[petsc-users] GAMG preconditioning

[Mark Adams]

Can you briefly describe your application,?

AMG usually only works well for straightforward elliptic problems, at least
right out of the box.


On Mon, Apr 12, 2021 at 11:35 AM Milan Pelletier via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> Dear all,
>
> I am currently trying to use PETSc with CG solver and GAMG preconditioner.
> I have started with the following set of parameters:
> -ksp_type cg
> -pc_type gamg
> -pc_gamg_agg_nsmooths 1
> -pc_gamg_threshold 0.02
> -mg_levels_ksp_type chebyshev
> -mg_levels_pc_type sor
> -mg_levels_ksp_max_it 2
>
> Unfortunately, the preconditioning seems to run extremely slowly. I tried
> to play around with the numbers, to check if I could notice some
> difference, but could not observe significant changes.
> As a comparison, the KSPSetup call with GAMG PC takes more than 10 times
> longer than completing the whole computation (preconditioning + ~400 KSP
> iterations to convergence) of the similar case using the following
> parameters :
> -ksp_type cg
> -pc_type ilu
> -pc_factor_levels 0
>
> The matrix size for my case is ~1,850,000*1,850,000 elements, with
> ~38,000,000 non-zero terms (i.e. ~20 per row). For both ILU and AMG cases I
> use matseqaij/vecseq storage (as a first step I work with only 1 MPI
> process).
>
> Is there something wrong in the parameter set I have been using?
> I understand that the preconditioning overhead with AMG is higher than
> with ILU, but I would also expect CG/GAMG to be competitive against CG/ILU,
> especially considering the relatively big problem size.
>
> For information, I am using the PETSc version built from commit
> 6840fe907c1a3d26068082d180636158471d79a2 (release branch from April 7,
> 2021).
>
> Any clue or idea would be greatly appreciated!
> Thanks for your help,
>
> Best regards,
> Milan Pelletier
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20210412/e8c07065/attachment-0001.html>