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

[Thomas Witkowski]

Jed Brown wrote:
> On Tue, Mar 29, 2011 at 15:29, Thomas Witkowski 
> <thomas.witkowski at tu-dresden.de 
> <mailto:thomas.witkowski at tu-dresden.de>> wrote:
>
>     Is there any example which makes use of it?
>
>
> src/snes/examples/tutorials/ex28.c is a (contrived) multi-physics 
> prototype that uses MatNest tranparently if you run with 
> -pack_dm_mat_type nest (currently, you should add -pc_type fieldsplit 
> -pc_fieldsplit_type additive because off-diagonal blocks are not 
> automatically preallocated).
>
> There are also two tests that use the low-level API:
>
> $ grep -l MatNest src/**/tests/*.c
> src/ksp/ksp/examples/tests/ex22.c
> src/snes/examples/tests/ex17.c
So, I reimplemented my code to assemble the matrices to a MatNest 
matrix. This seems to work fine. But how to define the FieldSplits? I 
did it in the following way:

    KSP solver;
    PC pc;
    KSPCreate(PETSC_COMM_WORLD, &solver);
    KSPGetPC(solver, &pc);
    const PetscInt interiorField[1] = {0};
    const PetscInt boundaryField[1] = {1};
    PCFieldSplitSetFields(pc, "interior", 1, interiorField);
    PCFieldSplitSetFields(pc, "boundary", 1, boundaryField);

When I run my code with the options "-ksp_type preonly -pc_type 
fieldsplit -pc_fieldsplit_type schur", I get the following errors in 
KSPSolve:

[0]PETSC ERROR: --------------------- Error Message 
------------------------------------
[0]PETSC ERROR: Petsc has generated inconsistent data!
[0]PETSC ERROR: Unhandled case, must have at least two fields!

Is the creation of the split wrong? If not, what is a good way to figure 
out the problem?

Thomas