a question about tolerance

[Barry Smith]

On Sep 15, 2009, at 6:04 PM, Ryan Yan wrote:

> Thanks Jed.
>
> And in the term ||Ae||/||Ax||,  PETSc impilictly assume that we use  
> zero as initial guess and here the x really means exact solution,  
> right?

    If you call KSPSetInitialGuessNonzero() then PETSc will use the  
initial value in x as the initial guess. The default convergence tests
use the ratio of || r_current||/ || b ||. Thus if you start with a  
great initial guess it may not iterate much at all.  You can use also - 
ksp_converged_use_initial_residual_norm
or KSPDefaultConvergedSetUIRNorm() to have it use the ratio of ||  
r_current||/|| r_initial|| for the relative convergence test.

    You can also provide your own convergence test with  
KSPSetConvergenceTest().

    Barry


>
> Yan
>
>
>
>
> On Tue, Sep 15, 2009 at 6:41 PM, Jed Brown <jed at 59a2.org> wrote:
> Ryan Yan wrote:
> > But can you say a little more about what happened in the first  
> scenario, I
> > did not quite fellow you. Let's say, If I set -ksp_rtol 1e-2, what  
> does this
> > mean in the left PC case. More specifically, which term will be  
> considered
> > as a stop creteria for the KSP solve.
>
> Look at the first column (preconditioned residual norm).  Notice that
> this is decreasing by 1e-2 and 1e-3 respectively in your examples.
>
> Jed
>
>