[petsc-users] SNES Line Search Monitor

[Su Yan]

Hi, I ran into something really weird when I tried to solve a nonlinear
equation with Newton method and line search. Specifically, I used
SNESSetType(snesMbr, SNESNEWTONLS); and
PetscOptionsSetValue("-snes_linesearch_type", "l2");

When I execute my program abc.bin with the following command:

./abc.bin -snes_monitor -snes_linesearch_monitor

I got the following output:

0 SNES Function norm 1.457697974866e+07
       Line search: lambdas = [1, 0.5, 0], fnorms = [669102, 7.35102e+06,
1.4577e+07]
       Line search terminated: lambda = 1.00553, fnorms = 652606
1 SNES Function norm 6.526060362905e+05
       Line search: lambdas = [1, 0.5, 0], fnorms = [3406.6, 326873, 652606]
       Line search terminated: lambda = 1.00171, fnorms = 2801.6
2 SNES Function norm 2.801596249480e+03
       Line search: lambdas = [1, 0.5, 0], fnorms = [2.51242, 1401.2,
2801.6]
       Line search terminated: lambda = 1.00029, fnorms = 2.09292
3 SNES Function norm 2.092918540169e+00
       Line search: lambdas = [1, 0.5, 0], fnorms = [0.000123295, 1.04646,
2.09292]
       Line search terminated: lambda = 1, fnorms = 0.000122588
4 SNES Function norm 1.225883678418e-04

Converged Reason: FNORM_RELATIVE

The nonlinear problem converged normally with a relative f_norm set as 1E-7.

However, if I execute exactly the same program, but with a slightly
different runtime command:

./abc.bin -snes_monitor

I got the following output:

 0 SNES Function norm 1.457697974975e+07
1 SNES Function norm 6.526060348917e+05
2 SNES Function norm 2.801608208510e+03
3 SNES Function norm 2.450488738084e+03
4 SNES Function norm 3.269507987119e+02
5 SNES Function norm 3.016606325384e+02
6 SNES Function norm 2.463851989463e+02
7 SNES Function norm 1.546266418976e+02
8 SNES Function norm 1.492518400407e+02
9 SNES Function norm 1.477122410995e+02
10 SNES Function norm 1.503359418680e+02
11 SNES Function norm 1.504759910776e+02
12 SNES Function norm 1.417592634863e+02
13 SNES Function norm 3.047096130411e+05

and the solver diverged.

The only difference was that whether I used "-snes_linesearch_monitor" or
not.

In my understanding, this runtime option only turns on the screen print. So
why did it make such a big difference? Is there anything special with this
option turned on? Hope someone could help me out.

Thanks a lot.

Regards,
Su
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