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Subject: Re: [OMPI devel] hcoll destruction via MPI attribute
From: George Bosilca (bosilca_at_[hidden])
Date: 2014-01-10 10:04:03

On Jan 10, 2014, at 15:55 , Jeff Squyres (jsquyres) <jsquyres_at_[hidden]> wrote:

> On Jan 10, 2014, at 9:49 AM, George Bosilca <bosilca_at_[hidden]> 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.


> --
> Jeff Squyres
> jsquyres_at_[hidden]
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