[petsc-users] different convergence behaviour

[Hoang Giang Bui]

Thanks Barry

This is a good comment. Since material behaviour depends very much on the
trajectory of the solution. I suspect that the error may concatenate during
time stepping.

I have re-run the simulation as you suggested and post the log file here:
https://www.dropbox.com/s/d6l8ixme37uh47a/log13Jul16?dl=0

However, I did not get what -ksp_monitor_true_solution used for? I see that
I have the same log that I had before.

Giang

On Wed, Jul 13, 2016 at 5:43 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
>   It is not uncommon for an iterative linear solver to work fine for some
> time steps but then start to perform poorly at a later timestep because the
> physics (mathematically the conditioning or eigenstructure of the Jacobian)
> changes over time; perhaps becomes singular. Another possibility is the
> trajectory of the solution is very sensitive to the solution of the
> nonlinear problem at each time step so that an iterative linear solver and
> a direct linear solver result in very difficult physical solutions after
> many time steps. In other words after many time-steps the computed
> solutions can be very different and if the computed solution for the
> iterative linear solver is eventually "non-physical" or ill-conditioned the
> nonlinear solver could break down.
>
>   Please run with the iterative solver (that eventually breaks) with the
> option -ksp_monitor_true_solution -ksp_converged_reason and and send ALL
> the output (it will be very large, don't worry about it). Then we can see
> if the linear solver is breaking down. Note that by default PETSc linear
> solvers do not generate an error that stops the program if the linear solve
> fails, hence your NR code should call KSPGetConvergedReason() after EVERY
> linear solve and if the reason is negative your code needs to do something
> different since the linear solve failed and your code should not just keep
> on running NR.
>
>   Barry
>
>
> > On Jul 12, 2016, at 9:52 AM, Matthew Knepley <knepley at gmail.com> wrote:
> >
> > 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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