[petsc-users] Fwd: SNES with Fieldsplit + MatNest in parallel

[Matthew Knepley]

On Tue, Nov 9, 2021 at 6:52 AM Nicolas Barnafi <nabw91 at gmail.com> wrote:

> Thanks for the help Matthew.
>
>
> The next step would be to verify that the operators you get in those slots
> are correct in parallel. I normally just print out the matrix
> for a small problem. This is the only way I know to debug assembly.
>
> To check the solver, you set it to an exact solver and see that it takes 1
> iterate. That means using LU as the subsolver
> for the first block, using the full Schur complement, and solving the
> Schur system with a residual tolerance of something like 1e-10.
>
>
> There is certainly something wrong in the nested matrix, as this is the
> output in serial (LU + full Schur LU):
>

Yes, this is why I recommend going to just 2 or 4 cells, so that you can
see explicitly what is wrong with the matrix.

  Thanks,

     Matt


>   0 SNES Function norm 3.240309155660e+03
>     0 KSP Residual norm 3.240309155660e+03
>     1 KSP Residual norm 3.219893365194e-08
>   1 SNES Function norm 3.999872813081e+03
>     0 KSP Residual norm 1.214145680994e+04
>     1 KSP Residual norm 2.742817643080e-04
>   2 SNES Function norm 3.663148395735e+01
>     0 KSP Residual norm 3.980103473350e+03
>     1 KSP Residual norm 1.814754406828e-05
>   3 SNES Function norm 2.136252100504e+01
>     0 KSP Residual norm 6.970609974245e+01
>     1 KSP Residual norm 1.982821753925e-05
>   4 SNES Function norm 1.059694009734e-02
>     0 KSP Residual norm 2.136131725219e+01
>     1 KSP Residual norm 4.545878835451e-06
>     2 KSP Residual norm 8.841427426699e-08
>     3 KSP Residual norm 3.305898584949e-09
>   5 SNES Function norm 3.729120062850e-03
>     0 KSP Residual norm 3.846880232871e-03
>     1 KSP Residual norm 1.740268533385e-06
>     2 KSP Residual norm 3.013108797112e-08
>     3 KSP Residual norm 1.105602265959e-09
>   6 SNES Function norm 1.117697448247e-09
>
> whereas in parallel it is this (two processes):
>
>   0 SNES Function norm 3.240309155660e+03
>     0 KSP Residual norm 3.240309155660e+03
>     1 KSP Residual norm 2.158162432432e-01
>     2 KSP Residual norm 2.156121452805e-01
>     3 KSP Residual norm 2.148639482595e-01
>     4 KSP Residual norm 2.148248193325e-01
>     5 KSP Residual norm 1.305140166188e-03
>   Linear solve converged due to CONVERGED_RTOL iterations 5
>   1 SNES Function norm 4.774611203034e+42
>
>
> Best,
> NB
>


-- 
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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