[petsc-dev] large cost of symmetrizing for GAMG?

[Mark Adams]

Symetrix graph make a transpose and then adds them.
I imagine adding two different matrices is expensive.

On Fri, Sep 16, 2022 at 8:30 PM Barry Smith <bsmith at petsc.dev> wrote:

>
> Mark,
>
>    I have runs of GAMG on one and two ranks with -pc_gamg_symmetrize_graph
> because the matrix is far from symmetric and some of GAMG is taking a huge
> amount more time with 2 ranks than one. (While other stuff like VecNorm
> shows improvement with two ranks). I've attached the two files
>
>   Have you seen this before, is there anything that can be done about? If
> going to two ranks causes almost a doubling in GAMG setup time that makes
> using parallelism not useful,
>
>   Barry
>
>
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