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@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() 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
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