Clarifying and extending the definition of MPI_Comm_disconnect

[Georg Bisseling <Georg.Bisseling@pallas.com>]

Dear Group,

I want to bring up an issue that was explained to us by application
programmers that use process spawning and MPI_Comm_disconnect
in a master-slave manner: The master is spawning child process
groups in a sequential order.

Due to resource limits the programmers needed to know somehow,
when exactly the child processes were gone and their respective
CPUs available again. So the programmers wanted a MPI_Comm_disconnect
that waits for the termination of the child processes.

As mentioned in the discussion about the (non)collective nature
of MPI_Finalize() there are other programming models (aka "fire
and forget") that are very useful and sensible also. But they 
would require a MPI_Comm_disconnect that does not wait for the
termination of the child process.

I think the application programmers would really appreciate to
have a predefined key (e.g. MPI_DISCONNECT_WAIT) for communicators 
to specify what flavor of MPI_Comm_disconnect to use for this comm.

There are some problems about that: 
	- one has to reference count when the last comm lets a 
	  certain process go and wait only then for its termination. 
	  This way the communicators can be disconnected
	  in any order without breaking the progress requirement.
	- what if a comm is disconnected last that does not contain
	  the father process in the OSes sense? Nobody can call wait!
	  This problem has many incarnations depending on the flavor
	  of (remote) execution used.
	  
Any implementation chosen would require careful documentation
about the behavior. Maybe one could guarantee expected behavior
only for spawned comms and intracomms that are merged spawned comms.	

Maybe there are other OS or batch system dependent requirements on
the behavior of MPI_Comm_disconnect that I did not even think about?  
	  
Regards
Georg


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