[petsc-users] Norm of RHS in BCGS

[Norihiro Watanabe]

What I wanted to do is displaying final converged errors without using
-ksp_monitor. Because my problem includes a lot of time steps and
nonlinear iterations, log output from -ksp_monitor for each linear
solve is sometimes too much. But you are right. It doesn't make sense
to call the expensive function just for the log output.

Thanks,
Nori

On Tue, Mar 22, 2016 at 2:45 PM, Matthew Knepley <knepley at gmail.com> wrote:
> On Tue, Mar 22, 2016 at 8:25 AM, Norihiro Watanabe <norihiro.w at gmail.com>
> wrote:
>>
>> Thank you Matt!
>>
>> Actually I don't want to change a norm type used in a convergence
>> check. I just want to output a relative error which PETSc actually
>> used for a convergence check (for log output in my program without
>> -ksp_*) and thought I need to have a norm of a preconditioned RHS to
>> compute it by myself. Or is there any function available in PETSc
>> which returns the relative error or the tolerance multiplied by the
>> norm of a preconditioned RHS? I couldn't find it.
>
>
> If you want the action of the preconditioner, you can pull it out
>
>   KSPGetPC()
>
> and apply it
>
>   PCApply()
>
> but I still do not understand why you want this. Do you want to check the
> norms
> yourself? The PCApply() could be expensive to calculate again.
>
>   Thanks,
>
>      Matt
>
>>
>>
>> Best,
>> Nori
>>
>>
>> On Tue, Mar 22, 2016 at 1:51 PM, Matthew Knepley <knepley at gmail.com>
>> wrote:
>> > On Tue, Mar 22, 2016 at 2:50 AM, Norihiro Watanabe
>> > <norihiro.w at gmail.com>
>> > wrote:
>> >>
>> >> Hi,
>> >>
>> >> Is it correct that a norm of a preconditioned RHS vector is used to
>> >> compute a relative error in BCGS?
>> >
>> >
>> > Yes, but you can verify this using -ksp_view
>> >
>> >>
>> >> I'm testing BCGS + BoomerAMG. With "-info", PETSc says "initial right
>> >> hand side norm" is 2.223619476717e+10 (see below) but an actual norm
>> >> of the RHS I passed is 4.059007e-02. If yes, is there any way to get a
>> >> norm of a preconditioned RHS?
>> >
>> >
>> > Do you mean unpreconditioned? You can try
>> >
>> >
>> >
>> > http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSPSetNormType.html
>> >
>> > or use
>> >
>> >   -ksp_monitor_true_residual
>> >
>> >   Thanks,
>> >
>> >     Matt
>> >
>> >>
>> >> [0] KSPConvergedDefault(): Linear solver has converged. Residual norm
>> >> 2.036064453512e-02 is less than relative tolerance 9.999999960042e-13
>> >> times initial right hand side norm 2.223619476717e+10 at iteration 6
>> >>
>> >>
>> >> Regards,
>> >> Nori
>> >>
>> >> --
>> >> Norihiro Watanabe
>> >
>> >
>> >
>> >
>> > --
>> > 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
>>
>>
>>
>> --
>> Norihiro Watanabe
>
>
>
>
> --
> 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



-- 
Norihiro Watanabe