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

[Barry Smith]

  Also always use -snes_linesearch_monitor in these cases.

  Also when something goes wrong in a nonlinear solver with a fancy preconditioner on the linear solver it is good to make a run with a direct solver to make sure the issue is not with the preconditioner; so -pc_type lu


> On May 15, 2015, at 2:04 PM, Dmitry Karpeyev <karpeev at mcs.anl.gov> wrote:
> 
> What does -ksp_monitor_true_residual show?
> 
> 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
> 
> 
> Any suggestions? Thanks.
> 
> Fande,
> 
> 
>