[petsc-users] Shell preconditioner within a fieldsplit

[Barry Smith]

   From the manual page for PCFieldSplitGetSubKSP()

   You must call KSPSetUp() before calling PCFieldSplitGetSubKSP().

   Please try this and let us know if it still doesn't call your PressureBlock().

    PETSc developers: we should figure out how to have it generate a very useful error message if the PCFieldSplitGetSubKSP() is called prematurely, currently it sets 

  ierr = PetscObjectComposeFunction((PetscObject)pc,"PCFieldSplitGetSubKSP_C",PCFieldSplitGetSubKSP_FieldSplit);CHKERRQ(ierr);

  in PCCreate_FieldSplit() which is premature and will lead to errors. Presumably we should not set this composition until the setup is called?


   Barry



> On Nov 13, 2016, at 11:36 PM, Safin, Artur <aks084000 at utdallas.edu> wrote:
> 
> Hello,
> 
> What is the proper way to set up a shell preconditioner within a fielsplit? I have tried it on my own, but do not get the proper behavior. The relevant portion looks like this:
> 
> __________________________________________________________________
> // Global System
> KSPSetOperators(ksp, A, A);
> 
> // Skipped code..
> 
> 
> // Shell Preconditioner for the pressure sub-block
> KSP *subksp;
> PCFieldSplitGetSubKSP(pc, NULL, &subksp);
> 
> Mat pressureA;
> KSPSetType(subksp[0], "preonly");
> MatCreateShell(MPI_COMM_WORLD, n_local_P_dofs, n_local_P_dofs, , PETSC_DETERMINE, PETSC_DETERMINE, &pressureA);
> MatShellSetOperation(pressureA, MATOP_MULT, (void(*)(void)) PressureBlock);
> KSPSetOperators(subksp[0], pressureA, pressureA);
> 
> // Skipped code..
> 
> 
> KSPSetUp(ksp);
> KSPSolve(ksp, b, x);
> __________________________________________________________________
> 
> The fieldsplit component works fine; the solver however does not go into the custom function PressureBlock(), so I am curious as to what the correct approach is.
> 
> Best,
> 
> Artur