[petsc-users] SNES + linesearch hackery?

[Andrew McRae]

On 24 March 2016 at 19:49, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> > On Mar 24, 2016, at 2:41 PM, Andrew McRae <A.T.T.McRae at bath.ac.uk>
> wrote:
> >
> > Apologies, in the end it seems this was more of a Firedrake question:
> with the help of Lawrence Mitchell, I now believe I should simply intercept
> SNESFormFunction().
> >
> > On 24 March 2016 at 17:39, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >
> > > On Mar 24, 2016, at 10:18 AM, Andrew McRae <A.T.T.McRae at bath.ac.uk>
> wrote:
> > >
> > > I have a finite element discretisation of the following nonlinear
> equation:
> > >
> > > m*(phi_xx * phi_yy - phi_xy^2) = const,
> > >
> > > solving for phi.  Unfortunately, the function m depends on phi in a
> complicated way -- let's assume I need to call my own function to handle
> this.
> >
> >   Andrew
> >
> >    So you are actually solving
> >
> >   m(phi)*(phi_xx * phi_yy - phi_xy^2) - const = 0
> >
> >   with finite elements for phi?
> >
> >
> >     What are you providing for a Jacobian?
> >
> > The Jacobian I give treats m as being independent of phi, so just
> whatever you get from linearising det(Hessian(phi)).
>
>   Ahh, a Picard iteration :-)
>

Not quite, I think.

Ah, that should have read "so just m*(whatever you get from
linearising...)", and I was updating m between nonlinear iterations :)


>
>
> >
> >
> >
> > >
> > > I'm using PETSc's SNES  in Python via petsc4py, within the wider
> environment of the software Firedrake.
> > >
> > > Currently I'm hacking in the m update (and various output diagnostics)
> by writing a Python function "fakemonitor" and calling
> snes.setMonitor(fakemonitor).  This allows me to update m each nonlinear
> iteration.
> >
> >     Hmm, I don't understand this. It sounds like you are passing (phi_xx
> * phi_yy - phi_xy^2) or something to SNES as the SNESFormFunction()? Why is
> this? Why not pass the entire function to SNES?
> >
> > I was passing in m(phi^n)(phi_xx * phi_yy - phi_xy^2) - const, i.e., m
> was effectively frozen from the last nonlinear iteration.  As stated above,
> I think it's as simple as arranging for m to be updated whenever
> SNESFormFunction() is called, which involves hacking Firedrake code but not
> PETSc code.
> >
> > Thanks,
> > Andrew
> >
> >
> >   Barry
> >
> > >
> > > While this is better than nothing, there's still some problems: if I
> use e.g. snes_linesearch_type: "l2", the fnorms for lambda = 1.0, 0.5 and
> 0.0 are calculated without updating m, and so the step length taken is
> (seemingly) far from optimal.  I tried adding a damping parameter, but all
> this does is change the lambdas used to generate the quadratic fit; it
> doesn't actually make the step length smaller.
> > >
> > > Is there some cleaner way to do what I want, perhaps by intercepting
> the fnorm calculation to update m, rather than abusing a custom monitor
> routine?
> > >
> > > Thanks,
> > > Andrew
> >
> >
>
>
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