[mpiwg-hybridpm] Can MPI processes request different threading levels?

[Jeff Hammond]

Nothing prohibits a use case where MPI_Init + process-specific environment
variables and/or MPI_Init_thread can initialize MPI with varying thread
support, and I think the MPMD use case clearly demands it.

Implementations are of course capable of (1) rounding up the thread level
or (2) returning an inadequate value of provided, if a precise
implementation of the application requirement cannot be satisfied.

On Tue, Aug 1, 2023 at 1:39 PM Michael Knobloch via mpiwg-hybridpm <
mpiwg-hybridpm at lists.mpi-forum.org> wrote:

> I'm not even sure it fully answers the question. This statement says
> that MPI processes can request different requirements in
> MPI_Init_thread, but it doesn't say whether all processes must be
> initialized with a call to the same routine or if some can be
> initialized with MPI_Init (and assume MPI_THREAD_SINGLE?) and some with
> MPI_Init_thread, which was the original question.
>
> -Michael
>
> On 01.08.23 12:24, Joseph Schuchart via mpiwg-hybridpm wrote:
> > Funny that this is inside an advice to implementors (which users like
> > Joachim are not expected to read) when it really should be part of the
> > normative text.
> >
> > Cheers
> > Joseph
> >
> > On 8/1/23 11:09, Jeff Hammond via mpiwg-hybridpm wrote:
> >>
> >> It is addressed here (in the text for MPI_INIT_THREAD):
> >>
> >> Advice to implementors. If provided is not MPI_THREAD_SINGLE then the
> >> MPI library should not invoke C or Fortran library calls that are not
> >> thread safe, e.g., in an environment where malloc is not thread safe,
> >> then malloc should not be used by the MPI library.
> >>
> >> Some implementors may want to use different MPI libraries for
> >> different levels of thread support. They can do so using dynamic
> >> linking and selecting which library will be linked when
> >> MPI_INIT_THREAD is invoked. If this is not possible, then
> >> optimizations for lower levels of thread support will occur only when
> >> the level of thread support required is specified at link time.
> >>
> >> *Note that required need not be the same value on all processes of
> >> MPI_COMM_WORLD.* (End of advice to implementors.)
> >>
> >> Jeff
> >>
> >> On Tue, Aug 1, 2023 at 11:47 AM Joachim Jenke via mpiwg-hybridpm
> >> <mpiwg-hybridpm at lists.mpi-forum.org> wrote:
> >> >
> >> > Hello,
> >> >
> >> > I'm not sure whether this is the right group to ask, but it is related
> >> > to hybrid execution :)
> >> >
> >> > Must all MPI processes collectively call MPI_Init or MPI_Init_thread -
> >> > or can some processes call one and other processes call the other
> >> function?
> >> >
> >> > Also, can the processes request different thread-support levels?
> >> >
> >> > Our use case is MPMD execution, where some processes run
> >> multi-threaded
> >> > and other single-threaded.
> >> >
> >> > Thanks,
> >> > Joachim
> >> >
> >> >
> >> > --
> >> > Dr. rer. nat. Joachim Jenke
> >> >
> >> > IT Center
> >> > Group: High Performance Computing
> >> > Division: Computational Science and Engineering
> >> > RWTH Aachen University
> >> > Seffenter Weg 23
> >> > D 52074  Aachen (Germany)
> >> > Tel: +49 241 80- 24765
> >> > Fax: +49 241 80-624765
> >> > jenke at itc.rwth-aachen.de
> >> > www.itc.rwth-aachen.de <http://www.itc.rwth-aachen.de>
> >> > _______________________________________________
> >> > mpiwg-hybridpm mailing list
> >> > mpiwg-hybridpm at lists.mpi-forum.org
> >> > https://lists.mpi-forum.org/mailman/listinfo/mpiwg-hybridpm
> >>
> >>
> >>
> >> --
> >> Jeff Hammond
> >> jeff.science at gmail.com
> >> http://jeffhammond.github.io/
> >>
> >> _______________________________________________
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> >
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>
> --
> Michael Knobloch
> Juelich Supercomputing Centre   (JSC)
> Institute for Advanced Simulation (IAS)
> Telefon: +49 2461 61-3546
> Telefax: +49 2461 61-6656
>
>
>
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-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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