[petsc-users] Nested Fieldsplit for custom index sets

[Barry Smith]

  Do you have a call to KSPSetFromOptions() before the call PCFieldSplitGetSubKSP()? I am guessing not which means that the PC does not yet know that it is of type fieldplit.

   Barry

> On Jul 26, 2016, at 9:00 PM, Safin, Artur <aks084000 at utdallas.edu> wrote:
> 
> Hello all,
> 
> I would like to work out how to get nested fieldsplit to work correctly. I have a submatrix (labeled fieldsplit_P) that I would like to block precondition with sub-blocks A & B. To do this, I access the PC object within fieldsplit_P, and pass index sets corresponding to these sub-blocks (P_A_IS, P_B_IS) that tell how the matrix should be split. This is what I have:
> 
> --------------------------------------------------------------------------------
>    KSP *ksp_all, ksp_P;
>    PCFieldSplitGetSubKSP(pc, &i, &ksp_all);
>    
>    ksp_P = ksp_all[0];
>    PC pc_P;
>    KSPGetPC(ksp_P, &pc_P);                     // This should extract the preconditioner for fieldsplit P
>    PCFieldSplitSetIS(pc_P, "A", P_A_IS);
>    PCFieldSplitSetIS(pc_P, "B", P_B_IS);
> --------------------------------------------------------------------------------
> 
> And these are the run-time arguments:
> 
> --------------------------------------------------------------------------------
>    -pc_type fieldsplit
>    -pc_fieldsplit_type multiplicative
> 
>    -fieldsplit_P_ksp_type gmres
>    -fieldsplit_P_pc_type fieldsplit
>    -fieldsplit_P_pc_fieldsplit_type multiplicative
> 
>    -fieldsplit_P_fieldsplit_A_ksp_type gmres
>    -fieldsplit_P_fieldsplit_B_pc_type lu
>    -fieldsplit_P_fieldsplit_B_ksp_type preonly
> --------------------------------------------------------------------------------
> 
> But that does not work:
> 
> --------------------------------------------------------------------------------
> [0]PETSC ERROR: --------------------- Error Message --------------------------------------------------------------
> [0]PETSC ERROR: No support for this operation for this object type
> [0]PETSC ERROR: Cannot locate function PCFieldSplitSetIS_C in object
> [0]PETSC ERROR: See http://www.mcs.anl.gov/petsc/documentation/faq.html for trouble shooting.
> [0]PETSC ERROR: Petsc Release Version 3.7.2, Jun, 05, 2016 
> [0]PETSC ERROR: ./main_2D on a x86_64 named artur-ubuntu by artur Tue Jul 26 18:55:29 2016
> [0]PETSC ERROR: Configure options --with-scalar-type=complex --with-mpi=1 --with-clanguage=c++ --with-cc=mpicc --with-fc=gfortran --with-cxx=mpic++ --with-fc=mpif90 --download-mumps --download-scalapack
> [0]PETSC ERROR: #1 PCFieldSplitSetIS() line 1756 in /home/artur/Rorsrach/Packages/petsc-3.7.2/src/ksp/pc/impls/fieldsplit/fieldsplit.c
> [0]PETSC ERROR: --------------------- Error Message --------------------------------------------------------------
> [0]PETSC ERROR: No support for this operation for this object type
> [0]PETSC ERROR: Cannot locate function PCFieldSplitSetIS_C in object
> [0]PETSC ERROR: See http://www.mcs.anl.gov/petsc/documentation/faq.html for trouble shooting.
> [0]PETSC ERROR: Petsc Release Version 3.7.2, Jun, 05, 2016 
> [0]PETSC ERROR: ./main_2D on a x86_64 named artur-ubuntu by artur Tue Jul 26 18:55:29 2016
> [0]PETSC ERROR: Configure options --with-scalar-type=complex --with-mpi=1 --with-clanguage=c++ --with-cc=mpicc --with-fc=gfortran --with-cxx=mpic++ --with-fc=mpif90 --download-mumps --download-scalapack
> [0]PETSC ERROR: #2 PCFieldSplitSetIS() line 1756 in /home/artur/Rorsrach/Packages/petsc-3.7.2/src/ksp/pc/impls/fieldsplit/fieldsplit.c
> --------------------------------------------------------------------------------
> 
> It seems that the preconditioner object for fieldsplit_P does not know that it is also of fieldsplit type. Does anyone have any idea of how I can specify the proper fieldsplit?
> 
> Best,
> 
> Artur