On Thu, Jul 7, 2022 at 7:03 AM Karabelas, Elias (elias.karabe...@uni-graz.at) <elias.karabe...@uni-graz.at> wrote:
> OK got it, well the "classical" option of GAMG removes this issue, and > also HYPRE does that out-of-the box. > Humm, so you are using hypre in PETSc with a Krylov KSP or using hypre directly. GAMG uses a polynomial smoother by default and hypre does not, I believe. That could be the difference, but I would think the outer Krylov in PETSc would still pollute this. If you are using hypre directly, I could imagine that they take the residual that they compute to check convergence and do a quick update of the solution, with the diagonal that they have access to, with u = u + D^-1 r (this is the sort of clever thing they would do) > I would slightly disagree with non-exact DirichletBCs, especially in > mechanics where I really assume a clamped node is clamped. > > I'm not saying exactness is not important (to you), I'm just saying many people seem to live with it. > Best > Elias > > Am 07.07.22 um 13:00 schrieb Mark Adams: > > I think PCCOMPOSITE is overkill here. > > First, I would only bother with this if this error is a problem. People > use your method all the time and accept error at the scale of the solver > tolerance. > > But if you want the exact solution, well, you could just clobber the > solution values after the solve if you have access to the Diri BC data > on hand. > > I was suggesting just creating a "Richardson/jacobi" solver, hardwire one > iteration, use an initial guess and solve it. > Not great, because this would have to grab the diagonal. If you had the > diagonal (eg, from the AMG smoother that does exist) then this would be > pretty cheap (a residual calculation mostly). > > Mark > > > On Thu, Jul 7, 2022 at 3:14 AM Karabelas, Elias ( > elias.karabe...@uni-graz.at) <elias.karabe...@uni-graz.at> wrote: > >> Hi Mark, >> thanks for the answer, but I'm struggling to translate your suggestion >> into solver options. >> From scrolling through the user manual I think this points towards >> PCCOMPOSITE. >> However, the User Manual is not very precise with composite PCs, so how >> would I achieve this on top of my existing solver options? >> Best regards >> Elias >> >> >> >> Am 06.07.22 um 16:41 schrieb 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.karabe...@uni-graz.at) <elias.karabe...@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.karabe...@uni-graz.at) <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 >>>> 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 >>> Web: https://ccl.medunigraz.at/ >>> >>> >> -- >> 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 >> Web: https://ccl.medunigraz.at/ >> >> > -- > 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 > Web: https://ccl.medunigraz.at/ > >
