Lawrence, Matt, I really do share your point. Nevertheless there are sometimes good reasons to do things "not the best way they should be done", at least in a first time (here PETSc is used within a huge fortran-based general purpose finite element solver and build and extract the pressure mass matrix is not a straightforward task). In the present case, I am looking for "the less worst approach" out of the fieldsplit built-in preconditioners. And I consider this is not an uninteresting question.
Best regards, Nicolas 2016-12-13 19:41 GMT+01:00 Matthew Knepley <[email protected]>: > On Tue, Dec 13, 2016 at 10:50 AM, Karin&NiKo <[email protected]> wrote: > >> Dear Petsc-gurus, >> >> I am solving Biot's poroelasticity problem : >> [image: Images intégrées 1] >> >> I am using a mixed P2-P1 finite element discretization. >> >> I am using the fieldsplit framework to solve the linear systems. Here are >> the options I am using : >> -pc_type fieldsplit >> -pc_field_split_type schur >> -fieldsplit_0_pc_type gamg >> -fieldsplit_0_pc_gamg_threshold -1.0 >> -fieldsplit_0_ksp_type gmres >> -fieldsplit_0_ksp_monitor >> -fieldsplit_1_pc_type sor >> -fieldsplit_1_ksp_type gmres >> -pc_fieldsplit_schur_factorization_type upper >> >> >> By increasing the mesh size, I get increasing numbers of outer >> iterations. >> >> According to your own experience, among all the features of fieldsplit, >> was is the "best" set of preconditioners for this rather classical problem >> in order to get an extensible solver (I would like to solve this problem >> on some tens millions of unknowns of some hundreds of procs)? >> > > Lawrence is right that you should construct the right preconditioner > matrix for the Schur complement, and its probably just something like I + > \Delta with > the correct multipliers. Without the mass matrix, it will likely be quite > bad. It should not take much time to code that up since you already have > the mass > matrix from your c_0 p term. > > Matt > > >> Thanks, >> Nicolas >> > > > > -- > 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 >
