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

[Matthew Knepley]

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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