[petsc-users] KSPBuildResidual in ex42.c

[Jed Brown]

On Wed, May 23, 2012 at 6:08 AM, Klaij, Christiaan <C.Klaij at marin.nl> wrote:

> > > Yes, only the norms. As I understand now, GMRES will compute the
> > > norm of the total system residual [U,V,W,P], but I would like the
> > > norms of [U], [V], [W] and [P] residuals separated. Based on
> > > ex42, I got the impression that I have to use KSPBuildResidual.
> > >
> >
> > Yes, the cost of building the residual (check -log_summary to see how big
> > it is) is a reason why people often avoid GMRES if they need the full
> > residual. It's directly available with GCR, for example (which is
> otherwise
> > similar in work to FGMRES) and with most of the non-residual-minimizing
> > methods.
>
> I find GCR to be less robust than FGMRES,


Right, it can break down in some cases that FGMRES does not.


> but ok, suppose I switch
> to GCR, how would I get the residual vector?


KSPBuildResidual(), but it's trivially fast with GCR.
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