Try using a multilevel method for the Poisson solve. If you use DM, you can just do -pc_type mg -pc_mg_levels 4 (or whatever) to do geometric multigrid. Otherwise, start with algebraic multigrid, -pc_type gamg, or configure with ML or Hypre and -pc_type ml or -pc_type hypre. The iteration count should be much lower, though each iteration will be somewhat more expensive.
On Sat, Jun 30, 2012 at 12:16 AM, Bao Kai <paeanball at gmail.com> wrote: > Hi, all, > > I am currently trying to solve a solute transport problem with PETSC > and finite difference method with structured grids. Two equations are > involved, one is a transport equation with advection and diffusion > terms, another one is the flow equation with gravity, which is > actually a poisson equation. Both the equations use the same solver. > > Since we eventually want to handle non-linear problems, we us SNES > here to solve the equations. I use the BICGSTAB ksp linear solver and > block Jacobi preconditioner. All the tolerances are the default ones > from PETSC. The range of the solution values for transport equation > is about 0-1100, and the range of the solution values of the flow > equations should be between -5000-5000. > > The problem is that the solution of flow equations takes about ten > times of the time than the transport equations. I am wondering if I > should set different tolerances for the two equations. > > Attached please find the convergence processes of the two equations. > Generally it takes about ten times of iterations to solve the flow > equations than transport equations. I only do applications and do not > have good knowledge in the solution of linear system, so I am here to > find some suggestions on this issue. > > The size of the mesh is 500X500X500 (125 million unknowns ) and 512 > nodes (2048 processors ) on BlueGene/P machine are used. > > Any suggestions will be much appreciated. > > Thank you . > > Kai > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120630/cd498916/attachment.html>
