[petsc-users] linear problem solved by snes in 2 iteration.

[Jed Brown]

It is because of the dafault convergence tolerances.

Try -ksp_rtol 1e-10
On Dec 8, 2011 10:12 AM, "Hailong Xiao" <xiaohl1986 at gmail.com> wrote:

> Hi
>
> I am using cell centered finite difference method for
> -div K (grad p) = f
> with Neumann boundary condtion.
>
> I called
> SNESGetKSP(snes, &ksp);
> MatNullSpaceCreate(PETSC_COMM_WORLD, PETSC_TRUE, 0, PETSC_NULL, &nullsp);
> KSPSetNullSpace(ksp, nullsp);
> MatNullSpaceDestroy(&nullsp);
>
> to handle the null space.
>
> But I got the result like the following
>   0 SNES Function norm 3.512407365520e+02
>   Linear solve converged due to CONVERGED_RTOL iterations 16
>   1 SNES Function norm 6.615335308038e-03
>   Linear solve converged due to CONVERGED_RTOL iterations 20
>   2 SNES Function norm 3.979662741066e-08
>
> I checked my numerical solution with analytical solution
> (p=x,y,z,cos(pi*x),cos(pi*y), cos(pi*z).
>
> These are the difference between the petsc result and analytical solution
> for
> -da_grid_x 5
> -da_grid_y 5
> -da_grid_z 100
>
> i j k 1 1 0
> petsc      0.499959
> true       0.495
> petsc-true 0.00495888
> i j k 1 1 1
> petsc      0.486626
> true       0.485
> petsc-true 0.00162554
> i j k 1 1 2
> petsc      0.475959
> true       0.475
> petsc-true 0.000958875
> i j k 1 1 3
> petsc      0.465673
> true       0.465
> petsc-true 0.000673159
>
> I think I got the correct solution.
>
> I do not understand why snes took 2 iterations for a linear problem? Is it
> normal?
>
>
> --
> Hailong
>
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