[petsc-users] performance regression with GAMG

[Mark Adams]

BTW, nice bug report ...

>
> So in the first step it coarsens from 150e6 to 5.4e6 DOFs instead of to
> 2.6e6 DOFs.


Yes, this is the critical place to see what is different and going wrong.

My 3D tests were not that different and I see you lowered the threshold.
Note, you can set the threshold to zero, but your test is running so much
differently than mine there is something else going on.
Note, the new, bad, coarsening rate of 30:1 is what we tend to shoot for
in 3D.

So it is not clear what the problem is.  Some questions:

* do you have a picture of this mesh to show me?
* what do you mean by Q1-Q2 elements?

It would be nice to see if the new and old codes are similar without
aggressive coarsening.
This was the intended change of the major change in this time frame as you
noticed.
If these jobs are easy to run, could you check that the old and new
versions are similar with "-pc_gamg_square_graph  0 ",  ( and you only need
one time step).
All you need to do is check that the first coarse grid has about the same
number of equations (large).

BTW, I am starting to think I should add the old method back as an option.
I did not think this change would cause large differences.

Thanks,
Mark




> Note that we are providing the rigid body near nullspace,
> hence the bs=3 to bs=6.
> We have tried different values for the gamg_threshold but it doesn't
> really seem to significantly alter the coarsening amount in that first
> step.
>
> Do you have any suggestions for further things we should try/look at?
> Any feedback would be much appreciated
>
> Best wishes
> Stephan Kramer
>
> Full logs including log_view timings available from
> https://github.com/stephankramer/petsc-scaling/
>
> In particular:
>
>
> https://github.com/stephankramer/petsc-scaling/blob/main/before/Level_5/output_2.dat
>
> https://github.com/stephankramer/petsc-scaling/blob/main/after/Level_5/output_2.dat
>
> https://github.com/stephankramer/petsc-scaling/blob/main/before/Level_6/output_2.dat
>
> https://github.com/stephankramer/petsc-scaling/blob/main/after/Level_6/output_2.dat
>
> https://github.com/stephankramer/petsc-scaling/blob/main/before/Level_7/output_2.dat
>
> https://github.com/stephankramer/petsc-scaling/blob/main/after/Level_7/output_2.dat
>
>
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