[petsc-users] why we have a zero residual?

[Matthew Knepley]

On Fri, May 15, 2015 at 1:45 PM, Fande Kong <fande.kong at colorado.edu> wrote:

> Hi all,
>
> I was wondering why we will get a zero residual when solving the linear
> step of a nonlinear problem? I am solving a fluid-structure interaction
> problems using inexact Newton method, within which fGMRES is used together
> with a multilevel Schwarz preconditioner. The algorithm works well when the
> mesh is small, but it catched a zero residual issue when we turn to use a
> little bit large mesh.  The output is listed as follows:
>
> 0 SNES Function norm 1.291775443677e-02
>   Jacobian: rows 5575441  Column 5575441
> call DMGetInterpolation_SpmcsDMMesh
>   Jacobian: rows 1298448  Column 1298448
>     0 KSP Residual norm 1.291775443677e-02
>     1 KSP Residual norm 0.000000000000e+00
> Nonlinear solve did not converge due to DIVERGED_LINE_SEARCH iterations 0
>

This looks like a mistake in the Jacobian or preconditioner, since you have
a 0 residual, but
a bad descent direction.

  Matt


> Any suggestions? Thanks.
>
> Fande,
>

-- 
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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