On Jan 10, 2014, at 15:55 , Jeff Squyres (jsquyres) <jsquy...@cisco.com> wrote:
> On Jan 10, 2014, at 9:49 AM, George Bosilca <bosi...@icl.utk.edu> 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. :-)
? We already had this discussion in the context of another proposal. Anyway
that’s an MPI Forum issue.
>>> 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.
Based on today’s MPI standard this code is incorrect as the MPI_Comm_free is
collective, and you can’t have matching blocking communications crossing a
collective line.
George.
>
> --
> Jeff Squyres
> jsquy...@cisco.com
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