[Mpi-forum] Question about the semantics of MPI_Comm_disconnect

[Rajeev Thakur]

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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