[petsc-users] Fwd: Problem running ex54f with GAMG

[Mark Adams]

On Fri, Mar 18, 2022 at 11:44 AM Fabio Durastante <fabio.durastante at unipi.it>
wrote:

> Hi everybody,
>
> I'm trying to run the rotated anisotropy example ex54f using CG and GAMG
> as preconditioner, I run it with the command:
>
> mpirun -np 2 ./ex54f -ne 1011 \
> -theta 18.0 \
> -epsilon 100.0 \
> -pc_type gamg \
> -pc_gamg_type agg \
> -log_view \
> -log_trace \
> -ksp_view \
> -ksp_monitor \
> -ksp_type cg \
> -mg_levels_pc_type jacobi \
> -mg_levels_ksp_type richardson \
> -mg_levels_ksp_max_it 4 \
> -ksp_atol 1e-9 \
> -ksp_rtol 1e-12
>
> But the KSP CG seems to stop just after two iterations:
>
>    0 KSP Residual norm 6.666655711717e-02
>    1 KSP Residual norm 9.859661350927e-03
>
> I'm attaching the full log, the problem seems to appear when I modify the
> value of epsilon, if I leave it to the default (1.0) it prints
>
>    0 KSP Residual norm 5.862074869050e+00
>    1 KSP Residual norm 5.132711016122e-01
>    2 KSP Residual norm 1.198566629717e-01
>    3 KSP Residual norm 1.992885901625e-02
>    4 KSP Residual norm 4.919780086064e-03
>    5 KSP Residual norm 1.417045143681e-03
>    6 KSP Residual norm 3.559622318760e-04
>    7 KSP Residual norm 9.270786187701e-05
>    8 KSP Residual norm 1.886403709163e-05
>    9 KSP Residual norm 2.940634415714e-06
>   10 KSP Residual norm 5.015043022637e-07
>   11 KSP Residual norm 9.760219712757e-08
>   12 KSP Residual norm 2.320857464659e-08
>   13 KSP Residual norm 4.563772507631e-09
>   14 KSP Residual norm 8.896675476997e-10
>
> that is very strange because the case with epsilon 1 should be easier.
>

As Barry said, it failed (use -ksp_converged_reason).

You are making it pretty hard with 100. The example uses .1 (ie, 10)

You are adding -mg_levels_ksp_type richardson which does work with jacobi.
Use sor (instead of jacobi) and it should work.
The default ksp_type is chebyshev.


> Any help with this would be great.
>
> Thank you very much,
>
> Fabio Durastante
>
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