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

[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/>
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