[Mpi-forum] Question about the semantics of MPI_Comm_disconnect

[Rajeev Thakur]

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.  

> 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.

Rajeev



> 
> -Nathan Hjelm
> HPC-5, LANL
> 
> On Tue, Nov 12, 2013 at 03:55:39PM -0600, Rajeev Thakur wrote:
>> It says "This requirement is the same as for MPI_FINALIZE." The text for Finalize on pg 359 has a clue on what the requirement is supposed to mean.
>> 
>> "Before an MPI process invokes MPI_FINALIZE, the process must perform all MPI calls needed to complete its involvement in MPI communications: It must locally complete all MPI operations that it initiated and must execute matching calls needed to complete MPI communications initiated by other processes. For example, if the process executed a nonblocking send, it must eventually call MPI_WAIT, MPI_TEST, MPI_REQUEST_FREE, or any derived function; if the process is the target of a send, then it must post the matching receive; if it is part of a group executing a collective operation, then it must have completed its participation in the operation."
>> 
>> Rajeev
>> 
>> 
>> On Nov 12, 2013, at 1:02 PM, Nathan Hjelm <hjelmn at lanl.gov> wrote:
>> 
>>> I am trying to understand the semantics of MPI_Comm_disconnect but there is a contradiction in
>>> the standard. On page 400, lines 29-31 the standard states:
>>> 
>>> 
>>> 
>>> MPI_COMM_DISCONNECT may be called only if all communication is complete and
>>> matched, so that buffered data can be delivered to its destination. This requirement is the
>>> same as for MPI_FINALIZE.
>>> 
>>> 
>>> 
>>> Which suggests that all communication MUST be complete before MPI_Comm_disconnect is called. But
>>> if I look at lines 41-43 I see this:
>>> 
>>> 
>>> 
>>> Rationale. It would be nice to be able to use MPI_COMM_FREE instead, but that
>>> function explicitly does not wait for pending communication to complete. (End of
>>> rationale.)
>>> 
>>> 
>>> 
>>> Which suggest that there may be pending communication on the communicator when MPI_Comm_disconnect
>>> is called.
>>> 
>>> 
>>> 
>>> A simple example that *could* be legal depending on the interpretation of the standard:
>>> 
>>> MPI_Isend (..., remote, ..., &req);
>>> 
>>> MPI_Comm_disconnect (remote);
>>> 
>>> MPI_Test (&req, &flag, ...);
>>> assert (flag);
>>> 
>>> 
>>> If 29-31 is correct this is illegal but if 41-43 is correct then the disconnect will complete
>>> the request and flag will be set to true. I assume the second interpretation is correct since
>>> MPI_Comm_disconnect would have the same semantics of MPI_Comm_free if all communication must
>>> be complete. Am I wrong?
>>> 
>>> 
>>> -Nathan Hjelm
>>> HPC-5, LANL
>>> _______________________________________________
>>> mpi-forum mailing list
>>> mpi-forum at lists.mpi-forum.org
>>> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum
>> 
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