[petsc-users] pc_gamg_threshol

[Mark Adams]

Thresholding is a heuristic and a crude algorithm (just chop weak edges
with the simplest measure of "weak"). I don't know of any abstract analysis
of it.

The original smoothed aggregation papers by Vanek, Mandel, Brezina were
fairly by fairly mathy folks.

Ray Tuminaro and Luke Olsen have done some nice work to develop a more
robust coarsening strategies that do some non-trivial analysis of the
matrix to try to identify genuinely strong connections. Crude methods are
not bad for M matrices but can get fooled by higher order discretizations.

On Thu, Jan 5, 2017 at 9:23 AM, Jeremy Theler <jeremy at seamplex.com> wrote:

> Yes, I read that page and it was that paragraph that made me want to
> learn more.
>
> For example, that pages says:
>
> “-pc_gamg_threshold 0.0 is the most robust option (the reason for this
> is not obvious) ...”
>
>
> Where can I find more math-based background on this subject? I mean,
> some text that describes the methods and not just the implementation as
> the source code at gamg/util.c so I can better understand what is going
> on.
>
>
> Thanks
>
>
>
> --
> Jeremy Theler
> www.seamplex.com
>
>
>
> On Thu, 2017-01-05 at 09:18 -0500, Mark Adams wrote:
> > You want the bottom of page 84 in the manual.
> >
> > On Wed, Jan 4, 2017 at 4:33 PM, Barry Smith <bsmith at mcs.anl.gov>
> > wrote:
> >
> >            The manual page gives a high-level description
> >         http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/
> PCGAMGSetThreshold.html the exact details can be found in the code here
> http://www.mcs.anl.gov/petsc/petsc-dev/src/ksp/pc/impls/gamg/util.c.html#
> PCGAMGFilterGraph  I'm adding a link from the former to the later in the
> documentation.
> >
> >            Barry
> >
> >
> >
> >         > On Jan 4, 2017, at 3:16 PM, Jeremy Theler
> >         <jeremy at seamplex.com> wrote:
> >         >
> >         > * Any reference to what pc_gamg_treshold means and/or does?
> >         >
> >         >
> >         >
> >         > On Wed, 2017-01-04 at 18:13 -0300, Jeremy Theler wrote:
> >         >> Hi! Any reference to what does -pc_gamg_threshold mean
> >         and/or?
> >         >>
> >         >
> >
> >
> >
> >
>
>


-- 
*The secret to doing good research is always be a little underemployed. You
waste years by not being able to waste hours* -- Amos Tversky
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