[petsc-users] different convergence behaviour

[Matthew Knepley]

On Tue, Jul 12, 2016 at 7:42 AM, Hoang Giang Bui <hgbk2008 at gmail.com> wrote:

> Hello
>
> I encountered different convergence behaviour of Newton Raphson when using
> different solver settings with PETSc
>
> For the first solver configuration, I used direct solver
> -ksp_type preonly
> -pc_type lu
> -pc_factor_mat_solver_package mumps
> -mat_mumps_icntl_1 6
> -mat_mumps_icntl_4 3
> -mat_mumps_icntl_7 4
> -mat_mumps_icntl_14 40
> -mat_mumps_icntl_23 0
>
> The simulation can run completely and the NR typically converged after 6/7
> iterations. Of course, it's very slow. For the second solver configuration:
> -ksp_type gmres
> -ksp_max_it 300
> -ksp_gmres_restart 300
> -ksp_gmres_modifiedgramschmidt
> -pc_view
> -pc_fieldsplit_type multiplicative
> -fieldsplit_u_pc_type hypre
> -fieldsplit_u_pc_hypre_type boomeramg
> -fieldsplit_u_pc_hypre_boomeramg_coarsen_type PMIS
> -fieldsplit_u_pc_hypre_boomeramg_strong_threshold 0.6
> -fieldsplit_u_pc_hypre_boomeramg_max_levels 25
> -fieldsplit_wp_ksp_rtol 1.0e-8
> -fieldsplit_wp_pc_type hypre
> -fieldsplit_wp_pc_hypre_type boomeramg
> -fieldsplit_wp_pc_hypre_boomeramg_coarsen_type PMIS
> -fieldsplit_wp_pc_hypre_boomeramg_strong_threshold 0.6
> -fieldsplit_wp_pc_hypre_boomeramg_max_levels 25
>
> The solver runs much faster, but the NR does not converge in 30 iterations
> after some time steps. I thought setting the solver tolerance -ksp_rtol
> 1.0e-12 but it doesn't help much because GMRES already terminate with
> tolerance 1e-30 (see sample log file). Can we set the tolerance of the
> sub-ksp of the Fieldsplit? I tried -fieldsplit_wp_ksp_rtol 1.0e-8 but it
> doesn't work.
>

1) In the log you sent, the linear solver converges due to the Relative
Tolerance, 1.0e-9, not the breakdown tolerance 1e-30. Change rtol will
affect the convergence.

2) What do you mean  -fieldsplit_wp_ksp_rtol 1.0e-8 does not work? ALWAYS
send the view output.

3) I can't tell you anything about Newton convergence if you do not send
the output, -snes_monitor -snes_view

4) If there is a difference between LU and an iterative solver with
residual 1e-9, then your system is very ill-conditioned.

  Thanks,

     Matt


> Sorry this problem is run with many time steps and is quite big so I
> cannot reproduce in a simple test case.
>
> Giang
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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