[petsc-dev] VecNest

[Mark Adams]

It looks like the RHS is zero in the 2nd case (0 KSP unpreconditioned resid
norm 0.000000000000e+00), but the true residual is the same.

It looks like you added "nest_subvec" to our example.

You can start by looking at the vectors with -vec_view (there is code that
you can view vectors explicitly in your code to get more control if this is
too noisy)

You could send us the diffs in ex111. We may be able to see something wrong.

Mark

On Mon, Jul 11, 2022 at 2:17 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
>
> Thanks,
> Pierre
>
>
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