[petsc-users] Norm of RHS in BCGS

[Matthew Knepley]

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
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