[petsc-users] PETSc 3.14 to PETSc 3.20: Different (slower) convergence for classical AMG (sequential and especially in parallel)

[Mark Adams]

Hi Pierre,

Sorry I missed this post and your issues were brought to my attention today.

First, the classic version is not supported well. The postdoc that wrote
the code is long gone and I don't know the code at all.
It is really a reference implementation that someone could build on and is
not meant for production.
In 10 years you are the first user that has connected us.

The hypre package is a very good AMG solver and it uses classical AMG as
the main solver.
I wrote GAMG ("agg") which is a smoothed aggregation AMG solver and is very
different from classical.
I would suggest you move to hypre or '-pc_gamg_type agg'.

The coarsening was developed in this time frame and there was a lot of
churn as a new strategy for aggressive coarsening did not work well for
some users and I had to add the old method in and then made it the default
(again).
This change missed v3.20, but you can get the old aggressive strategy with
'-pc_gamg_aggressive_square_graph'.
Check with -options_left to check that it is being used.

As far as your output (nice formatting, thank you), the coarse grid is
smaller in the new code.
            rows=41, cols=41   |          rows=30, cols=30
"square graph" should fix this.

You can also try not using aggressive coarsening with:
You could try '-pc_gamg_aggressive_coarsening 0'

Let me know how it goes and let's try to get you into a more sustainable
state ... I really try not to change this code but sometimes need to.

Thanks,
Mark





On Mon, Oct 9, 2023 at 10:43 AM LEDAC Pierre <Pierre.LEDAC at cea.fr> wrote:

> Hello all,
>
>
> I am struggling to find the same convergence in iterations when using
> classical algebric multigrid in my code with PETSc 3.20 compared to PETSc
> 3.14.
>
>
> I am using in order to solve a Poisson system:
>
> *-ksp_type cg -pc_type gamg -pc_gamg_type classical*
>
>
> I read the different releases notes between 3.15 and 3.20:
>
> https://petsc.org/release/changes/317
>
> https://petsc.org/main/manualpages/PC/PCGAMGSetThreshold/
>
>
> And have a look at the archive mailing list (especially this one:
> https://www.mail-archive.com/petsc-users@mcs.anl.gov/msg46688.html)
>
> so I added some other options to try to have the same behaviour than PETSc
> 3.14:
>
>
> *-ksp_type cg -pc_type gamg -pc_gamg_type classical *-mg_levels_pc_type
> sor -pc_gamg_threshold 0.
>
>
> It improves the convergence but there still a different convergence though
> (26 vs 18 iterations).
>
> On another of my test case, the number of levels is different (e.g. 6 vs
> 4) also, and here it is the same, but with a different coarsening according
> to the output from the -ksp_view option
>
> The main point is that the convergence dramatically degrades in parallel
> on a third test case, so I can't upgrade to PETSc 3.20 for now unhappily.
>
> I send you the partial report (petsc_314_vs_petsc_320.ksp_view) with
> -ksp_view (left PETSc 3.14, right PETSc 3.20) and the configure/command
> line options used (in petsc_XXX_petsc.TU files).
>
>
> Could my issue related to the following 3.18 change ? I have not tried the
> first one.
>
>
>    -
>
>    Remove PCGAMGSetSymGraph() and -pc_gamg_sym_graph. The user should now
>    indicate symmetry and structural symmetry using MatSetOption
>    <https://petsc.org/release/manualpages/Mat/MatSetOption/>() and GAMG
>    will symmetrize the graph if a symmetric options is not set
>    -
>
>    Change -pc_gamg_reuse_interpolation default from false to true.
>
>
> Any advice would be greatly appreciated,
>
>
> Pierre LEDAC
> Commissariat à l’énergie atomique et aux énergies alternatives
> Centre de SACLAY
> DES/ISAS/DM2S/SGLS/LCAN
> Bâtiment 451 – point courrier n°43
> F-91191 Gif-sur-Yvette
> +33 1 69 08 04 03
> +33 6 83 42 05 79
>
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