[petsc-users] SNES with constraint diverges

[Josh L]

Hi,

my solution is mostly very close to 1. only  for a very small area where
solution goes from 0 to 1(a smeared crack).

I set -snes_atol 1e-7 and it is converging.

I've noticed the following:

There is a difference between the function norm.

I calculate the function norm in FormFunction, so every time it is called
it gives the function norm
, and the result is different from the function norm given by -snes_monitor
if i set

     SNESSetType(snes,SNESVINEWTONRSLS,ierr)
     SNESVISetVariableBounds(snes,xl,xu,ierr)

The function norm calculated in FromFunction is NOT reducing, however, the
function norm given by -snes_monitor is reducing
They are the same if I just use regular SNES without setting variable
bounds.


Thanks,
Josh

2018-09-12 12:02 GMT-05:00 Smith, Barry F. <bsmith at mcs.anl.gov>:

>
>      You have too tight a convergence tolerance for your problem.  You
> can't expect to get more than 1.e-12 as the minimum residual norm or even
> less.
>
>       How close is your solution to 1 and -1?
>
>       If you really need much higher convergence you can try ./configure
> --with-precision=__float128
>
>
>       Barry
>
> > On Sep 11, 2018, at 11:53 PM, Josh L <ysjosh.lo at gmail.com> wrote:
> >
> > Yes, I initialize all u_i to 1.0
> >
> >
> >
> > 2018-09-11 23:37 GMT-05:00 Smith, Barry F. <bsmith at mcs.anl.gov>:
> >
> >    Do you start with initial conditions of  0 <= u_i <= 1 ?
> >
> >     Run with -snes_monitor -snes_converged_reason
> -ksp_monitor_true_residual -info -snes_linesearch_monitor and send all the
> output
> >
> >   Barry
> >
> >
> > > On Sep 11, 2018, at 11:33 PM, Josh L <ysjosh.lo at gmail.com> wrote:
> > >
> > > Hi,
> > >
> > > I am using SNES to solve an nonlinear equation f(u), and I know all
> the u_i should be 0 and 1.
> > >
> > > First, I use SNES without constraint, and it converges.
> > >
> > > But, If I set
> > >      SNESSetType(snes,SNESVINEWTONRSLS,ierr)
> > >      SNESVISetVariableBounds(snes,xl,xu,ierr)
> > >
> > > where xl and xu is vector, and xl_i=0 and xu_i=1
> > >
> > > then SNES fails to converge, because linesearch fails(snes reason =
> -6), and the norm of residual is not reducing(the norm of incremental
> solution is reducing)
> > >
> > > The reason to add constraint is that I want to implement some
> irreversibility.
> > >
> > >
> > > Thanks,
> > > Josh
> > >
> >
> >
>
>
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