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

[Josimar Alves da Silva]

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.

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 ?

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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