[petsc-dev] KSP rtol

[Matthew Knepley]

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

  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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