[petsc-users] different convergence behaviour

[Matthew Knepley]

On Tue, Jul 12, 2016 at 8:44 AM, Hoang Giang Bui <hgbk2008 at gmail.com> wrote:

> 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
>

KSP "preonly" does no iterations, so it does not read the tolerance. If you
want to lower the tolerance,
choose a solver like GMRES

  -fieldsplit_wp_ksp_type gmres -fieldsplit_wp_ksp_rtol 1e-8


> 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.
>

It is hard to debug an iteration which we did not code. It could be you
have a bug. If not, then very small changes in
the iterates are making a difference, which means your Jacobians are close
to singular. A problem reformulation would
probably help more than solver tweaking.

  Thanks,

    Matt


> 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
>>
>
>


-- 
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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