Please send -log_view for the ilu and GAMG case. Barry
On Apr 12, 2021, at 10:34 AM, Milan Pelletier via petsc-users <[email protected]> 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