[petsc-users] Correlation between da_refine and pg_mg_levels

[Matthew Knepley]

On Sun, Apr 2, 2017 at 2:13 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> > On Apr 2, 2017, at 9:25 AM, Justin Chang <jychang48 at gmail.com> wrote:
> >
> > Thanks guys,
> >
> > So I want to run SNES ex48 across 1032 processes on Edison, but I keep
> getting segmentation violations. These are the parameters I am trying:
> >
> > srun -n 1032 -c 2 ./ex48 -M 80 -N 80 -P 9 -da_refine 1 -pc_type mg
> -thi_mat_type baij -mg_coarse_pc_type gamg
> >
> > The above works perfectly fine if I used 96 processes. I also tried to
> use a finer coarse mesh on 1032 but the error persists.
> >
> > Any ideas why this is happening? What are the ideal parameters to use if
> I want to use 1k+ cores?
> >
>
>    Hmm, one should never get segmentation violations. You should only get
> not completely useful error messages about incompatible sizes etc. Send an
> example of the segmentation violations. (I sure hope you are checking the
> error return codes for all functions?).


He is just running SNES ex48.

  Matt


>
>   Barry
>
> > Thanks,
> > Justin
> >
> > On Fri, Mar 31, 2017 at 12:47 PM, Barry Smith <bsmith at mcs.anl.gov>
> wrote:
> >
> > > On Mar 31, 2017, at 10:00 AM, Jed Brown <jed at jedbrown.org> wrote:
> > >
> > > Justin Chang <jychang48 at gmail.com> writes:
> > >
> > >> Yeah based on my experiments it seems setting pc_mg_levels to
> $DAREFINE + 1
> > >> has decent performance.
> > >>
> > >> 1) is there ever a case where you'd want $MGLEVELS <= $DAREFINE? In
> some of
> > >> the PETSc tutorial slides (e.g., http://www.mcs.anl.gov/
> > >> petsc/documentation/tutorials/TutorialCEMRACS2016.pdf on slide
> 203/227)
> > >> they say to use $MGLEVELS = 4 and $DAREFINE = 5, but when I ran this,
> it
> > >> was almost twice as slow as if $MGLEVELS >= $DAREFINE
> > >
> > > Smaller coarse grids are generally more scalable -- when the problem
> > > data is distributed, multigrid is a good solution algorithm.  But if
> > > multigrid stops being effective because it is not preserving sufficient
> > > coarse grid accuracy (e.g., for transport-dominated problems in
> > > complicated domains) then you might want to stop early and use a more
> > > robust method (like direct solves).
> >
> > Basically for symmetric positive definite operators you can make the
> coarse problem as small as you like (even 1 point) in theory. For
> indefinite and non-symmetric problems the theory says the "coarse grid must
> be sufficiently fine" (loosely speaking the coarse grid has to resolve the
> eigenmodes for the eigenvalues to the left of the x = 0).
> >
> > https://www.jstor.org/stable/2158375?seq=1#page_scan_tab_contents
> >
> >
> >
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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