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

[Garth N. Wells]

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.

>    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

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