>> matched
>> 
>> The standard defines what matching means for point-to-point communications, but:
>> 
>> 1. Does it define how an application is able to tell if a communication *has been matched* by the peer process?
>> 
>> 2. What about non-point-to-point communication?  E.g., is there a definition for "match" for collective file IO?
>> 
>> 
>> 
>> 
>> 
>> On Nov 12, 2013, at 5:40 PM, Rajeev Thakur <thakur at mcs.anl.gov> wrote:
>> 
>>> Let's take this sequence
>>> 
>>> MPI_Isend
>>> MPI_Wait
>>> MPI_Comm_disconnect
>>> 
>>> After MPI_Wait returns, it doesn't mean that the data has gone over to the other side. It could be buffered locally. Comm_disconnect will ensure that it gets communicated to the other side. If the Wait wasn't called at all in the above sequence, it would be similar to calling MPI_Finalize without a Wait (i.e., incorrect).
>>> 
>>> Think of Comm_disconnect as "whatever connection was there between client and server is gone".
>>> 
>>> Rajeev
>>> 
>>> 
>>> 
>>> 
>>> 
>>> On Nov 12, 2013, at 4:27 PM, Nathan Hjelm <hjelmn at lanl.gov> wrote:
>>> 
>>>> On Tue, Nov 12, 2013 at 04:20:05PM -0600, Rajeev Thakur wrote:
>>>>> On Nov 12, 2013, at 4:08 PM, Nathan Hjelm <hjelmn at lanl.gov> wrote:
>>>>> 
>>>>>> That doesn't match with the wording on p 400 32-34:
>>>>>> 
>>>>>> "MPI_COMM_DISCONNECT has the same action as MPI_COMM_FREE, except that it
>>>>>> waits for pending communication to finish internally and enables the guarantee about the
>>>>>> behavior of disconnected processes."
>>>>> 
>>>>> The above sentence says that MPI_Comm_free does not wait for pending communication to complete, whereas MPI_Comm_disconnect does.  
>>>> 
>>>> That makes absolutely no sense if MPI_Wait/MPI_Test cannot be called after MPI_Comm_disconnect. If
>>>> neither of those functions can be called after MPI_Comm_disconnect then it would be better wording
>>>> that all communication MUST be complete before the call the MPI_Comm_disconnect without any
>>>> qualification that MPI_Comm_disconnect with wait until all communication is complete. There should
>>>> be no communication otherwise we have to allow MPI_Wait/MPI_Test after the call to MPI_Comm_disconnect.
>>>> You see why this is confusing/bad wording in the standard? As an implementor I can not tell what
>>>> was intended here.
>>>> 
>>>>>> Which suggests that some communication may not be finished when MPI_Comm_disconnect is called. Note
>>>>>> that is is safe to call MPI_Wait after MPI_Comm_disconnect but not after MPI_Finalize.
>>>>> 
>>>>> You cannot call MPI_Wait after MPI_Comm_disconnect. You can call it after MPI_Comm_free.
>>>> 
>>>> I don't see that anywhere in the description of MPI_Comm_disconnect. As far as I can tell the
>>>> code snippet I provided is 100% correct MPI code.
>>>> 
>>>> -Nathan
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>> 
>> 
>> -- 
>> Jeff Squyres
>> jsquyres at cisco.com
>> For corporate legal information go to: http://www.cisco.com/web/about/doing_business/legal/cri/
>> 
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-- 
Jeff Squyres
jsquyres at cisco.com
For corporate legal information go to: http://www.cisco.com/web/about/doing_business/legal/cri/