[petsc-users] Can we force SNES solver to do at least Newton step?

[David Knezevic]

>
> Hmm, it sounds like the convergence measure is bad. Maybe using a weighted
> norm would be better?


That's a good thought, I'd like to look into that idea too. Could you
please give me some guidance on how to use a weighted norm in the
convergence test? (Or are there any examples of doing that in the example
suite?)

Thanks,
David

On Sat, Sep 2, 2023 at 5:54 PM Matthew Knepley <knepley at gmail.com> wrote:

> On Sat, Sep 2, 2023 at 5:45 PM David Knezevic <david.knezevic at akselos.com>
> wrote:
>
>> OK, thanks, I'll look into the custom convergence test.
>>
>> I do not understand this comment. What do you mean by "inaccurate"? Since
>>> we do not have the true solution, we usually say "inaccurate" for large
>>> residual, but you already said that the residual is small. Why would you
>>> want to do another iterate?
>>
>>
>> I agree with your comments, but the specific case I'm considering is very
>> numerically sensitive since it includes creep (which unfortunately has
>> large exponential terms in it) which is the root cause of the issues I'm
>> facing. Based on test cases with a known reference solution we're finding
>> that we get inaccurate results due to steps with "zero iterations". We can
>> fix this by tightening the tolerance but then we do an excessive number of
>> iterations in other steps. So it seems to me that ensuring that we do at
>> least one iteration will help here, so that's what I wanted to try.
>>
>
> Hmm, it sounds like the convergence measure is bad. Maybe using a weighted
> norm would be better?
>
>   Thanks,
>
>      Matt
>
>
>> Thanks again for your help.
>>
>> Best,
>> David
>>
>> On Sat, Sep 2, 2023 at 3:23 PM Matthew Knepley <knepley at gmail.com> wrote:
>>
>>> On Sat, Sep 2, 2023 at 3:05 PM David Knezevic via petsc-users <
>>> petsc-users at mcs.anl.gov> wrote:
>>>
>>>> Hi all,
>>>>
>>>> I'm using the SNES solver for a plasticity model, and the issue I've
>>>> run into is that in some time steps the solver terminates after "NL step 0"
>>>> since the initial residual (based on the solution from the previous time
>>>> step) is below the specified tolerance.
>>>>
>>>> I gather that "NL step 0" only checks the residual and doesn't actually
>>>> do a Newtown update, and hence it seems that this is leading to inaccurate
>>>> results in some cases.
>>>>
>>>
>>> I do not understand this comment. What do you mean by "inaccurate"?
>>> Since we do not have the true solution, we usually say "inaccurate" for
>>> large residual, but you already said that the residual is small.
>>> Why would you want to do another iterate?
>>>
>>>
>>>> I can of course specify a smaller convergence tolerance to avoid this
>>>> issue, but I've found it difficult to find a smaller tolerance that works
>>>> well in all cases (e.g. it leads to too many iterations or
>>>> non-convergence). So instead what I would like to do is ensure that the
>>>> solver does at least 1 Newton iteration instead of terminating at "NL step
>>>> 0". Is there a way to enforce this behavior, e.g. by skipping "NL step 0",
>>>> or specifying a "minimum number of iterations"? I didn't see anything like
>>>> this in the documentation, so I was wondering if there are any suggestions
>>>> on how to proceed for this.
>>>>
>>>
>>> The easiest way to do this is to write a custom convergence test that
>>> looks like this
>>>
>>> PetscErrorCode SNESConvergedDefault(SNES snes, PetscInt it, PetscReal
>>> xnorm, PetscReal snorm, PetscReal fnorm, SNESConvergedReason *reason, void
>>> *dummy)
>>> {
>>>   PetscFunctionBeginUser;
>>>   if (!it) {
>>>     *reason = SNES_CONVERGED_ITERATING;
>>>     PetscFunctionReturn(PETSC_SUCCESS);
>>>   }
>>>   PetscCall(SNESConvergedDefault(snes, it, xnorm, snorm, fnorm, reason,
>>> dummy));
>>>   PetscFunctionReturn(PETSC_SUCCESS);
>>> }
>>>
>>>   Thanks,
>>>
>>>      Matt
>>>
>>>
>>>> Thanks,
>>>> David
>>>>
>>>
>>>
>>> --
>>> 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/>
>>>
>>
>
> --
> 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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