Jed Brown wrote:

On Tue, Mar 29, 2011 at 15:29, Thomas Witkowski <thomas.witkowski@tu-dresden.de <mailto:thomas.witkowski@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?

This is almost certainly a problem with the setup order. In this code, the PC type is not yet set

so the FieldSplit calls are ignored. When it finally is set, there are no fields defined. I would call

KSPSetFromOptions() before retrieving the PC.

Matt

Thomas