[petsc-users] Snes behavior

[Barry Smith]

On Jan 10, 2010, at 3:52 PM, Ryan Yan wrote:

> Hi Barry,
> Yes, exactly. The original multi-components system scale quite  
> unevenly. I will try to rescale it.
> Could this be helpful to show some promise on quadratic convergence?

    I won't be concerned about "quadratic convergence" I'd only be  
concerned that it is converging to the correct answer and that you are  
getting close enough to the correct answer.

     You can run with -snes_mf_operator and -snes_type test to verify  
if the Jacobian being computed is accurate. Perhaps in your function  
evaluation you are not using the stencil that you set with the DA,  
this would cause the wrong Jacobian to be computed.


    Barry

>
> Thanks a lot,
>
> Yan
>
> On Sun, Jan 10, 2010 at 4:35 PM, Barry Smith <bsmith at mcs.anl.gov>  
> wrote:
>
>  You already got a 10^16 drop in the residual norm. It is not  
> realistic to expect to get much more than that for double precision  
> calculations. Perhaps your original F() has some funky scaling of  
> different components that you can fix.
>
>
>
>   Barry
>
>
> On Jan 10, 2010, at 2:55 PM, Ryan Yan wrote:
>
> Hi All,
> I am solving a nonlinear system using snes. The -snes_monitor option  
> has the following output:
>
>  0 SNES Function norm 2.640163923729e+09
>  1 SNES Function norm 1.047643565314e+08
>  2 SNES Function norm 1.712732074788e+06
>  3 SNES Function norm 1.002169173269e+04
>  4 SNES Function norm 1.655878303433e+03
>  5 SNES Function norm 3.746498305706e+02
>  6 SNES Function norm 8.317435704773e+01
>  7 SNES Function norm 1.857639969641e+01
>  8 SNES Function norm 4.149691057773e+00
>  9 SNES Function norm 9.265604042412e-01
>  10 SNES Function norm 2.069527103214e-01
>  11 SNES Function norm 4.624186491082e-02
>  12 SNES Function norm 1.035558432688e-02
>  13 SNES Function norm 2.341362958811e-03
>  14 SNES Function norm 5.507445427277e-04
>  15 SNES Function norm 1.485123568354e-04
>  16 SNES Function norm 5.180043781814e-05
>  17 SNES Function norm 2.341966514486e-05
>  18 SNES Function norm 1.344936158651e-05
>  19 SNES Function norm 1.054812641176e-05
> Number of Newton iterations = 19
> Converged reason is 4
>
> It looks like the iterate never falls into a quadratic convergence  
> region before it converges. Is there any hint to understand this  
> behavior?
>
> Thanks a lot,
>
> Yan
>
>
>
>