[petsc-users] Line search Ended due to ynorm, nondeterministic stagnation

[Mark Lohry]

Symptom: running the same code on the same core count multiple times, 80%
of the time it converges out no problem. 20% of the time SNES seems to
reset to the previous step, showing
"Line search: Ended due to ynorm < stol*xnorm"

Sample output below. Everything is healthy until TS step 7, which starts
with a norm of 1.181275684011e-04. After that point, every KSP converges to
RTOL and I see the line search message, and the subsequent timesteps all
seem like they don't use the previous update, because they have exactly the
same residuals.

Running the same thing again, it happily proceeds through and converges out
without the stagnation.

Does this sound familiar to anyone?


5 TS dt 30. time 150.
    0 SNES Function norm 2.654593713313e-03
      0 KSP Residual norm 2.654593713313e-03
...
      41 KSP Residual norm 2.515907124549e-04
    Linear solve converged due to CONVERGED_RTOL iterations 41
        Line search: gnorm after quadratic fit 2.445531672458e-03
        Line search: Quadratically determined step,
lambda=1.5043681801723077e-01
    1 SNES Function norm 2.445531672458e-03
      0 KSP Residual norm 2.445531672458e-03
...
     40 KSP Residual norm 2.281075491298e-04
    Linear solve converged due to CONVERGED_RTOL iterations 40
        Line search: gnorm after quadratic fit 2.158781959371e-03
        Line search: Quadratically determined step,
lambda=2.1593140643569805e-01
    2 SNES Function norm 2.158781959371e-03
      0 KSP Residual norm 2.158781959371e-03
     40 KSP Residual norm 1.902363860750e-04
    Linear solve converged due to CONVERGED_RTOL iterations 40
        Line search: gnorm after quadratic fit 1.727564041943e-03
        Line search: Quadratically determined step,
lambda=3.7179194957755984e-01
    3 SNES Function norm 1.727564041943e-03
      0 KSP Residual norm 1.727564041943e-03
...
     38 KSP Residual norm 1.572811703375e-04
    Linear solve converged due to CONVERGED_RTOL iterations 38
        Line search: Using full step: fnorm 1.727564041943e-03 gnorm
1.074289112389e-03
    4 SNES Function norm 1.074289112389e-03
      0 KSP Residual norm 1.074289112389e-03
...
      18 KSP Residual norm 1.047939125382e-04
    Linear solve converged due to CONVERGED_RTOL iterations 18
        Line search: Using full step: fnorm 1.074289112389e-03 gnorm
1.049627557624e-04
    5 SNES Function norm 1.049627557624e-04
  Nonlinear solve converged due to CONVERGED_FNORM_RELATIVE iterations 5
      TSAdapt none beuler 0: step   5 accepted t=150        + 3.000e+01
dt=3.000e+01
 6 TS dt 30. time 180.
    0 SNES Function norm 1.181275684011e-04
      0 KSP Residual norm 1.181275684011e-04
...

     44 KSP Residual norm 1.107203720458e-05
    Linear solve converged due to CONVERGED_RTOL iterations 44
        Line search: Ended due to ynorm < stol*xnorm (1.034132241459e-04 <
2.190637006857e-04).
    1 SNES Function norm 1.181275684011e-04
  Nonlinear solve converged due to CONVERGED_SNORM_RELATIVE iterations 1
      TSAdapt none beuler 0: step   6 accepted t=180        + 3.000e+01
dt=3.000e+01
7 TS dt 30. time 210.
    0 SNES Function norm 1.181275684011e-04
      0 KSP Residual norm 1.181275684011e-04
...
44 KSP Residual norm 1.110103600083e-05
    Linear solve converged due to CONVERGED_RTOL iterations 44
        Line search: Ended due to ynorm < stol*xnorm (1.047067861804e-04 <
2.190637006857e-04).
    1 SNES Function norm 1.181275684011e-04
  Nonlinear solve converged due to CONVERGED_SNORM_RELATIVE iterations 1
      TSAdapt none beuler 0: step   7 accepted t=210        + 3.000e+01
dt=3.000e+01
8 TS dt 30. time 240.
    0 SNES Function norm 1.181275684011e-04
      0 KSP Residual norm 1.181275684011e-04
...
     44 KSP Residual norm 1.102709658960e-05
    Linear solve converged due to CONVERGED_RTOL iterations 44
        Line search: Ended due to ynorm < stol*xnorm (1.035351833216e-04 <
2.190637006857e-04).
    1 SNES Function norm 1.181275684011e-04
  Nonlinear solve converged due to CONVERGED_SNORM_RELATIVE iterations 1
      TSAdapt none beuler 0: step   8 accepted t=240        + 3.000e+01
dt=3.000e+01
         9  2.700e+02  3.000e+01  1.72412e-09  2.28859e-07  2.39889e-07
 9.69858e-08  4.71512e-06  3.30532e-02  1.83203e+00 -3.38502e-01
-3.09701e+00  1.96301e-02  3.53675e-02  3.96395e+01
9 TS dt 30. time 270.
    0 SNES Function norm 1.181275684011e-04
      0 KSP Residual norm 1.181275684011e-04
...
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