[petsc-users] Behavior of KSP iterations when using Restart

[Matthew Knepley]

On Thu, Jun 8, 2023 at 9:13 PM Nicolas Garcia Guzman <nicolas.garcia at tum.de>
wrote:

> Hello,
>
>
> I am solving a linear system using petsc4py, with the following command:
>
>
> python main.py -ksp_type gmres -ksp_gmres_restart 16 -ksp_max_it 180000
> -ksp_monitor -ksp_converged_reason -ksp_rtol 1e-15 -pc_type asm
> -sub_pc_type ilu -sub_pc_factor_levels 1 -sub_ksp_type preonly
>
>
> In the script all I do is import the libraries, load the linear system,
> set options and solve.
>
>
> However, simply changing the restart parameter to something like 20, will
> make the solution iterate to exactly 2 times the restart parameter, and
> then say the iteration diverged. This happens every time no matter the
> parameter chosen, unless it's 16 or less. Is this expected behavior or is
> the problem coming from my linear system?
>

1) Always send the complete output

2) I am guessing that the iteration stops because the restarted residual is
too large.This could be due to your linear system. What kind of system are
you solving?

  Thanks,

     Matt

-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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