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