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

[Barry Smith]

  Sure, but have you ever seen such a large jump in time in going from one to two MPI ranks, and are there any algorithms to do the aggregation that would not require this very expensive parallel symmetrization?

> On Sep 17, 2022, at 9:07 AM, Mark Adams <mfadams at lbl.gov> wrote:
> 
> 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 <mailto: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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