Le jeu. 15 avr. 2021 à 11:57, bunel...@gmail.com <bunel.fe...@gmail.com> a écrit :
> Hi, > > thank you very much for your answer. > I have actually tried to solve using GMRES like this : > > SolverControl > solver_control(phi_system_rhs.size()*2,1e-10); > PreconditionJacobi<> preconditioner; > preconditioner.initialize(phi_system_matrix, 1.0); > SolverGMRES<Vector <double>> solver(solver_control ); > solver.solve(phi_system_matrix, phi_update, phi_system_rhs, > preconditioner); > > and with this, I still get an error because the solver reached the max > number of iteration and did not converge... : > > "Iterative method reported convergence failure in step 164354. The > residual in the last step was 0.348567." > Let me rephrase what I said. Non-restarted GMRES is guaranteed to converge. deal.II uses restarted gmres. You can increase the number of iterations before a restart (see https://dealii.org/developer/doxygen/deal.II/structSolverGMRES_1_1AdditionalData.html#ac385db0e8c853c02d828469d5a5bdcca ) > > Unfortunately, I did not find any literature on preconditioner for my > specific system so I guess I am down to trying them all and seeing if one > of them works... > If you have no idea which preconditioner to use, you can try a black-box preconditioner, such as incomplete LU (ILU), algebraic multigrid (AMG), and geometric multigrid (GMG). ILU and AMG require Trilinos or PETSc. For GMG, there are a couple of tutorials you can look at. Best, Bruno > > Le jeudi 15 avril 2021 à 17:41:10 UTC+2, bruno.t...@gmail.com a écrit : > >> Hi, >> >> Bicgstab is not guaranteed to converge especially when using a bad >> preconditioner. You are using Jacobi as preconditioner which is a simple >> but not very good preconditioner. As you can see from the error message, >> you performed 1959 bicgstab iterations. That's a lot and it probably >> indicates that there was a breakdown. The easy fix to your problem is to >> use gmres instead of bicgstab. Gmres is guaranteed to converge. The real >> fix is to use a better preconditioner. With a bad preconditioner, gmres >> will converge slowly. You should look in the literature what kind of >> preconditioner you should use. >> >> Best, >> >> Bruno >> >> On Thursday, April 15, 2021 at 4:21:21 AM UTC-4 bunel...@gmail.com wrote: >> >>> >>> Hi, >>> I'm having a problem for some values of parameters in my code. >>> I get an error "dealii::SolverControl::NoConvergence", almost instantly >>> after the start of the solving process. The status at the end is this : >>> >>> "Iterative method reported convergence failure in step 1959. The >>> residual in the last step was nan." >>> >>> Here is some details about my problem. >>> >>> I'm solving this equation in phi : >>> [image: Screenshot from 2021-04-15 10-03-35.png] >>> with u and v, speeds that are calculated in another part of the code. >>> >>> Since it is a non linear problem in phi, i'm using a Newton method to >>> solve it. >>> I have developped my Newton Method and calculated the part that I'm >>> assembling. >>> [image: Screenshot from 2021-04-15 10-05-29.png] >>> As you can see, it is a non symmetric problem because of the advection >>> term and as such, i'm using the Bicgstab solver like this : >>> >>> SolverControl >>> solver_control(phi_system_rhs.size()*2,1e-10); >>> SolverBicgstab<Vector<double>> solver(solver_control); >>> PreconditionJacobi<> preconditioner; >>> >>> preconditioner.initialize(phi_system_matrix, 1.0); >>> solver.solve(phi_system_matrix, phi_update, phi_system_rhs, >>> preconditioner); >>> phi_constraints.distribute(phi_update); >>> >>> Note that if I use a direct solver like this : >>> >>> SparseDirectUMFPACK A_direct; >>> A_direct.initialize(phi_system_matrix); >>> A_direct.vmult(phi_update, phi_system_rhs); >>> >>> phi_constraints.distribute(phi_update); >>> >>> I don't get an error but it is of course much slower (and the newton >>> method painfully converge but I knew this was gonna be difficult). >>> >>> >>> "The other situation where this error may occur is when your matrix is >>> not invertible (e.g., your matrix has a null-space), or if you try to apply >>> the wrong solver to a matrix (e.g., using CG for a matrix that is not >>> symmetric or not positive definite). In these cases, the residual in the >>> last iteration is likely going to be large." >>> >>> This message at the end of the error made me wonder if I was choosing a >>> bad solver for this task and tried to find the Bicgstab recquirements. >>> Unfortunately, I was not able to find the recquirements for the Bicgstab in >>> the documentation. >>> >>> I found this page >>> <https://www.dealii.org/current/doxygen/deal.II/classSolverBicgstab.html>that >>> tells me to go to the solver base for requirements but I could not find the >>> solverBase page with this information. >>> >>> So could someone point me in the right direction and/or tell me if they >>> have an idea of why this solver is not converging in my case ? >>> >>> Thanks again for developing dealii that is very useful to my research. >>> >>> -- > The deal.II project is located at http://www.dealii.org/ > For mailing list/forum options, see > https://groups.google.com/d/forum/dealii?hl=en > --- > You received this message because you are subscribed to a topic in the > Google Groups "deal.II User Group" group. > To unsubscribe from this topic, visit > https://groups.google.com/d/topic/dealii/ehsEIhSqksU/unsubscribe. > To unsubscribe from this group and all its topics, send an email to > dealii+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/dealii/2570a596-bc4c-401a-81e1-03ee929ef46fn%40googlegroups.com > <https://groups.google.com/d/msgid/dealii/2570a596-bc4c-401a-81e1-03ee929ef46fn%40googlegroups.com?utm_medium=email&utm_source=footer> > . > -- The deal.II project is located at http://www.dealii.org/ For mailing list/forum options, see https://groups.google.com/d/forum/dealii?hl=en --- You received this message because you are subscribed to the Google Groups "deal.II User Group" group. 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