[petsc-dev] Seeming performance regression with GAMG

[Mark Adams]

>
> 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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