[petsc-users] SNES with constraint diverges

[Smith, Barry F.]

   The function norm given by the -snes_monitor is the function norm for the DVI problem (which depends on where the solution is constrained) while FormFunction() norm you compute is for the original unconstrained problem.

    I am still not sure why you solve this as a DVI. If you solve it without the constraints do you get the "wrong" answer?

    Barry


> On Sep 12, 2018, at 3:32 PM, Josh L <ysjosh.lo at gmail.com> wrote:
> 
> 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  
> > >  
> > 
> > 
> 
>