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

[Matthew Knepley]

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
>>>
>>>
>>>
>>> _______________________________________________
>>> CIG-SHORT mailing list
>>> CIG-SHORT at geodynamics.org
>>> http://lists.geodynamics.org/cgi-bin/mailman/listinfo/cig-short
>>>
>>
>>
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>
>
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