[petsc-users] Nullspaces for schur complement PCs

[Lawrence Mitchell]

On 10 Nov 2014, at 21:48, Matthew Knepley <knepley at gmail.com> wrote:

> On Fri, Nov 7, 2014 at 11:04 AM, Lawrence Mitchell <lawrence.mitchell at imperial.ac.uk>wrote:
> Hi petsc-dev,
> 
> I'm solving a pure Neumann mixed Poisson-like problem, preconditioning with a schur complement.  The pressure space has a nullspace of the constant functions and so I attach the appropriate nullspace to the krylov solver, and compose the constant nullspace with the IS defining the pressure space.  My RHS is consistent.
> 
> That is supposed to work, and I think it does in my tests. The code is here
> 
>   https://bitbucket.org/petsc/petsc/src/1f0d623c8336219eb98f7ded6f95c151ca603fe7/src/ksp/pc/impls/fieldsplit/fieldsplit.c?at=master#cl-562
> 

That code attaches the nullspace attached to the IS to jac->pmat if appropriate.  However, if I'm using -pc_fieldsplit_schur_precondition selfp then the schur complement is preconditioned with jac->schurp (rather than jac->pmat[1]) which is not setup until here https://bitbucket.org/petsc/petsc/src/1f0d623c8336219eb98f7ded6f95c151ca603fe7/src/ksp/pc/impls/fieldsplit/fieldsplit.c?at=master#cl-656

at which point the nullspace is not attached, no?

Cheers,

Lawrence
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