[petsc-users] -snes_type test

[Gianluca Meneghello]

Yes, my starting state is 0, not such a good guess...

what do you mean by constant +1 and constant -1? Is it the
starting/user-defined state vector +1 and -1?

My problem should not be ill posed --- it's the Jacobian of the Navier
Stokes operator, with good boundary conditions, at least theoretically
--- but still I tried what you suggested. Here is the output

using -mat_fd_type_ds -snes_type test:

   Testing hand-coded Jacobian, if the ratio is
   O(1.e-8), the hand-coded Jacobian is probably correct.
   Run with -snes_test_display to show difference
   of hand-coded and finite difference Jacobian.
   Norm of matrix ratio 9.17279e-09 difference 2.58883e-06
   Norm of matrix ratio 8.79304e-08 difference 2.59233e-05
   Norm of matrix ratio 8.87045e-08 difference 2.58875e-05

while using -ksp_rtol 1e-12 -pc_type lu -snes_monitor

  0 SNES Function norm 2.981436587230e+00
  1 SNES Function norm 1.487397936082e+00
  2 SNES Function norm 3.490136703474e-01
  3 SNES Function norm 1.303397000344e-01
  4 SNES Function norm 3.181190030949e-02
  5 SNES Function norm 5.280858553562e-03
  6 SNES Function norm 3.168168082141e-04
  7 SNES Function norm 1.217775965490e-06
  8 SNES Function norm 1.559322624068e-11


do I have to deduce that it's ill posed?
thanks

gianluca

On 18 March 2011 12:54, Jed Brown <jed at 59a2.org> wrote:
> On Fri, Mar 18, 2011 at 12:41, Gianluca Meneghello <gianmail at gmail.com>
> wrote:
>>
>> One more thing: what does the three lines stand for then? I was
>> thinking it was three Newton steps, but I understand it's not...
>
> It is your Jacobian evaluated at user-defined state, constant -1.0, and
> constant 1.0. I added this to the output for petsc-dev:
> $ ./ex5 -snes_type test
> Testing hand-coded Jacobian, if the ratio is
> O(1.e-8), the hand-coded Jacobian is probably correct.
> Run with -snes_test_display to show difference
> of hand-coded and finite difference Jacobian.
> Norm of matrix ratio 2.77166e-08 difference 2.17871e-07 (user-defined state)
> Norm of matrix ratio 1.34123e-08 difference 1.23213e-07 (constant state
> -1.0)
> Norm of matrix ratio 1.31875e-07 difference 9.05494e-07 (constant state 1.0)
> [0]PETSC ERROR: SNESSolve() line 2360 in
> /home/jed/petsc/src/snes/interface/snes.c
> [0]PETSC ERROR: main() line 220 in src/snes/examples/tutorials/ex5.c
>
>>
>> The output I sent was actually the output of one of your examples. The
>> one from my own code is
>>
>> Testing hand-coded Jacobian, if the ratio is
>> O(1.e-8), the hand-coded Jacobian is probably correct.
>> Run with -snes_test_display to show difference
>> of hand-coded and finite difference Jacobian.
>> Norm of matrix ratio 9.17257e-08 difference 2.58876e-05
>
> Is your starting state zero?
>
>>
>> Norm of matrix ratio 1.45865e-06 difference 0.000430034
>> Norm of matrix ratio 1.47148e-06 difference 0.000429436
>
> If your problem is ill-conditioned, this might come from differencing error.
> Try with -mat_fd_type ds to use a slightly more robust scheme. You can also
> run with -ksp_rtol 1e-12 -pc_type lu -snes_monitor and check if you see
> quadratic convergence.



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