[petsc-users] Non-linear solve: DIVERGED_LINE_SEARCH

[Karthikeyan Chockalingam - STFC UKRI]

I am trying to solve the non-linear scalar problem

-\nabla \cdot \big( (1+u^2)\nabla u\big)=f

Using finite element formulation.



The linear iteration converged – does that mean my Jacobian is correct??



I am hoping the non-linear residual goes down to 1-e11.



Kind regards,

Karthik.


From: Stefano Zampini <stefano.zampini at gmail.com>
Date: Thursday, 7 September 2023 at 11:49
To: Chockalingam, Karthikeyan (STFC,DL,HC) <karthikeyan.chockalingam at stfc.ac.uk>
Cc: petsc-users at mcs.anl.gov <petsc-users at mcs.anl.gov>
Subject: Re: [petsc-users] Non-linear solve: DIVERGED_LINE_SEARCH
The solver did not diverge.
It was the line search that was not able to make further progress in minimizing the 2-norm of the residual.
This is common in nonlinear solvers. It would help if you tell us what you are trying to solve.
Note that at the first step, your residual norm is already 1.e-6. What kind of accuracy do you want?


Il giorno gio 7 set 2023 alle ore 13:08 Karthikeyan Chockalingam - STFC UKRI via petsc-users <petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov>> ha scritto:
Hello,

The non-linear solution diverged. The final solution seems right and I believe the Jacobian is correct (not 100% certain).

I am not sure if I doing something wrong in the solver setting.



 0 SNES Function norm 3.890991210938e-03

    0 KSP Residual norm 9.037762538598e+00

    1 KSP Residual norm 2.120375403775e-01

    2 KSP Residual norm 5.155439334511e-03

    3 KSP Residual norm 1.394364169369e-04

    4 KSP Residual norm 9.233543407204e-06

  Linear solve converged due to CONVERGED_RTOL iterations 4



      Line search: Using full step: fnorm 3.890991210938e-03 gnorm 7.701511565083e-06

  1 SNES Function norm 7.701511565083e-06

    0 KSP Residual norm 5.630229687829e-03

    1 KSP Residual norm 1.030475601271e-04

    2 KSP Residual norm 2.576454714319e-06

    3 KSP Residual norm 6.669316846898e-08

    4 KSP Residual norm 3.215810984829e-09

  Linear solve converged due to CONVERGED_RTOL iterations 4



      Line search: gnorm after quadratic fit 1.805500533481e-05

      Line search: Cubic step no good, shrinking lambda, current gnorm 2.563759884284e-05 lambda=3.0804668685096816e-02

      Line search: Cubic step no good, shrinking lambda, current gnorm 3.332721829751e-05 lambda=3.0804668685096817e-03

      Line search: Cubic step no good, shrinking lambda, current gnorm 4.102754045833e-05 lambda=3.0804668685096822e-04

      Line search: Cubic step no good, shrinking lambda, current gnorm 4.872893294880e-05 lambda=3.0804668685096822e-05

      Line search: Cubic step no good, shrinking lambda, current gnorm 5.643043250787e-05 lambda=3.0804668685096822e-06

      Line search: Cubic step no good, shrinking lambda, current gnorm 6.413194279696e-05 lambda=3.0804668685096827e-07

      Line search: Cubic step no good, shrinking lambda, current gnorm 7.183345417492e-05 lambda=3.0804668685096828e-08

      Line search: Cubic step no good, shrinking lambda, current gnorm 7.953496567312e-05 lambda=3.0804668685096829e-09

      Line search: Cubic step no good, shrinking lambda, current gnorm 8.723647719173e-05 lambda=3.0804668685096831e-10

      Line search: Cubic step no good, shrinking lambda, current gnorm 9.493798871875e-05 lambda=3.0804668685096832e-11

      Line search: Cubic step no good, shrinking lambda, current gnorm 1.026395002516e-04 lambda=3.0804668685096835e-12

      Line search: Cubic step no good, shrinking lambda, current gnorm 1.103410117889e-04 lambda=3.0804668685096835e-13

      Line search: unable to find good step length! After 12 tries

      Line search: fnorm=7.7015115650831560e-06, gnorm=1.1034101178892401e-04, ynorm=6.2052357872955976e-03, minlambda=9.9999999999999998e-13, lambda=3.0804668685096835e-13, initial slope=-5.9313318983096354e-11

Nonlinear solve did not converge due to DIVERGED_LINE_SEARCH iterations 1


Thank you for your help.

Kind regards,
Karthik.

--
Dr. Karthik Chockalingam
Senior Research Software Engineer
High Performance Systems Engineering Group
Hartree Centre | Science and Technology Facilities Council
karthikeyan.chockalingam at stfc.ac.uk<mailto:karthikeyan.chockalingam at stfc.ac.uk>

 [signature_3970890138]



--
Stefano
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