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

[Matthew Knepley]

On Tue, Apr 5, 2011 at 8:38 AM, Thomas Witkowski <
thomas.witkowski at tu-dresden.de> wrote:

> 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.


Then there is no choice but to use the debugger to see what is going on.

   Matt


>
> 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
>>
>
>


-- 
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
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