[petsc-users] KSPBuildResidual in ex42.c

[Jed Brown]

On Tue, May 22, 2012 at 9:54 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.
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