On Jan 10, 2014, at 9:49 AM, George Bosilca <[email protected]> wrote:
> As I said, this is the case today. There are ongoing discussion in the MPI
> Forum to relax the wording of the MPI_Comm_free as most of the MPI
> implementations do not rely on the strict “collective” behavior of the
> MPI_Comm_free (in the sense that it has to be called by all processes but not
> necessarily in same time).
That will be an interesting discussion. I look forward to your proposal. :-)
>> I still agree with this point, though — even though COMM_FREE is collective,
>> you could still get into ordering / deadlock issues if you're (effectively)
>> doing communication inside it.
>
> As long as the call is collective and the same attributes exists on all
> communicators I don’t see how the deadlock is possible. My wording was more a
> precaution for the future than a restriction for today.
Here's an example:
-----
MPI Comm comm;
// comm is setup as an hcoll-enabled communicator
if (rank == x) {
MPI_Send(..., y, tag, MPI_COMM_WORLD);
MPI_Comm_free(comm);
} else if (rank == y) {
MPI_Comm_free(comm);
MPI_Recv(..., x, tag, MPI_COMM_WORLD);
}
------
If the hcoll teardown in the COMM_FREE blocks waiting for all of its peer
COMM_FREEs in other processes in the communicator (e.g., due to blocking
communication), rank x may block in MPI_SEND waiting for rank y's MPI_RECV, and
therefore never invoke its COMM_FREE.
--
Jeff Squyres
[email protected]
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