[Mpi-forum] Question about the semantics of MPI_Comm_disconnect

[Nathan Hjelm]

Thanks for the clarification. The wording is a little weird but the semantics make sense.

-Nathan

On Tue, Nov 12, 2013 at 04:40:17PM -0600, Rajeev Thakur 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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