[petsc-users] Test convergence with non linear preconditioners

Fri Aug 7 11:19:58 CDT 2020

Great, that works. What would be the way to change the ilu level, I need to
use ilu(1). I would assume that I can accomplish that by means of:

-npc_fas_coarse_pc_type ilu
-npc_fas_coarse_pc_ilu_levels 1

I noticed that the first line actually works, but I am not sure about the
second one.

Thanks,
Adolfo

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On Thu, Aug 6, 2020 at 9:23 PM Matthew Knepley <knepley at gmail.com> wrote:

> 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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