[petsc-users] Snes failled with reason -6

[Barry Smith]

  ratio is || A - Afd || /||A|| where |||| is the frobenus norm and difference is || A - Afd ||. 

  Try using -snes_mf -pc_type lu 

   This can often converge even if the A you provide is wrong, so if this convergence sine then likely the Jacobian is wrong.

   It is also possible that the function evaluation has a bug.  

http://www.mcs.anl.gov/petsc/documentation/faq.html#newton


   Barry


   
On Dec 19, 2013, at 4:26 PM, Que Cat <quecat001 at gmail.com> wrote:

> Hello,
> 
> I use SNES to solve the non linear coupled equations. I have checked the hand-coded jacobian with -snes_type test and received the result:
> Norm of matrix ratio 1.39606e-23 difference 1.53372e-14 (user-defined state)
> Norm of matrix ratio 3.03157e-10 difference 0.33305 (constant state -1.0)
> Norm of matrix ratio 3.03157e-10 difference 0.33305 (constant state 1.0)
> It said that "  if the ratio is O(1.e-8), the hand-coded Jacobian is probably correct." What is the second paramenter "difference 1.53372e-14 ...."? Does it play any role in checking the accuracy of hand-coded jacobian?
> 
> The SNES solver for failed with the reason -6 "the line search failed". I tried with all other line search method stated in the PETSC manual, but they all failed. 
> 
> I tried to figure out what is wrong with my code? Could you please give me any recommendation to direct me how to check my code? I appreciate your help.
> 
> QUe
>