[petsc-users] Fieldsplit PC null pointer error on getksp

[Matthew Knepley]

On Tue, Jun 30, 2015 at 11:43 AM, Sander Land <sander.land at gmail.com> wrote:

> I am trying to use the Schur complement preconditioner in petsc, but am
> encountering a null argument error calling PCFieldSplitGetSubKSP.
> This only happens on PC_COMPOSITE_SCHUR, the multiplicative/additive
> options do return a KSP array.
>
> Error message:
>
> [0]PETSC ERROR: --------------------- Error Message
> --------------------------------------------------------------
> [0]PETSC ERROR: Null argument, when expecting valid pointer
> [0]PETSC ERROR: Null Object: Parameter # 1
> [0]PETSC ERROR: See http://www.mcs.anl.gov/petsc/documentation/faq.html
> for trouble shooting.
> [0]PETSC ERROR: Petsc Release Version 3.6.0, Jun, 09, 2015
> [0]PETSC ERROR: ./elecmech on a arch-linux2-cxx-debug
> [0]PETSC ERROR: Configure options --with-shared-libraries=1
> --with-debugging=1 --download-openmpi=1 --with-clanguage=c++
> --download-fblaslapack=1 --download-scalapack=1 --download-blacs=1
> --download-suitesparse=1 --download-pastix=1 --download-superlu=1
> --dowload-essl=1 --download-ptscotch=1 --download-mumps=1 --download-lusol=1
> [0]PETSC ERROR: #1 MatSchurComplementGetKSP() line 317 in
> petsc-3.6/src/ksp/ksp/utils/schurm.c
> [0]PETSC ERROR: #2 PCFieldSplitGetSubKSP_FieldSplit_Schur() line 1264 in
> petsc-3.6/src/ksp/pc/impls/fieldsplit/fieldsplit.c
> [0]PETSC ERROR: #3 PCFieldSplitGetSubKSP() line 1668 in
> petsc-3.6/src/ksp/pc/impls/fieldsplit/fieldsplit.c
>
> Code snippet:
>
> ISCreateGeneral(PETSC_COMM_SELF,xp_dofs,&ii[0],PETSC_COPY_VALUES,&is_xp);
> ISCreateGeneral(PETSC_COMM_SELF,all_dofs -
> xp_dofs,&ii[xp_dofs],PETSC_COPY_VALUES,&is_wk);
>
> PCSetType(pc,PCFIELDSPLIT);
> PCFieldSplitSetType(pc,PC_COMPOSITE_SCHUR);
> PCFieldSplitSetIS(pc,"xp",is_xp);
> PCFieldSplitSetIS(pc,"wk",is_wk);
> int n;
> KSP* ksps;
> PC   subpc;
> PCFieldSplitGetSubKSP(pc,&n,&ksps);
>
>
> ii here is simply an array with ii[i] = i. There is probably a better way
> to simply indicate two blocks of different size, but I couldn't find it.
>

It looks like the PC is not setup. Can you try calling PCSetUp(pc) before
GetSubKSP()?

  Thanks,

     Matt


> Thanks,
> Sander
>



-- 
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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