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

[LEDAC Pierre]

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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