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Subject: Re: [OMPI devel] RFC: about dynamic/intercomm_create test from ibm test suite
From: Ralph Castain (rhc_at_[hidden])
Date: 2014-05-28 08:35:48


On May 28, 2014, at 4:31 AM, Jeff Squyres (jsquyres) <jsquyres_at_[hidden]> wrote:

> 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 just pulling you chain, Jeff :-)

> 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.

Agreed - however, I find it concerning that loop_spawn, which does have every process calling comm_free, is showing the same symptom upon the parent calling finalize.

>
> 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...").

Also agreed, assuming we can find the right place to correctly determine that is what is happening.

> But that's definitely extra bonus points, and not required.
>
> --
> Jeff Squyres
> jsquyres_at_[hidden]
> For corporate legal information go to: http://www.cisco.com/web/about/doing_business/legal/cri/
>
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