[petsc-dev] VecNest

[Matthew Knepley]

On Mon, Jul 11, 2022 at 1:17 PM Pierre Jolivet <pierre at joliv.et> wrote:

> Hello,
> Could anyone help me understand what is going on in the following example,
> please?
> I have a VecNest.
> I either: a) initialize all values to 0.0, then set a specific part of the
> vector to nonzero or b) initialize a part of the vector to 0.0 and set the
> other part to nonzero.
> I don’t see why a) and b) produce different results.
>
> $ ./ex1111 -pc_type fieldsplit -ksp_monitor_true_residual
> -ksp_converged_reason -fieldsplit_pc_type jacobi -ksp_pc_side right
> -ksp_view_final_residual -nest_subvec true
>   0 KSP unpreconditioned resid norm 8.375635517980e-01 true resid norm
> 8.375635517980e-01 ||r(i)||/||b|| 1.000000000000e+00
>   1 KSP unpreconditioned resid norm 4.748816884247e-01 true resid norm
> 4.748816884247e-01 ||r(i)||/||b|| 5.669798875623e-01
>   2 KSP unpreconditioned resid norm 4.713006778679e-01 true resid norm
> 4.713006778679e-01 ||r(i)||/||b|| 5.627043784990e-01
>   3 KSP unpreconditioned resid norm 7.092979927129e-02 true resid norm
> 7.092979927129e-02 ||r(i)||/||b|| 8.468587144106e-02
>   4 KSP unpreconditioned resid norm 1.457836310255e-02 true resid norm
> 1.457836310255e-02 ||r(i)||/||b|| 1.740567992870e-02
>   5 KSP unpreconditioned resid norm 1.625040500524e-14 true resid norm
> 1.633468028779e-14 ||r(i)||/||b|| 1.950261595401e-14
> Linear solve converged due to CONVERGED_RTOL iterations 5
> KSP final norm of residual 1.63347e-14
> $ ./ex1111 -pc_type fieldsplit -ksp_monitor_true_residual
> -ksp_converged_reason -fieldsplit_pc_type jacobi -ksp_pc_side right
> -ksp_view_final_residual -nest_subvec false
>   0 KSP unpreconditioned resid norm 0.000000000000e+00 true resid norm
> 8.375635517980e-01 ||r(i)||/||b||            inf
> Linear solve converged due to CONVERGED_ATOL iterations 0
> KSP final norm of residual 0.837564
>

I find if I assemble the vector, I get the same answers. Will try to figure
out what assembly is doing.

  Thanks,

    Matt


> Thanks,
> Pierre
>
>

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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