[petsc-users] local dimensions

[Massoud Rezavand]

Thank you very much,

So, if I am using PetscSplitOwnership() and then MatGetOwnershipRange() to
be prepared for preallocation, then MatSetSizes(A, local_size, local_size,
N, N) should be called with the calculated  local_size from
PetscSplitOwnership() ?

Thanks,
Massoud


On Mon, Dec 12, 2016 at 12:35 AM, Jed Brown <jed at jedbrown.org> wrote:

> Massoud Rezavand <msdrezavand at gmail.com> writes:
>
> > Dear PETSc team,
> >
> > What is the difference between the following two methods to get the local
> > dimensions of a square matrix A? If they do the same, which one is
> > recommended? Should I use MPI_Scan after both?
>
> I would typically use 1 because it's fewer calls and automatically uses
> the correct communicator.  You can use MatGetOwnershipRange() instead of
> manually using MPI_Scan.
>
> > 1)
> >
> > PetscInt local_size = PETSC_DECIDE;
> >
> > MatSetSizes(A, local_size, local_size, N, N);
> >
> >
> > 2)
> >
> > PetscInt local_size = PETSC_DECIDE;
> >
> > PetscSplitOwnership(PETSC_COMM_WORLD, &local_size, &N);
> >
> > MPI_Scan(&local_size, &end_row, 1, MPIU_INT, MPI_SUM, PETSC_COMM_WORLD);
> > begin_row = end_row - local_size;
> >
> >
> > Thanks in advance,
> > Massoud
>
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