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

[Matthew Knepley]

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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