On Tue, Aug 20, 2013 at 12:50 PM, Garth N. Wells <[email protected]> wrote:
> On 19 August 2013 14:03, Matthew Knepley <[email protected]> wrote: > > On Mon, Aug 19, 2013 at 7:40 AM, Jed Brown <[email protected]> wrote: > >> > >> "Garth N. Wells" <[email protected]> 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
