On 2 Dec 2013, at 15:20, Matthew Knepley <[email protected]> wrote:
> On Mon, Dec 2, 2013 at 6:31 AM, Lawrence Mitchell
> <[email protected]> wrote:
>
> On 2 Dec 2013, at 12:23, Matthew Knepley <[email protected]> wrote:
>
> > On Mon, Dec 2, 2013 at 2:26 AM, Lawrence Mitchell
> > <[email protected]> wrote:
> > Dear petsc-users,
> >
> > I have a 3x3 block system built as a single MatNest (with 9 Mats in it).
> > I'd like to treat this as a 2x2 system:
> > [ A B
> > C D ]
> >
> > where A is 2x2 and precondition the A block with a schur complement. Since
> > I have a mat nest, the ISes for the three fields are just
> > ISCreateStride(..., mat_i_rows, offset, 1, ...) and these are set on the
> > fieldsplit pc. If I understand the documentation correctly, I think I
> > should now be able to do:
> >
> > -pc_type fieldsplit -fieldsplit_0_fields 0,1 -fieldsplit_1_fields 2
> > -fieldsplit_0_pc_type field split -fieldsplit_0_pc_fieldsplit_type schur
> >
> > ...
> >
> > However, when doing so, I get an error: "To use Schur complement
> > preconditioner you must have exactly 2 fields". Which suggests to me I
> > have failed to inform PETSc that I want the first two fields to be treated
> > as 1.
> >
> > Note that I am not using a DM to build any of these objects. I build a
> > SNES, pull the KSP out of the SNES and then the PC out of the KSP. I never
> > explicitly call SetFromOptions on the PC. Instead, before the SNES solve I
> > call SNESSetFromOptions. Might this be the problem?
> >
> > This is an unfortunately limitation of the implementation right now. This
> > option works if you are on a DA with collocation, or if you
> > use a DM, but not if you just specify the ISes. We should probably write
> > that code. However, the idea is for people to be moving
> > to using DM. Could you tell us why DM did not work for you here?
>
> At the moment, our mesh infrastructure is not plumbed in to use petsc data
> structures. We're attempting to migrate to dmplex at the moment but
> currently we're building Mats from third-party connectivity information.
> It's possible that the move to dmplex will happen soon enough that it isn't a
> big issue, but I don't know the timescales on which we'll be done with that.
>
> I was feeling bad about not implementing this, but I remembered the problem
> with the pure IS solution. The obvious solution
> is to just combine several ISes to create the field for PCFIELDSPLIT.
> However, once this is done, they lose their identity as
> separate fields. Thus, it is not possible to untangle for the second level of
> FieldSplit that you want. The DM version maintains
> the field identity at the next level so that we can split hierarchically. So,
> for the above to work, I think you must use a DM.
OK, I think I see. I wonder if, until I have a DM in place, I can hack this
together by building my MatNest recursively? That is build
A = [A' B'
C' D']
and then build
F = [A B
C D]
would that work, or am I barking up the wrong tree?
Cheers,
Lawrence
