[petsc-dev] Preconditioner for Schur complement (pcfieldsplit)

[Thomas Witkowski]

Jed Brown wrote:
> On Tue, Mar 29, 2011 at 13:12, Thomas Witkowski 
> <thomas.witkowski at tu-dresden.de 
> <mailto:thomas.witkowski at tu-dresden.de>> wrote:
>
>     PCFieldsplitSchurPrecondition seems to do what I want. But how to
>     create the matrix for the preconditioner? I'm a little bit
>     confused about it because the matrix/operator S acts on an index
>     range that does not start from 0 and is not continuous, because
>     each rank owns only some parts of the interior domain boundaries.
>
>
> We do not currently have a way to set attributes on a "submatrix" that 
> you will extract in the future. You can implement MatGetSubMatrix to 
> return your special matrix for that block when called with that pair 
> of ISs. Or you can use MatNest for the preconditioning matrix, it is 
> frequently a good storage format when doing fieldsplit and you can use 
> it unintrusively.
MatNest sounds good. Is there any example which makes use of it?

Thomas