[petsc-users] Norm of RHS in BCGS

[Norihiro Watanabe]

Can't I use KSPGetResidualNorm()? I mean if I'm interested only in the
last residual.

On Tue, Mar 22, 2016 at 3:20 PM, Matthew Knepley <knepley at gmail.com> wrote:
> On Tue, Mar 22, 2016 at 9:08 AM, Norihiro Watanabe <norihiro.w at gmail.com>
> wrote:
>>
>> Unfortunately -ksp_converged_reason prints the number of iterations
>> but no information about final errors.
>
>
> If you want the actual residuals (not errrors), you could use
>
>
> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSPGetResidualHistory.html
>
>     Matt
>
>>
>> Thanks,
>> Nori
>>
>> On Tue, Mar 22, 2016 at 3:06 PM, Matthew Knepley <knepley at gmail.com>
>> wrote:
>> > On Tue, Mar 22, 2016 at 9:02 AM, Norihiro Watanabe
>> > <norihiro.w at gmail.com>
>> > wrote:
>> >>
>> >> 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.
>> >
>> >
>> > Maybe something like -ksp_converged_reason?
>> >
>> >   Thanks,
>> >
>> >      Matt
>> >
>> >>
>> >> 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
>> >
>> >
>> >
>> >
>> > --
>> > 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