On Mon, Jul 30, 2012 at 4:06 PM, Michele Rosso <mrosso at uci.edu> wrote:
> Hi, > > I am solving a variable coefficients Poisson equation with periodic BCs. > The equation is discretized by using the standard 5-points stencil finite > differencing scheme. > I managed to solve the system successfully with the PCG method and now I > would like to add > a preconditioner to speed up the calculation. My idea is to use the > multigrid preconditioner. > > Example ex22f.F implements what I think I need. > If I understand correctly example ex22f.F, the subroutines "ComputeRHS" > and "ComputeMatrix" define how the > matrix and rhs-vector have to be computed at each level. > In my case tough, both the jacobian and the rhs-vector cannot be computed > "analytically", that is, they depend on variables > whose values are available only at the finest grid. > > How can I overcome this difficulty? > Two possibilities: 1. homogenize on your own and rediscretize 2. use Galerkin coarse operators (possibly with algebraic multigrid) Option 2 is much more convenient because it never For geometric multigrid using DMDA, just use -pc_type mg -pc_mg_galerkin For algebraic multigrid, use -pc_type gamg -pc_gamg_agg_nsmooths 1 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120730/93592b83/attachment.html>
