[petsc-users] Problem running ex54f with GAMG

[Mark Adams]

MG is not stable with richardson/jacobi, or at least it is almost not
stable.
Yea I would guess gmres will go forever because it does not care.
I think CG exists when it gets a negative number for beta or whatever and
it was probably -eps.
That is my guess.

On Fri, Mar 18, 2022 at 3:17 PM Barry Smith <bsmith at petsc.dev> wrote:

>
>   The GAMG produced an indefinite operator. I don't know if there is a way
> to detect why this happened or how to stop it.
>
>   You can try -ksp_type gmres and see how that goes since it can handle an
> indefinite preconditioner.
>
>
>
> > On Mar 18, 2022, at 11:53 AM, Fabio Durastante <
> fabio.durastante at unipi.it> wrote:
> >
> > For the default case:
> >
> > Linear solve converged due to CONVERGED_ATOL iterations 14
> >
> > for the other it tells
> >
> > Linear solve did not converge due to DIVERGED_INDEFINITE_PC iterations 2
> >
> > that again seems a bit strange to me, since this should be a symmetric
> V-cycle built on smoothed aggregation that should be definite (and
> symmetric).
> >
> > Fabio
> >
> > Il 18/03/22 16:47, Barry Smith ha scritto:
> >>   Run with -ksp_converged_reason to have it print why it has stopped
> the iteration.
> >>
> >>
> >>> On 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.
> >>>
> >>> Any help with this would be great.
> >>>
> >>> Thank you very much,
> >>>
> >>> Fabio Durastante
> >>> <log_ex54f.txt>
>
>
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