[petsc-dev] Seeming performance regression with GAMG

Mark Adams mfadams at lbl.gov
Sat May 2 07:28:46 CDT 2015

BTW, I have pushed a fix in mark/gamg-square.  The parameter is now an
integer.  You will want at least 2.  Inf should give you the old, pre
January, semantics.

On Thu, Apr 30, 2015 at 3:10 PM, Mark Adams <mfadams at lbl.gov> wrote:

>> Yes, thanks, this is the change.  If I switch back to using the square
>> of the adjacency graph on all levels then I recover the previous
>> behaviour.  Mark, any comments?
> The intent here is that you need to get faster coarsening on most
> problems, especially not high order discretizations. After the first coarse
> grid the number of non-zeros increases and this seems to be enough to
> coarsen a good rate for the problems that I test: lowish order FE elements
> (8 and maybe 27 node hexes).
> You have about 10 non-zeros per row.  This is very low and squaring is not
> even really enough.  BTW, if you can add fake zeros to make a large stencil
> and use a negative threshold value, GAMG will use that fake graph for
> coarsening, that is, it will not drop the zeros from the coarsening graph.
> BTW, I hated making this change for this reason but made a mistake in
> implementing this.  I was always intending to do turn the squaring off
> after the first level, but realized earlier this year that I was not
> turning it off.
> Anyway, I would suggest that we turn the square graph argument into an
> integer for the number of levels that you want.  Zero would be like false,
> one would be like true, and then you can add what you want.
> If there are no objections I will do this.  It will break everyone that
> uses this parameter but it is a better interface.
> Mark
>> Cheers,
>> Lawrence
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