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

[Thomas Witkowski]

Matthew Knepley wrote:
> On Tue, Apr 5, 2011 at 4:28 AM, Thomas Witkowski 
> <thomas.witkowski at tu-dresden.de 
> <mailto:thomas.witkowski at tu-dresden.de>> wrote:
>
>     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>
>         <mailto: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?
>
>
> 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.
This generates exactly the same error message.

Thomas
>
>    Matt
>  
>
>
>     Thomas
>
>
>
>
> -- 
> What most experimenters take for granted before they begin their 
> experiments is infinitely more interesting than any results to which 
> their experiments lead.
> -- Norbert Wiener