[petsc-dev] Mark, what's the rational for keeping the Chebyshev tuning inside GAMG?

[Tobin Isaac]

On Wed, Feb 25, 2015 at 11:02:00AM -0500, Mark Adams wrote:
> On Mon, Feb 23, 2015 at 7:19 AM, Tobin Isaac <tisaac at ices.utexas.edu> wrote:
> 
> > On Sun, Feb 22, 2015 at 07:48:51PM -0700, Jed Brown wrote:
> > > Barry Smith <bsmith at mcs.anl.gov> writes:
> > > >   Jed,
> > > >
> > > >    Wrong, wrong, wrong. You are looking at the wrong place in the
> > > >    code. (Yes, this is a problem with duplicate code in THREE!
> > > >    places). I am referring to the code around line 795 in gamg.c /* do
> > > >    my own cheby */ which I actually cut and pasted into my previous
> > > >    email. You are referring to the code in agg.c around line 1209
> > > >    which does what you said.
> > >
> > > Okay, yes.
> > >
> > > >    The code I am referring to computes the estimates used by the
> > > >    smoother on each level and thus forces a bypass of the code in
> > > >    Chebyshev.c it seems completely unneeded to me (aside from maybe
> > > >    the hack for crazy matrices) since Chebyshev.c will do it next
> > > >    automatically anyways.
> > >
> > > Yeah. It would be nice to avoid repeating the estimate when the smoother
> > > matches the AMG smoothing.
> >
> > If we just keep the part that calls KSPChebyshevSetEigenvalues()
> > (gamg.c:801) when the smoother PC is Jacobi or SOR, that would
> > accomplish this.  The used would have to recognize what's happening
> > and not call -mg_levels_ksp_chebysev_estimate_eigenvalues, which would
> > recompute/override the values that gamg sets.
> >
> 
> That is a reasonable approach.
> 
> 
> >
> > As for crazy matrices, I think the most general solution is for
> > KSPChebyshev to try a MatProjectDirichlet(Mat,Vec) method on the rhs
> > it uses for estimation.
> >
> >
> That sounds promising.  Is that basically the same code that I have?

We could make what you have (finding rows with just diagonals) the
default.  For, e.g., vector-valued problems with unaligned partial
Dirichlet conditions, the user could specify something else.

  Toby

> 
> 
> 
> >   Toby
> >
> >
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