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

[David Knezevic]

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.

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/>
>
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