[MPICH2-dev] reference counting communicators

[David Gingold]

I've been puzzling over this one this afternoon:

Is it a programming error to call MPI_Comm_free() when there are  
outstanding unmatched sends that reference the communicator?

The MPI spec seems to say that a communicator is not internally  
deallocated until these sends are completed at the receive end.  But  
how is the implementation to know?  The processes could try to check  
their early send queues (or increment reference counts as early sends  
arrive), but with an eager message implementation it's possible that  
the unmatched sends could be in flight, not accounted for by the  
sender or the receiver.

(And if MPI deallocates and later re-uses the communicator before the  
sends are matched, we can construe a scenario where a receive  
mistakenly matches a send from the old communicator.)

I believe this has implications for how MPICH2 must reference count  
communicators in send and receive requests.  If the MPI  
implementation doesn't need to account for these stranded sends, then  
we might avoid creating references to the communicator in some cases.

-dg

--
David Gingold
Principal Software Engineer
SiCortex
One Clock Tower Place, Suite 100
Maynard MA 01754
(978) 897-0214 x224