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Subject: Re: [OMPI devel] RFC: about dynamic/intercomm_create test from ibm test suite
From: Jeff Squyres (jsquyres) (jsquyres_at_[hidden])
Date: 2014-05-28 07:31:57

On May 27, 2014, at 9:11 PM, Gilles Gouaillardet <gilles.gouaillardet_at_[hidden]> wrote:

> in the case of intercomm_create, the children free all the communicators and then MPI_Disconnect() and then MPI_Finalize() and exits.
> the parent only MPI_Disconnect() without freeing all the communicators. MPI_Finalize() tries to disconnect and communicate with already exited processes.
> my understanding is that there are two ways of seeing things :
> a) the "R-way" : the problem is the parent should not try to communicate to already exited processes
> b) the "J-way" : the problem is the children should have waited either in MPI_Comm_free() or MPI_Finalize()

I didn't ignore Ralph's email; I was pointing out what the MPI semantics are supposed to be.

I had only a short time this morning to look at the intercomm_create test program, and it looks like Gilles might be correct -- the children are freeing all relevant communicators *but the parent is not*. If this is, indeed, correct, then a) OMPI's implementation might be fine because the test program is erroneous (i.e., the children *think* that they are disconnected and therefore allow themselves to exit, but the parents *think* that they are still connected and therefore try to contact the children during the parents' MPI_FINALIZE), and b) his original patch to the test program could well be correct.

I won't have time to investigate this today; if someone else could look at the test code and confirm whether this is correct or not, that would be appreciated.

> as far as i am concerned, i have no opinion on which of a) or b) is the correct/most appropriate approach.

To be totally clear: MPI says it is erroneous for only some (not all) processes in a communicator to call MPI_COMM_FREE. So if that's the real problem, then the discussion about why the parent(s) is(are) trying to contact the children is moot -- the test is erroneous, and erroneous application behavior is undefined.

All that being said, if we want to make this error case a bit friendlier to the user, that would be great (i.e., a show_help something like "It looks like an MPI process is trying to contact another MPI process that has already exited/called MPI_FINALIZE. This is almost certainly an error in the application..."). But that's definitely extra bonus points, and not required.

Jeff Squyres
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