[Mpi-forum] Question about the semantics of MPI_Comm_disconnect
thakur at mcs.anl.gov
Tue Nov 12 17:25:57 CST 2013
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.)
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
> 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
>> 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".
>> 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.
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> Jeff Squyres
> jsquyres at cisco.com
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