[pooma-dev] Re: [PATCH] Support hybrid MPI/OpenMP if available

[Richard Guenther]

On Thu, 8 Jan 2004, Jeffrey D. Oldham wrote:

> Richard Guenther wrote:
> >
> >  #if POOMA_MPI
> > +# ifdef _OPENMP
> > +  int provided;
> > +  MPI_Init_thread(&argc, &argv, MPI_THREAD_FUNNELED, &provided);
> > +  PInsist(provided >= MPI_THREAD_FUNNELED, "No MPI support for OpenMP");
> > +# else
> >    MPI_Init(&argc, &argv);
> > +# endif
> >  #elif POOMA_CHEETAH
> >    controller_g = new Cheetah::Controller(argc, argv);
> >  #endif
>
> OpenMP does not support MPI_init?  I'd prefer to initialize OpenMP using
> the same mechanism as for MPI implementations.

It's somewhat difficult.  The MPI-1 standard does not support any sort of
threading and has only MPI_Init.  The MPI-2 standard does support many
levels of thread support which needs to be specified through
MPI_Init_thread - though using MPI_Init is still possible, which is
equivalent to initializing with no thread support.

Nearly all implementation support the MPI_Init_thread call as part of
(usually incomplete) MPI-2 support.  To allow using OpenMP if MPI is used,
too, we need at least make the MPI library aware of this.  So, if no
OpenMP is used, MPI_Init suffices and allows for MPI-1 only
implementations.  For OpenMP support we absolutely need MPI_Init_threads,
so we use it.

> Also, does finalization also need to change?

No.

Ok with this explanation?

Thanks,
Richard.