[petsc-dev] KSP rtol

[Mark F. Adams]

On Dec 14, 2011, at 4:39 PM, Matthew Knepley wrote:

> On Wed, Dec 14, 2011 at 3:13 PM, Mark F. Adams <mark.adams at columbia.edu> wrote:
> 
> On Dec 14, 2011, at 3:44 PM, Barry Smith wrote:
> 
> >
> > On Dec 14, 2011, at 12:45 PM, Mark F. Adams wrote:
> >
> >> I have a nonlinear problem and am finding the the convergence test for the interior PETSc linear solver is apparently testing against the norm of the first linear solve (see appended).
> >
> >   No it is || r = b - Ax ||/|| b||   with a B in there if left preconditioned
> >
> >   It has nothing to do with the "first linear solve". Each linear solve is self contained and it does not use anything from a previous linear solve to determine convergence of a later linear solve.
> >
> 
> Matts suggestion -ksp_converged_use_initial_residual_norm did the trick.
> 
> >> This is clearly not what anyone would want,
> >
> >    Why not?
> >
> 
> OK, I should say I don't know why anyone would want this and if you pop a KSP into a nonlinear loop, which is a very common thing to do obviously, then you get behavior that is probably not what you want.  Namely, an approximate Newton where the outer nonlinear tolerance is much smaller than the inner linear tolerance.  Just my perspective.
> 
> We put it in because there is a very common case, where you give an initial guess, its great
> say 10^-7, and you really do not want to iterate to 10^-12. I think what you really want is

OK, I'll buy that,

> something like EW since it is meaningfully connected to the nonlinear tolerance, which is
> physical.
> 

What is EW.  What I am doing is classic inexact Newton (or it looks like these folks are doing Picard).

Mark

>   Matt
>  
> 
> Mark
> 
> >   Barry
> >
> >
> >> is this the intended semantics?
> >
> >
> >> Is there perhaps a method to reset rtol?
> >>
> >> Mark
> >>
> >> Picard iteration 0 max(resid) = 781020
> >> 0 KSP Residual norm 2.499264167662e+07
> >> 1 KSP Residual norm 6.330915810095e+06
> >> 2 KSP Residual norm 1.856999435466e+06
> >> 3 KSP Residual norm 3.967325725053e+05
> >> 4 KSP Residual norm 9.037332775808e+04
> >> 5 KSP Residual norm 1.826960320400e+04
> >> Linear solve converged due to CONVERGED_RTOL iterations 5
> >> Picard iteration 1 max(resid) = 727577 ------- Rate = 1.07345
> >> 0 KSP Residual norm 6.885328126772e+06
> >> 1 KSP Residual norm 2.150319244570e+06
> >> 2 KSP Residual norm 2.297759417371e+05
> >> 3 KSP Residual norm 3.321819214451e+04
> >> 4 KSP Residual norm 4.155970071625e+03
> >> Linear solve converged due to CONVERGED_RTOL iterations 4
> >> Picard iteration 2 max(resid) = 486788 ------- Rate = 1.49465
> >> 0 KSP Residual norm 2.987417661953e+06
> >> 1 KSP Residual norm 5.046119985008e+05
> >> 2 KSP Residual norm 3.882757664605e+04
> >> 3 KSP Residual norm 4.106091678345e+03
> >> Linear solve converged due to CONVERGED_RTOL iterations 3
> >> Picard iteration 3 max(resid) = 137348 ------- Rate = 3.54418
> >> 0 KSP Residual norm 7.285359985607e+05
> >> 1 KSP Residual norm 9.132700545850e+04
> >> 2 KSP Residual norm 6.834608381922e+03
> >> Linear solve converged due to CONVERGED_RTOL iterations 2
> >> Picard iteration 4 max(resid) = 25082.7 ------- Rate = 5.47582
> >> 0 KSP Residual norm 1.207523847817e+05
> >> 1 KSP Residual norm 1.286174706329e+04
> >> Linear solve converged due to CONVERGED_RTOL iterations 1
> >> Picard iteration 5 max(resid) = 5723.21 ------- Rate = 4.38264
> >> 0 KSP Residual norm 2.211945822133e+04
> >> Linear solve converged due to CONVERGED_RTOL iterations 0
> >>
> >
> >
> 
> 
> 
> 
> -- 
> 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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