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

[Matthew Knepley]

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.
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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