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

[Matthew Knepley]

On Fri, Nov 22, 2013 at 6:09 PM, Geoffrey Irving <irving at naml.us> wrote:

> On Fri, Nov 22, 2013 at 3:41 PM, Matthew Knepley <knepley at gmail.com>
> wrote:
> > 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.
>
> Here are results with -ksp_monitor_true_residual -pc_type none:
>
>     http://naml.us/random/laplace-rtol.txt # with -ksp_rtol 1e-5
>     http://naml.us/random/laplace-atol.txt # with -ksp_atol 1e-5
>

Okay, if you have an inconsistent RHS I do not think that true_residual
will work
since it uses the unprojected b, but the solve should be fine.

   Matt


> Both versions converge in 3 iterations for the first SNES iteration,
> but the relative one starts to churn after that since the residual
> starts very small.  The true residual goes down to 4/3 and stagnates.
> Is there a convenient way to print out the RHS to see whether it has a
> component in the nullspace (which seems likely given the true residual
> stagnation)?
>
> I suppose I already do print the result of SNESComputeFunction on the
> zero vec, which is
>
>     RHS = [  6.66666667e-01   1.33333333e+00   6.66666667e-01
> 1.11022302e-16]
>
> Thanks,
> Geoffrey
>



-- 
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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