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

[Karabelas, Elias (elias.karabelas@uni-graz.at)]

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<mailto:elias.karabelas at uni-graz.at>) <elias.karabelas at uni-graz.at<mailto: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<mailto: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<mailto:elias.karabelas at uni-graz.at>
Web:  https://ccl.medunigraz.at/
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