[petsc-users] SNES Line Search Monitor

[Matthew Knepley]

On Sun, Aug 18, 2013 at 8:28 AM, Peter Brune <prbrune at gmail.com> wrote:

>
>
>
> 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?
>

Please repeat the runs with -snes_view. That way we can see what line
search is used in each run.

   Matt


> 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
>>>>
>>>>
>>>
>>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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