[petsc-dev] SNESConvergedDefault() side effect?

[Dmitry Karpeyev]

On Wed, Jul 24, 2013 at 3:24 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> Dmitry Karpeyev <karpeev at mcs.anl.gov> writes:
>
> > It appears that SNESConvergedDefault() sets snes->ttol here
> >
> https://bitbucket.org/petsc/petsc/src/40bc6eb114e6a8e7f27fd216b5c1226efddf337a/src/snes/interface/snesut.c?at=master#cl-462
> >
> > That should be harmless, albeit unnecessary, since snes->ttol is also set
> > at the beginning of the solve in SNESSolve_NEWTONLS() (and other
> > SNESSolve_XXX) right before calling the convergence test:
> >
> https://bitbucket.org/petsc/petsc/src/40bc6eb114e6a8e7f27fd216b5c1226efddf337a/src/snes/impls/ls/ls.c?at=master#cl-202
> >
> >
> > However, this might make for some fragile code and may confuse the user
> > modeling a custom convergence test on SNESConvergedDefault().  In
> > particular, a local reading of SNESConvergedDefault() creates the
> > impression that it is the convergence monitor that is responsible for
> > setting snes->ttol on the zeroth iteration -- snes->ttol is used outside
> of
> > SNES, for example,  in one of the variants of the EW code (maybe only
> > there).  That would require access to SNES internals by user code.
>
> Well, when using SNESSkipConverged (should have been named
> SNESConvergedSkip when changing to SNESConvergedDefault!), we don't need
> an initial function norm, so setting ttol outside the convergence test
> is wasteful.  (I'm thinking in terms of reductions when using an
> inexpensive SNES iteration like a point smoother.)  If the convergence
> test implementation wants to use a relative tolerance, then it should
> store the target tolerance (or equivalently, the initial residual) in
> its own context.  I think all reference to ttol should be removed from
> outside the convergence tests.
>
Yes.  How should EW version 3 be implemented then?
Currently EW3 piggybacks on snes->ttol to get a measure of the initial norm
of the residual,
and relies on it having been set elsewhere.  If no references to snes->ttol
in SNESKSPEW_PreSolve()
are allowed, what should take its place?

>
> > More generally, should this sort of thing be handled everywhere using
> > snes->norm_init for consistency and to prevent potential breakage?
>
> snes->norm_init goes with SNESSetInitialFunctionNorm and I don't think
> it's used otherwise.
>
I was just thinking of a single cache for the initial residual norm.
Currently there are at least two, it seems to me.

Dmitry.

>
> > At the very least, should snes->ttol = ... be removed from
> > SNESConvergedDefault()?
>
>
>
> > Thanks.
> > Dmitry.
>
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