[petsc-users] difference between solns for np=1 and np>1.

[Jed Brown]

On Thu, 18 Mar 2010 13:50:32 -0400, "(Rebecca) Xuefei YUAN" <xy2102 at columbia.edu> wrote:
> Dear Jed,
> 
> I excluded the bug from CreateNullSpace(), but still have different  
> convergence history for np=1 and np=2, both with -pc_type none, and  
> -pc_type jacobi.
> 
> The convergence history for np=1, -pc_type none is
> 
> 
>    0 SNES Function norm 3.277654936380e+02
> Linear solve converged due to CONVERGED_RTOL iterations 1
>    1 SNES Function norm 1.010694930474e+01
> Linear solve converged due to CONVERGED_RTOL iterations 9
>    2 SNES Function norm 1.456202001578e+00
> Linear solve converged due to CONVERGED_RTOL iterations 23
>    3 SNES Function norm 6.670544108392e-02
> Linear solve converged due to CONVERGED_RTOL iterations 28
>    4 SNES Function norm 1.924506428876e-04
> Linear solve did not converge due to DIVERGED_ITS iterations 10000
>    5 SNES Function norm 3.554534723246e-05
> Linear solve did not converge due to DIVERGED_ITS iterations 10000
>    6 SNES Function norm 3.554534511905e-05
> Linear solve did not converge due to DIVERGED_ITS iterations 10000
>    7 SNES Function norm 3.554534511895e-05
> Nonlinear solve converged due to CONVERGED_PNORM_RELATIVE

It looks like this has stagnated.  You said you have checked that the
matrices are the same, what did you do to confirm this?  How did you
check that the null spaces are the same?  What do the unpreconditioned
residuals look like (e.g. -ksp_type fgmres or -ksp_type lgmres
-ksp_right_pc)?  If you are working in unpreconditioned residuals, then
how are you implementing boundary conditions?

Jed