[petsc-users] SNES Line Search Monitor

[Peter Brune]

On Sat, Aug 17, 2013 at 6:46 PM, Su Yan <suyan0 at gmail.com> wrote:

> I am using PETSc 3.4.1. This problem is reproducible.
>

This is not reproduction of the problem.  Does it happen with a simple test
problem that you can allow us to debug?

Additionally, does your problem have the same convergence history every
time that it is run with the same options?  If not, then the
-snes_linesearch_monitor is most likely a misdiagnosis of some other
difficulty.

- Peter

I tested several other cases, almost every case has the similar problem.
> Take this one as an example:
>
> with ./abc.bin -snes_monitor -snes_linesearch_monitor
>
>  0 SNES Function norm 1.470975405802e+06
>       Line search: lambdas = [1, 0.5, 0], fnorms = [7051.02, 735338,
> 1.47098e+06]
>       Line search terminated: lambda = 0.999762, fnorms = 7026.38
>   1 SNES Function norm 7.026377609722e+03
>       Line search: lambdas = [1, 0.5, 0], fnorms = [189.711, 3514.55,
> 7026.38]
>       Line search terminated: lambda = 0.999295, fnorms = 189.783
>   2 SNES Function norm 1.897827226353e+02
>       Line search: lambdas = [1, 0.5, 0], fnorms = [32.3811, 100.302,
> 189.783]
>       Line search terminated: lambda = 1.01592, fnorms = 32.5125
>   3 SNES Function norm 3.251248928384e+01
>       Line search: lambdas = [1, 0.5, 0], fnorms = [0.00236829, 16.2564,
> 32.5125]
>       Line search terminated: lambda = 1.00001, fnorms = 0.00233087
>   4 SNES Function norm 2.330867616400e-03
>
> with ./abc.bin -snes_monitor
>
>   0 SNES Function norm 1.470975405802e+06
>   1 SNES Function norm 7.026377609170e+03
>   2 SNES Function norm 1.897827231670e+02
>   3 SNES Function norm 3.251248934919e+01
>   4 SNES Function norm 2.171496483661e-02
>
> Significant difference can be observed. The equation I am solving is quite
> ill-conditioned. Without L2 line search it is hard to converge in some
> cases. Still try to figure out the reason.
>
> Thanks,
> Su
>
> On Sat, Aug 17, 2013 at 8:49 AM, Peter Brune <brune at mcs.anl.gov> wrote:
>
>>
>>
>>
>> On Sat, Aug 17, 2013 at 1:40 AM, Su Yan <suyan0 at gmail.com> wrote:
>>
>>> 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");
>>>
>>>
>> Which version of PETSc are you using?
>>
>>
>>>  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
>>>
>>>
>> Is this problem reproducible?  Note that even your 0th norm is very
>> slightly different, and the first three norms are quite similar.
>>
>>
>>> 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.
>>>
>>
>> This is correct.  It should not influence the solution at all, and all it
>> does is enable printing.  Also, the l2 line search takes significantly more
>> work for well-behaved Newton convergence than bt, as bt automatically
>> accepts the full step.  l2 is mostly meant for cases where the step is
>> automatically ill-scaled.
>>
>> - Peter
>>
>>
>>>
>>> Thanks a lot.
>>>
>>> Regards,
>>> Su
>>>
>>>
>>
>
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