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Subject: Re: [OMPI devel] Thread safety levels
From: N.M. Maclaren (nmm1_at_[hidden])
Date: 2010-05-10 09:02:32


On May 10 2010, Kawashima wrote:
>
>Because MPI_THREAD_FUNNELED/SERIALIZED doesn't restrict other threads to
>call functions other than those of MPI library, code bellow are not
>thread safe if malloc is not thread safe and MPI_Allreduce calls malloc.
>
> #pragma omp parallel for private(is_master)
> {
> MPI_Is_thread_main(&is_master);
> if (is_master == 0) { /* master thread */
> MPI_Allreduce(...);
> } else { /* other threads */
> /* work that calls malloc */
> }
> }

That's one case. It also applies to ordinary memory accesses, where
they might use data that overlaps the MPI_Allreduce. That's probably
impossible for the blocking MPI calls, but it isn't for the one-sided
communication ones, and probably not for the non-blocking ones. Mixing
non-blocking (and, worse, one-sided communication) and threading is a
nightmare area.

>> On others, they use a completely different (and seriously incompatible,
>> at both the syntactic and semantic levels) set of libraries. E.g. AIX.
>
>Sorry, I don't know these issue well.
>Do you mean the case I wrote above about malloc?

No. You have to compile using different paths if you want threading,
and that generated incompatible code (in a very few ways, but ones
that hit my users).

>In C, omp parallel region ends with for-block.
>So I think that would call MPI_Allreduce once per process.
># In Fortran, it may require omp end parallel directive to end parallel
># region. But I don't know Fortran well, sorry.

You're right. I had got myself confused - what I was saying is true
for Fortran coarrays but not OpenMP (in either C or fortran)!

>> I can't comment on that, though I doubt it's quite that simple. There's
>> a big difference between MPI_THREAD_FUNNELED and MPI_THREAD_SERIALIZED
>> in implementation impact.
>
>I can't imagine difference between those two, unless MPI library uses
>something thread local. Ah, there may be something on OSes that I don't
>know....

See my other message. It's evil. It's also rare, and I don't know how
widespread it is today.

Regards,
Nick Maclaren.