[petsc-users] neumann failure in my version of snes ex12

[Matthew Knepley]

On Fri, Nov 22, 2013 at 5:36 PM, Geoffrey Irving <irving at 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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20131122/024ea80b/attachment.html>