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

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

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.

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/
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