[petsc-users] PCFIELDSPLIT on a MATNEST with MATSHELL off-diagonal blocks

[Matthew Knepley]

On Fri, Apr 29, 2016 at 7:34 AM, Le Chenadec, Vincent <vlc at illinois.edu>
wrote:

> Dear all,
>
> I'm trying to use PCFIELDSPLIT as a preconditioner on a block matrix A,
>
> [A_{00}   A_{01}]
> [A_{10}   A_{11}]
>
> where the diagonal blocks (A_{00} and A_{11}) are diagonally dominant. I
> would like to use either one of the block preconditioners (not even based
> on the Schur complement, simply either one of the Jacobi/Gauss-Seidel ones).
>

For any questions about solvers, we need to see the output of -ksp_view.

Below, it seems clear that either a) One of your diagonal blocks is a
MatShell, or b) you are trying to invert
an off-diagonal block.

   Matt


> The issue I'm running into concerns the formats I would like to use, so
> I'll start with that:
>
>    1. A_{00} and A_{11} are both in MATMPIAIJ format (created using
>    DMCreateMatrix on 2D DMDAs of different sizes),
>    2. A_{10} and A_{01} are both in MATSHELL format (created using
>    DMCreateShell). The reason why I used this format is that it seemed
>    convenient, since the 2D DMDAs are of different sizes and I assumed I had
>    to provide the MATOP_MULT operation only,
>    3. Finally the blocks are stored in MATNEST format (MatCreateNest).
>
> With this choice, matrix-vector multiplications work fine (the DMDAs being
> packed in a DMComposite), which makes me think that so far the code is ok.
> Also, when I attempt to invert the matrix without preconditioner, I get
> the correct result.
>
> Now here is where things do not work: if I try to use a preconditioner, I
> get the error pasted below (for a simple PCJACOBI)
>
> [0]PETSC ERROR: No support for this operation for this object type
>
> [0]PETSC ERROR: Mat type shell
>
> [0]PETSC ERROR: #1 MatGetDiagonal() line 4306 in
> petsc/3.6.3_dbg/src/mat/interface/matrix.c
>
> [0]PETSC ERROR: #2 PCSetUp_Jacobi() line 180 in
> petsc/3.6.3_dbg/src/ksp/pc/impls/jacobi/jacobi.c
>
> [0]PETSC ERROR: #3 PCSetUp_Jacobi_NonSymmetric() line 261 in
> petsc/3.6.3_dbg/src/ksp/pc/impls/jacobi/jacobi.c
>
> [0]PETSC ERROR: #4 PCApply_Jacobi() line 286 in
> petsc/3.6.3_dbg/src/ksp/pc/impls/jacobi/jacobi.c
>
> [0]PETSC ERROR: #5 PCApply() line 483 in
> petsc/3.6.3_dbg/src/ksp/pc/interface/precon.c
>
> [0]PETSC ERROR: #6 KSP_PCApply() line 242 in
> petsc/3.6.3_dbg/include/petsc/private/kspimpl.h
>
> [0]PETSC ERROR: #7 KSPSolve_CG() line 139 in
> petsc/3.6.3_dbg/src/ksp/ksp/impls/cg/cg.c
>
> [0]PETSC ERROR: #8 KSPSolve() line 604 in
> petsc/3.6.3_dbg/src/ksp/ksp/interface/itfunc.c
>
> [0]PETSC ERROR: #9 PCApply_FieldSplit() line 988 in
> petsc/3.6.3_dbg/src/ksp/pc/impls/fieldsplit/fieldsplit.c
>
> [0]PETSC ERROR: #10 PCApply() line 483 in
> petsc/3.6.3_dbg/src/ksp/pc/interface/precon.c
>
> [0]PETSC ERROR: #11 KSP_PCApply() line 242 in
> petsc/3.6.3_dbg/include/petsc/private/kspimpl.h
>
> [0]PETSC ERROR: #12 KSPSolve_CG() line 139 in
> petsc/3.6.3_dbg/src/ksp/ksp/impls/cg/cg.c
>
> [0]PETSC ERROR: #13 KSPSolve() line 604 in
> petsc/3.6.3_dbg/src/ksp/ksp/interface/itfunc.c
>
> I may not be calling the appropriate routines, here is what I'm basically
> doing:
>
>    1. KSP type set to KSPCG (A is symmetric), PC type to PCFIELDSPLIT,
>    2. I then use MatNestGetISs on A to supply the ISs to
>    PCFieldSplitSetIS,
>    3. Finally I use PCFieldSplitGetSubKSP to use set the subKSP for each
>    diagonal block.
>
> As I mentioned, as long as the subKSPs don't have any preconditioning
> (PCNONE), KSPSolve does not complain, only when I try to using one (even
> PCJACOBI) do I get similar errors). That makes me suspect that I'm doing
> something wrong, but I could not find an example that was doing the same
> thing...
>
> Any pointers would be much appreciated!
>
> Thanks,
> Vincent
>



-- 
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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