[petsc-users] Test convergence with non linear preconditioners

[Matthew Knepley]

On Thu, Aug 6, 2020 at 10:08 PM Adolfo Rodriguez <adantra at gmail.com> wrote:

> Considering the output produced by snes_view (attachment), would be
> possible to change the linear solver tolerances and the preconditioning
> level or type?
>

Yes. The options prefix is shown for each subsolver. For example, you can
change the linear solver type for the coarse level of FAS using

  -npc_fas_coarse_ksp_type bigcg

Notice that you are using 1 level of FAS, so its the same as just Newton.

  Thanks,

     Matt


> Adolfo
>
>
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> On Wed, Aug 5, 2020 at 8:31 PM Barry Smith <bsmith at petsc.dev> wrote:
>
>>
>>    Turtles on top of turtles on top of turtles.
>>
>>    It is probably easiest for you to look at the actual code to see how
>> it handles things
>>
>>   1) the SNESFAS uses SNES for each of the levels, for each of these
>> level SNES you can control the convergence criteria (either from the
>> command lineor with appropriate prefix (highly recommended) or with
>> function calls (not recommended)); and even provide your own convergence
>> functions  run with -snes_view to see the various solvers and their
>> prefixes).
>>
>>   2) at the finest level of SNESFAS it does call
>>
>>     /* Test for convergence */
>>     if (isFine) {
>>       ierr =
>> (*snes->ops->converged)(snes,snes->iter,0.0,0.0,snes->norm,&snes->reason,snes->cnvP);CHKERRQ(ierr);
>>       if (snes->reason) break;
>>     }
>>
>> src/snes/impls/fas/fas.c line 881 so at least in theory you can provide
>> your own convergence test function.
>>
>>   It was certainly our intention that users can control all the
>> convergence knobs for arbitrary imbedded nonlinear solvers including FAS
>> but, of course, there may be bugs so let us know what doesn't work.
>>
>>  Generally the model for FAS is to run a single (or small number of)
>> iteration(s) on the level solves and so not directly use convergence
>> tolerances like rtol to control the number of iterations on a level but you
>> should be able to set any criteria you want.
>>
>>   You should be able to run with -snes_view and change some of the
>> criteria on the command line and see the changes presented in the
>> -snes_view output, plus see differences in convergence behavior.
>>
>>
>>
>>   Barry
>>
>>
>>
>> On Aug 5, 2020, at 8:10 PM, Adolfo Rodriguez <adantra at gmail.com> wrote:
>>
>> It looks like I cannot really change the test function or anything else
>> for this particular SNES solver  (I am using SNESFas). Basically, I am
>> trying to use the ideas exposed in the paper on Composing scalable solvers
>> but it seems that SNESFas does not allow to change the function for testing
>> convergence, or anything else. Is this correct?
>>
>> Adolfo
>>
>>
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>> On Wed, Aug 5, 2020 at 3:41 PM Barry Smith <bsmith at petsc.dev> wrote:
>>
>>>
>>>    Adolfo,
>>>
>>>      You can also just change the tolerances for the inner solve using
>>> the options data base and the prefix for the inner solve.
>>>
>>>      When you run with -snes_view it will show the prefix for each of
>>> the (nested) solvers. You can also run with -help to get all the possible
>>> options for the inner solvers.
>>>
>>>      In this case I think the prefix is npc so you can set tolerances
>>> with -npc_snes_rtol <rtol> -npc_ksp_rtol <rtol> etc.  From the program you
>>> can use SNESGetNPC() and then call SNESSetXXX() to set options, for the
>>> linear solver (if there is one) call SNESGetKSP() on the npc and then set
>>> options on that KSP.
>>>
>>>
>>>    Barry
>>>
>>>
>>> On Aug 5, 2020, at 3:30 PM, Adolfo Rodriguez <adantra at gmail.com> wrote:
>>>
>>> Jed,
>>>
>>> I tred your suggestion
>>>
>>>   SNESGetNPC(snes, &inner);
>>>   SNESSetConvergenceTest(inner, YourFunc, ...);
>>>
>>> and it is working as expected. I had not pieced together the fact that
>>> "inner" is a "snes" object as well.
>>>
>>> Thanks!
>>>
>>>
>>>
>>>
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>>>
>>> On Wed, Aug 5, 2020 at 3:11 PM Jed Brown <jed at jedbrown.org> wrote:
>>>
>>>> Adolfo Rodriguez <adantra at gmail.com> writes:
>>>>
>>>> > Actually I can set the non-linear pc. My problem relates to the
>>>> convergence
>>>> > test. I have not found a way to set the tolerances for the inner
>>>> problem,
>>>> > are they hard coded?
>>>>
>>>> I suggested code to set a custom convergence test.  Nothing is
>>>> hard-coded.  What have you done?
>>>>
>>>
>>>
>>

-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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