[petsc-users] SNES convergence failure (line search) for Neumann boundary condition

[Matthew Knepley]

On Wed, Aug 10, 2011 at 4:10 PM, Graf, Peter <Peter.Graf at nrel.gov> wrote:

> Hi Barry,
> Thanks for your response.  I did not see a "fgmre" option, but "fgmres"
> works, and here is the output (including -info).
> Thanks in advance for any help you can offer
>

Okay, the linear system is being solved, but Newton convergence is stalling.
It may be that Newton just cannot solve
this system, but it is more likely that there is an error in the Jacobian.
In order to test this, you can use

  -snes_fd

which is very slow, but forms the entire Jacobian using finite differences.

  Thanks,

     Matt


> -Peter
>
> On 8/10/11 8:53 AM, "Barry Smith" <bsmith at mcs.anl.gov> wrote:
>
>
>
>   Peter,
>
>    Matt gave some bad advice. Please rerun with -ksp_type fgmre
> -ksp_monitor -ksp_converged_reason as well as the -snes_monitor and send the
> output.  (With the preonly it doesn't produce enough useful information
> about how the linear solver did).
>
> > Along the way (with -info) I get messages of the form:
> > SNESLSCheckResidual_Private(): ||J^T(F-Ax)||/||F-AX|| 208.031 near zero
> implies inconsistent rhs
> >
> > At the end, I get:
> > [0] SNESLSCheckLocalMin_Private(): || J^T F|| 80.3 near zero implies
> found a local minimum
>
>   These are fine, there is no local minimum. By "near zero" we mean like
> 10^-7.  These are no where near zero.
>
>
>   Barry
>
> On Aug 10, 2011, at 9:06 AM, Graf, Peter wrote:
>
> > Hi Matt,
> > Thanks for your reply.  Attached is the output as requested,
> specifically, of
> >
> > ./snes_test -snes_monitor -snes_converged_reason -ksp_converged_reason
> -snes_view -snes_monitor -ksp_monitor -ksp_type preonly -pc_type lu
> >
> > For good measure (?), I am also attaching the case with "-info".
> >
> > (R.e. terminology, I was contrasting the case here with the Dirichlet
> case of, e.g., u(0) =1, u(1) = 1.  Actually I think I'm still wrong, in that
> my BCs are mixed type,... but thanks for being clear)
> >
> > Again, thank you very much for any help.
> > -Peter
> >
> > On 8/10/11 7:13 AM, "Matthew Knepley" <knepley at gmail.com> wrote:
> >
> > On Wed, Aug 10, 2011 at 12:39 PM, Graf, Peter <Peter.Graf at nrel.gov>
> wrote:
> > Dear Petsc-Users,
> > I am implementing a 1D solution of the (drift/diffusion) semiconductor
> equations.  The case of Dirichlet boundaries works fine.  The case of
> Neumann boundaries (i.e. current boundary conditions such as occur at
> semiconductor/metal contacts) does not.   In sorting this out, I have
> modified one of your examples to recreate the problem:
> >
> > The (modified) example seeks to solve
> > u`` + u^{2} = f
> > on [0,1] with
> > u'(0) = U0
> > u(1) = 1
> >
> > With U0 = 0, the SNES solver converges.  With U0=-2 (for example) it does
> not:
> >
> > Just to be clear, both of these are Neumann conditions.
> >
> > 11 SNES Function norm 5.018284941187e+00
> > Nonlinear solve did not converge due to DIVERGED_LS_FAILURE
> >
> > When asking about convergence, please send the entire output of
> -snes_view -snes_monitor -ksp_monitor -snes_converged_reason
> >
> > Since the Jacobian becomes more ill-conditioned, its possible the linear
> solver is not converging. When testing, its best to always use
> > -ksp_type preonly -pc_type lu so that the solves are exact to start out.
> >
> >  Thanks,
> >
> >      Matt
> >
> > (interestingly, it still comes up with roughly good solutions, e.g. at
> least visually, which suggests to me my discretization is not _total_
> garbage)
> >
> > Along the way (with -info) I get messages of the form:
> > SNESLSCheckResidual_Private(): ||J^T(F-Ax)||/||F-AX|| 208.031 near zero
> implies inconsistent rhs
> >
> > At the end, I get:
> > [0] SNESLSCheckLocalMin_Private(): || J^T F|| 80.3 near zero implies
> found a local minimum
> >
> > I have used -snes_type test to verify that my Jacobian is accurate.
> > I have also dumped the Jacobians to a file and examined their condition
> numbers.  The nonconverged cases have condition numbers around 10^7, whereas
> the converged cases have condition numbers around 10^3.  This seems like a
> clue, but I'm not sure what to do about it.
> >
> > All this is completely analogous to the real case of interest (in that
> case, my condition numbers go to 10^13 or more, and there is similar visual
> evidence that the solver is "trying" to do the right thing).
> > I would love to have help getting this to work.  Specifically, what does
> the message about the "inconsistent rhs" tell me I'm doing wrong?  What is
> the evidence of the condition number telling me?  Do I have an error
> somewhere, or do I have a legitimately ill-conditioned Jacobian (in which
> case, what should I do about that?)?
> >
> > Thank you very much for any advice,
> >
> > Peter Graf
> > NREL Scientific Computing Center
> > Golden, CO
> >
> >
> > <verbose_output2><verbose_output_info>
>
>
>


-- 
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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