[petsc-users] TS (arkimex) question

[Constantinescu, Emil M.]

On 1/19/20 11:38 AM, Mark Adams wrote:
Can you recommend a higher order method that I might try?


Mark, all of 2e, 3, 4, 5 are high order with really good properties. They have error estimators that are cheaper but less reliable (most of the time work well enough).

Emil

On Sun, Jan 19, 2020 at 10:37 AM Jed Brown <jed at jedbrown.org<mailto:jed at jedbrown.org>> wrote:
Use -ts_adapt_monitor to see the rationale.

Note that 1bee is backward Euler with an extrapolation error estimator
(for adaptive control).  It's still only first order accurate, and the
longer step may be part of your SNES issues.

You can set a maximum time step (-ts_adapt_dt_max) or be more aggressive
about reducing time step in response to SNES failure
(-ts_adapt_scale_solve_failed) or remember that failure for longer
before increasing the step again (-ts_adapt_time_step_increase_delay) or
more gradually increase time step when permitted (-ts_adapt_clip).

Mark Adams <mfadams at lbl.gov<mailto:mfadams at lbl.gov>> writes:

> I am using -ts_type arkimex -ts_arkimex_type 1bee -ts_max_snes_failures -1
> -ts_rtol 1e-6 -ts_dt 1.e-7
>
> First ,Jed gave me these parameters. This is not a DAE, just a fully
> implicit solve. Advice on parameters welcome.
>
> Second, TS is reporting a large time step (0.0505357) that is wrong.
>
> Third, it repeatedly takes this extra one or two (its a 3 step method) step
> due to SNES failure. I wonder if that can be optimized.
>
> Thanks,
> Mark
>
>
> ....
>     9 SNES Function norm 1.438395286712e-06
>    10 SNES Function norm 8.050454869525e-07
>   Nonlinear solve converged due to CONVERGED_SNORM_RELATIVE iterations 10
> [0] TSAdaptChoose_Basic(): Estimated scaled local truncation error
> 0.00461254, *accepting step of size 0.00304954*
> 600 TS *dt 0.0304954* time 0.697817
>     0 SNES Function norm 1.018387577463e-02
> ...
>    23 SNES Function norm 6.583420045281e-05
>    24 SNES Function norm 5.959294539241e-05
>    25 SNES Function norm 5.394347124131e-05
>   Nonlinear solve did not converge due to *DIVERGED_MAX_IT* iterations 25
>     0 SNES Function norm 1.018387577468e-02
>   ...
>    24 SNES Function norm 1.000717662032e-06
>    25 SNES Function norm 7.741622573808e-07
>   Nonlinear solve converged due to *CONVERGED_SNORM_RELATIVE* iterations 25
>     0 SNES Function norm 1.014795904701e-02
> ...
>    15 SNES Function norm 1.334407891279e-06
>    16 SNES Function norm 9.148934277015e-07
>   Nonlinear solve converged due to *CONVERGED_SNORM_RELATIVE* iterations 16
>     0 SNES Function norm 1.016588008759e-02
>   ...
>    16 SNES Function norm 9.144418053264e-07
>   Nonlinear solve converged due to *CONVERGED_SNORM_RELATIVE* iterations 16
> [0] TSAdaptChoose_Basic(): Estimated scaled local truncation error
> 0.0184347, *accepting step of size 0.00762384*
> 601 TS *dt 0.0505357 *time 0.705441
>     0 SNES Function norm 1.014792968017e-02
>     1 SNES Function norm 1.026477259201e-03
>     2 SNES Function norm 6.170336507030e-04
>     3 SNES Function norm 5.433176612554e-04
>     4 SNES Function norm 5.196626557375e-04
>     5 SNES Function norm 4.977855046309e-04
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