KSP Tolerances

[Matthew Knepley]

Run with -ksp_view -ksp_monitor -info and send us all the output.

  Matt

On Mon, Dec 14, 2009 at 8:36 AM, <jarunan at ascomp.ch> wrote:

>
> Dear Petsc Team,
>
> There is something not clear for me about divergence tolerance. Firstly, my
> computation diverges with the reason of dtol (-4), which was set by default
> (10000). When I increased dtol to 1.0e+8, it became converged. However, the
> relative residual where it was diverged is less than the default dtol. I
> would like to know, how it is exactly calculated.
>
> ----------I describe my problem more in detail here.-------------------
> As I set the tolerances for the solver as below, my problem diverges with
> converged reason -4 (from divergence tolerance)
>
> call KSPSetTolerances(ksp,1.0e-06,1.0e-20,
> PETSC_DEFAULT_DOUBLE_PRECISION,10000,ierr)
>
> I monitor the residual. it printed the information below and quit the
> computation.(I delete the true residual norm and Ax/Ae. Please tell me if
> you need them.)
>
>   0 KSP preconditioned resid norm 5.139790011051e+02
>
> With -ksp_view, it shows that default value of dtol is 10000. I increased
> dtol to 1.0e+8, then the solver solved the problem. With
> -ksp_monitor_true_residual, the first 5 iterations read:
>
>  0 KSP preconditioned resid norm 5.139790011051e+02
>  1 KSP preconditioned resid norm 1.840751662590e+01
>  2 KSP preconditioned resid norm 3.653339750400e+00
>  3 KSP preconditioned resid norm 2.538794826933e+00
>  4 KSP preconditioned resid norm 2.299844715275e-01
>
> Here, from iteration 0 to iteration 1,the relative residual is 0.03580,
> much less than 1.0e+4 the default dtol value. So, why was it diverged at the
> first time?
>
> ----------------------------------------------------------------------------
>
> Regards,
> Jarunan
>
>
>
>
> --
> Jarunan Panyasantisuk
> Development Engineer
> ASCOMP GmbH, Technoparkstr. 1
> CH-8005 Zurich, Switzerland
> Phone : +41 44 445 4072
> Fax   : +41 44 445 4075
> E-mail: jarunan at ascomp.ch
> www.ascomp.ch
>



-- 
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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