[petsc-dev] KSP rtol

[Mark F. Adams]

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.

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
>> 
> 
>