[petsc-users] Attach approximate null space to a fieldsplit block

[Matthew Knepley]

On Tue, Aug 20, 2013 at 1:41 PM, Garth N. Wells <gnw20 at cam.ac.uk> wrote:

> On 20 August 2013 19:14, Matthew Knepley <knepley at gmail.com> wrote:
> > On Tue, Aug 20, 2013 at 12:50 PM, Garth N. Wells <gnw20 at cam.ac.uk>
> wrote:
> >>
> >> On 19 August 2013 14:03, Matthew Knepley <knepley at gmail.com> wrote:
> >> > On Mon, Aug 19, 2013 at 7:40 AM, Jed Brown <jedbrown at mcs.anl.gov>
> wrote:
> >> >>
> >> >> "Garth N. Wells" <gnw20 at cam.ac.uk> writes:
> >> >>
> >> >> > I'm using PCFIELDSPLIT for a mixed problem, with fields indicated
> via
> >> >> > an index set (using PCFieldSplitSetIS). I'd like to attach an
> >> >> > approximate null space to the A00 block of the system. Is there a
> way
> >> >> > to attach the null space to one block?
> >> >
> >> >
> >> > There is a better way. You can attach the null space to the IS that
> >> > forms
> >> > the split
> >> >
> >> >   PetscObjectAttach((PetscObject) is0, "nullspace", nullspace);
> >> >
> >> > and PCFIELDSPLIT will pull it out and attach it to the preconditioner
> >> > for
> >> > block 0.
> >> >
> >>
> >> I tried this and it didn't seem to work - the solver didn't converge at
> >> all.
> >
> >
> > Is this helpless week :) Run with -ksp_view so we can see what actually
> > happened.
>
> The string should be 'nearnullspace' in place of 'nullspace' as above, i.e.
>
>     PetscObjectCompose((PetscObject) is0, "nearnullspace",
> (PetscObject) nullspace);
>
> rather than
>
>     PetscObjectCompose((PetscObject) is0, "nullspace", (PetscObject)
> nullspace);
>
> With the former it works as expected.


I did not realize you wanted the AMG starter. Glad it works.

  Matt


>
> Garth
>
>
> > Here is the code that gets out the null space:
> >
> >
> >
> https://bitbucket.org/petsc/petsc/src/d45619dec29bfb59cf96225a84e0a74106da50ca/src/ksp/pc/impls/fieldsplit/fieldsplit.c?at=master#cl-531
> >
> > You can break there and see if it gets it.
> >
> >   Thanks,
> >
> >       Matt
> >
> >>
> >> >    Matt
> >> >
> >> >>
> >> >> With MatNest, you can use MatSetNearNullSpace on sub-blocks and it
> will
> >> >> be used automatically.  This is also the low-memory way to use
> >> >> PCFieldSplit.
> >>
> >> We're working on support for this.
> >>
> >> >> Note that if you use MatSetValuesLocal, you can use the
> >> >> NEST format with identical code in assembly; only matrix creation is
> >> >> different, see src/snes/example/tutorials/ex28.c for an example that
> >> >> works with both AIJ and NEST formats.
> >> >>
> >>
> >> Is there a reason why MatSetValues can't be used? We rely on PETSc
> >> caching and then communicating off-process entries.
> >>
> >> >> Otherwise, you should be able to PCSetUp, then get out the
> sub-solvers
> >> >> PCFieldSplitGetSubKSP, pull out their matrices, and call
> >> >> MatSetNearNullSpace.  Those submatrices should not be overwritten as
> >> >> long as you use SAME_NONZERO_PATTERN, so the null space would still
> >> >> work
> >> >> in future iterations.
> >> >
> >>
> >> I'm using the above and it's working great - thanks.
> >>
> >> Garth
> >>
> >> >
> >> >
> >> >
>



-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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