[Mpi-forum] Question about the semantics of MPI_Comm_disconnect

Jeff Squyres (jsquyres) jsquyres at cisco.com
Wed Nov 13 15:00:06 CST 2013

On Nov 12, 2013, at 5:40 PM, Rajeev Thakur <thakur at mcs.anl.gov> wrote:

> Think of Comm_disconnect as "whatever connection was there between client and server is gone".

Rajeev --

This sentence has been bugging me, and I just figured out why.  

I've always thought of COMM_DISCONNECT just as you describe: whatever connection was there between client and server is gone.  But what about other types of MPI handles -- like MPI_File and MPI_Window?  For example:

MPI_Comm_spawn(..., &child);
MPI_Intercomm_merge(child, 0, &child_intra);
MPI_File_open(child_intra, ..., &file);

At this point, all communicators between parent and child are gone, but there's still a File (the same example could be done with an RMA window).

So the processes are *not* actually disconnected (in the MPI sense of the word "disconnected).  Indeed, in the above example, if there was no File, you still have to do *2* disconnects before the parent and child are truly disconnected.

Hence, I think that this is a very poorly named function -- it *doesn't* necessarily disconnect MPI processes.  It really performs and operation that is something like MPI_COMM_FREE_WAIT.

I wonder if this is worth an AtoU...

Jeff Squyres
jsquyres at cisco.com
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