[petsc-users] SNESSetFunctionDomainError

[Dmitry Karpeyev]

On Aug 24, 2014 9:40 PM, "Barry Smith" <bsmith at mcs.anl.gov> wrote:
>
>
> On Aug 24, 2014, at 9:06 PM, Derek Gaston <friedmud at gmail.com> wrote:
>
> > There must be something I'm missing about SNESSetFunctionDomainError().
> >
> > I'm using a matrix free newton solve and I was hoping that calling
SNESSetFunctionDomainError() would completely stop the nonlinear solver...
but that doesn't appear to be the case.
> >
> > Now - if I call that function during the initial residual computation
it appears to work (the solve never starts).  However, if I call it during
the _middle_ of a matrix free nonlinear solve during a residual calculation
- it appears to do absolutely nothing.
>
>    You are not missing anything. The way SNESSetFunctionDomainError()
works is it merely sets a flag in the SNES object. The solver code then can
check that flag after a function evaluation and generate an if that flag is
set (or perhaps do some kind of recovery). So, for example, at the
beginning of SNESSolve_NewtonLS() we have
>
>  } else {
>     if (!snes->vec_func_init_set) {
>       ierr = SNESComputeFunction(snes,X,F);CHKERRQ(ierr);
>       ierr = SNESGetFunctionDomainError(snes, &domainerror);CHKERRQ(ierr);
>       if (domainerror) {
>         snes->reason = SNES_DIVERGED_FUNCTION_DOMAIN;
>         PetscFunctionReturn(0);
>
> Now the reason it is not erroring out for you is because
>
> 1) our MatMult_MFFD() totally ignores the SNES domain error business
because it doesn’t even know about SNES

This could potentially be fixed by letting domain errors be treated as
regular PETSc errors. In particular, solvers could push and pop (e.g.,
before the linear solve) custom error handlers that would set an
appropriate converged reason. I think there would be no noticeable
performance penalty, since the error handler would only fire when there is
an actual domain error, and then the expectation of high performance is out.

>
> 2) inside the various nonlinear solvers and many complicated line search
codes we may not always be checking the domainerror flag.
>
>   Both of these things, of course, should be fixed. We need to add
support to the MatMult_MFFD() to indicate domain errors and we need to
check all the code to make sure we always handle the domain error flag.
>
>   Barry
>
>
> >
> > Here's my stupid debugging code that's at the top of my residual
callback method:
> >
> >     static int count = 0;
> >     count++;
> >
> >     if (count == 5)
> >     {
> >       std::cout<<"stopping solve!"<<std::endl;
> >       SNESSetFunctionDomainError(snes);
> >     }
> >
> >
> > During the solve "stopping solve!" gets printed out after computing the
3rd linear iteration of the first newton step... but the solve just
continues without stopping...
> >
> > Is there something more I need to do here?
> >
> > Thanks!
> >
> > Derek
>
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