On Fri, Nov 22, 2013 at 5:36 PM, Geoffrey Irving <irv...@naml.us> wrote:
> I have a duplicate of snes ex12 (FEM Poisson) which works with
> Dirichlet boundary conditions, but it's breaking for me with Neumann
> conditions. In particular, with Neumann conditions I get results
> which explode even though I believe I am setting a constant nullspace.
>
> For example, if I use two first order elements (the unit square
> divided into two triangles), the resulting solution has
>
> L2 error = 1.75514e+08
> u = [-175513825.75680602, -175513825.66302037,
> -175513825.48390722, -175513824.84436429]
>
> This looks rather a lot like the null space isn't getting through. I
> am creating the constant nullspace with
>
> MatNullSpace null;
> CHECK(MatNullSpaceCreate(comm(),PETSC_TRUE,0,0,&null));
> CHECK(MatSetNullSpace(m,null));
> CHECK(MatNullSpaceDestroy(&null));
>
> If I pass "-ksp_view -mat_view", I get the following. The matrix
> entries seem right (they do indeed have the constant nullspace), and
> ksp_view shows that a nullspace is attached. Is attaching the
> nullspace to the matrix with MatSetNullSpace enough, or do I need to
> additionally attach it to the KSP object?
>
1) I always run with -ksp_monitor_true_residual now when debugging. This
can give
you an idea whether you have a singular PC, which I suspect here.
2) Can you try using -pc_type jacobi? I think ILU might go crazy on a
deficient matrix.
Thanks,
Matt
> Thanks,
> Geoffrey
>
> ---------------------------------------------
>
> Mat Object: 1 MPI processes
> type: seqaij
> row 0: (0, 1) (1, -0.5) (2, -0.5)
> row 1: (0, -0.5) (1, 1) (2, 0) (3, -0.5)
> row 2: (0, -0.5) (1, 0) (2, 1) (3, -0.5)
> row 3: (1, -0.5) (2, -0.5) (3, 1)
> KSP Object: 1 MPI processes
> type: gmres
> GMRES: restart=30, using Classical (unmodified) Gram-Schmidt
> Orthogonalization with no iterative refinement
> GMRES: happy breakdown tolerance 1e-30
> maximum iterations=10000, initial guess is zero
> tolerances: relative=1e-05, absolute=1e-50, divergence=10000
> left preconditioning
> has attached null space
> using PRECONDITIONED norm type for convergence test
> PC Object: 1 MPI processes
> type: ilu
> ILU: out-of-place factorization
> 0 levels of fill
> tolerance for zero pivot 2.22045e-14
> using diagonal shift on blocks to prevent zero pivot [INBLOCKS]
> matrix ordering: natural
> factor fill ratio given 1, needed 1
> Factored matrix follows:
> Mat Object: 1 MPI processes
> type: seqaij
> rows=4, cols=4
> package used to perform factorization: petsc
> total: nonzeros=14, allocated nonzeros=14
> total number of mallocs used during MatSetValues calls =0
> using I-node routines: found 3 nodes, limit used is 5
> linear system matrix = precond matrix:
> Mat Object: 1 MPI processes
> type: seqaij
> rows=4, cols=4
> total: nonzeros=14, allocated nonzeros=14
> total number of mallocs used during MatSetValues calls =0
> has attached null space
> using I-node routines: found 3 nodes, limit used is 5
>
--
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
