[petsc-users] snes linesearch bt vs basic?

[Xiangdong]

I did not use the TS, but I did adjust my time step based on the number of
snes iterations as well as the solution changes in the current time step.
We want to minimize the time cuts (failed snes) as well as not using too
small dt.

If I want to know how petsc computes the dt_next based on dt_current,
namely, dt_next = dt_current*func(snesits, etc), which function/file should
I look into?

Thanks.

Xiangdong

On Mon, Feb 1, 2016 at 3:16 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> > On Feb 1, 2016, at 1:46 PM, Xiangdong <epscodes at gmail.com> wrote:
> >
> > Hello everyone,
> >
> > I have a question on snes with line search. Under what circumstance,
> applying the (globalized) line search can perform WORSE than without line
> search (i.e. full newton step ls type basic)?
> >
> > When I apply snes with my problem, I found that at some time steps, snes
> diverges with linesearch_bt, but can converge with linesearch_basic (using
> same initial guess). For the same problem, there are also some steps, at
> which linearsearch_basic fails, but linesearch_bt converges.
>
>    Here are you referring to "time steps"?  When doing time stepping,
> depending on the evolution of the equations, the nonlinear systems can
> become more difficult to solver or easier to solve. One can often adjust
> the time-step size to make sure the nonlinear equations are never to
> difficult. So for example if the nonlinear solver is taking more than, say,
> 10 Newton steps, or is failing, decrease the timestep by some factor and
> try again. If the nonlinear solver needs only 1 or 2 Newton steps one can
> try increasing the time-step. Ideally this is also done in conjunction with
> an ODE error estimator to control the error by adjusting the timestep. Some
> of the PETSc implicit TS ODE integrators do this automatically. Generally
> adjusting the timestep is a more reliable way to insure convergence than
> trying to produce a completely robust nonlinear solver (as Matt points out
> if you did develop a completely robust nonlinear solver you could retire on
> your fame).
>
>    Barry
>
>
> > Is there a general guide line to decide whether linesearch should be
> used?
> >
> > Thanks.
> >
> > Xiangdong
>
>
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