[CIG-SHORT] PyLith Convergence issue - DIVERGED_FUNCTION_COUNT and CONVERGED_SNORM_RELATIVE

[Josimar Alves da Silva]

Matt,

Thanks for the help. Here is the log file with more information:
https://www.dropbox.com/s/qnpk055o9xg38j1/log.out?dl=0

Let me know if you need anything else.

thank you,
Josimar

On Thu, Jun 22, 2017 at 3:21 PM, Matthew Knepley <knepley at rice.edu> wrote:

> On Thu, Jun 22, 2017 at 12:10 PM, Josimar Alves da Silva <
> jsilva.mit at gmail.com> wrote:
>
>> Matt,
>>
>> Thank you so much for the help. I followed up on your suggestions:
>>
>> 1) Using snes_max_funcs=10000 did not help. The simulation does not
>> go beyond iteration #5000. It stopped at the same time step as the previous
>> one.
>>
>
> Note this has to be in the [petsc] section. Please add snes_view. Then we
> can see exactly what the setting were.
> It would also be good to add snes_converged_reason
>
> I looked at the log. It makes as much as progress as it will ever make
> after 2 iterates, thus I think there
> is a problem with interplay of tolerances here.
>
>
>> 2) Can you comment a little bit on how can I improve my initial guess, as
>> you mentioned here : "if your initial guess is not good enough they can fail".
>> What you mean about it ?
>>
>
> I mean if you are taking time steps so big that the last solution is not
> close to the next one, the solves will be harder.
>
>   Matt
>
>
>> The log file is attached.
>>
>> thank you so much in advance,
>> Josimar
>>
>>
>>
>>
>> On Thu, Jun 22, 2017 at 11:33 AM, Matthew Knepley <knepley at rice.edu>
>> wrote:
>>
>>> On Wed, Jun 21, 2017 at 6:25 PM, Josimar Alves da Silva <
>>> jsilva.mit at gmail.com> wrote:
>>>
>>>> Dear Brad, Matt and Charles,
>>>>
>>>> I am working on a 2-D quasi-static, elastic, simulation using rate and
>>>> state friction model.
>>>>
>>>> The simulation runs fine for many time steps, then at some point I get
>>>> the error below related to DIVERGED_FUNCTION_COUNT.
>>>>
>>>> Nonlinear solve did not converge due to DIVERGED_FUNCTION_COUNT
>>>> iterations 5000
>>>>
>>>> All the log files, along with the .cfg files, model geometry and
>>>> boundary conditions as well as a convergence plot for the SNES residual can
>>>> be found on the attached .zip file.
>>>>
>>>> I have some questions that I would like to have your input:
>>>>
>>>> 1) Is it possible to extend the maximum value for
>>>> DIVERGED_FUNCTION_COUNT  ? Note that I have snes_max_it = 10000  but SNES
>>>> still stops at 5000 value.
>>>>
>>>
>>> snes_max_funcs 10000
>>>
>>>
>>>> 2) Looking at the convergence plots that I have attached, I noticed
>>>> that the time step that fails contains iterations that actually increase
>>>> the residual,instead of decreasing it. Would you know what is causing this
>>>> behavior ?
>>>>
>>>
>>> There is no guarantee of decrease with a general nonlinear problem.
>>>
>>>
>>>> 3) There are several time steps that converge with tolerances smaller
>>>> than what I ask for using the snes_atol (see figure attached). At these
>>>> time steps I get the following
>>>>
>>>> Line search: Aborted due to ynorm < stol*xnorm (9.933794721748e-10 <
>>>> 1.046238089915e-09) and inadequate full step.
>>>>
>>>> Nonlinear solve converged due to CONVERGED_SNORM_RELATIVE iterations 694
>>>>
>>>
>>> This means that even a very short step length gave no residual decrease,
>>> also known as "stagnation". Here is the nonlinear
>>> solver is just not working.
>>>
>>> We have no convergence theory for the friction problems, and if your
>>> initial guess is not good enough they can fail. We want to explore
>>> more powerful solvers for these, but after we get a revised formulation
>>> in place.
>>>
>>>  Thanks,
>>>
>>>     Matt
>>>
>>>
>>>> Note that I am using snes_error_if_not_converged = true.
>>>>
>>>> Could you please comment on why this is happening and how I would fix
>>>> this issue ? Does it affect the next time steps after it ?
>>>>
>>>> Let me know if you need any further information.
>>>>
>>>> Thank you so much for you help in advance,
>>>> Best
>>>> Josimar
>>>>
>>>>
>>>>
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>>>>
>>>
>>>
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>>
>>
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