[petsc-users] [PCGAMG + AGG + GMRES] Non-Exact Dirichlet Boundary Conditions

[Mark Adams]

And one iteration of undamped Jacobi after the solve should fix this.

On Wed, Jul 6, 2022 at 8:42 AM Karabelas, Elias (elias.karabelas at uni-graz.at)
<elias.karabelas at uni-graz.at> wrote:

> Dear Matt,
>
> thanks for the fast response. That makes perfect sense to me.
>
> Best regards
> Elias
>
> Am 06.07.22 um 14:35 schrieb Matthew Knepley:
>
> On Wed, Jul 6, 2022 at 7:46 AM Karabelas, Elias (
> elias.karabelas at uni-graz.at) <elias.karabelas at uni-graz.at> wrote:
>
>> Dear all,
>>
>> I don't know if this is a bug, but I observed that when using GMRES with
>> AGG-PCGAMG as preconditioner Dirichlet boundary conditions don't seem to be
>> exactly fulfilled.
>>
>> My Matrix has zero rows and cols with 1 on the diagonal where I have
>> dirichlet-bcs in my FE-mesh and I would expect that the eqs in this rows
>> can be exactly fulfilled (as u_i = g_i) there.
>>
> I would not expect aggregation to be exact here, but only within the
> iteration tolerance. If instead you eliminate those variables, you can
> maintain algebraic exactness.
> This is what we do in examples, like SNES ex56.
>
>   Thanks,
>
>      Matt
>
>> However, when I solve A*x = b with the above solver I only get u_i = g_i
>> + error in that part of the vector. Switching from pc_gamg_type agg to
>> pc_gamg_type classical cures this problem, but the classical is not
>> advertised in the user manual.
>>
>> These are the options I'm currently using:
>>
>> -ksp_type gmres
>> -ksp_pc_side right
>> -pc_type gamg
>> -pc_gamg_type agg [or classical]
>> -pc_gamg_sym_graph 1
>> -pc_gamg_square_graph 1
>> -pc_gamg_agg_nsmooths 1
>> -pc_gamg_threshold 0.01
>> -pc_mg_cycles v
>>
>> Iteration counts are basically the same.
>>
>> Best regards
>>
>> Elias
>>
>> --
>> Dr. Elias Karabelas
>> Research Associate
>> University of Graz
>> Institute of Mathematics and Scientific Computing
>> Heinrichstraße 36
>> A-8010 Graz
>> Austria
>>
>> Phone: +43 316 380 8546
>> Email: elias.karabelas at uni-graz.at
>> Web:  https://ccl.medunigraz.at/
>>
>>
>
> --
> 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/>
>
>
> --
> Dr. Elias Karabelas
> Research Associate
> University of Graz
> Institute of Mathematics and Scientific Computing
> Heinrichstraße 36
> A-8010 Graz
> Austria
>
> Phone: +43 316 380 8546
> Email: elias.karabelas at uni-graz.at
> Web:  https://ccl.medunigraz.at/
>
>
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