[petsc-users] different convergence behaviour

[Hoang Giang Bui]

Hi Matt

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.

Sorry i got it wrong in the previous email, the ksp_rtol 1.0e-12 DOES
affect the convergence, and it took more iterations. But the simulation
still failed at a definite time step.

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

In the log file I sent previously, the line

    KSP Object:    (fieldsplit_wp_)     8 MPI processes
      type: preonly
      maximum iterations=10000, initial guess is zero
      tolerances:  relative=1e-05, absolute=1e-50, divergence=10000
      left preconditioning
      using NONE norm type for convergence test

impressed me that the rtol for fieldsplit_wp is still 1.0e-5

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

I did not yet use SNES, instead using my NR iterator so I have no view for
SNES.

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







Giang

On Tue, Jul 12, 2016 at 2:49 PM, Matthew Knepley <knepley at gmail.com> wrote:

> 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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