On Mon, Jul 30, 2012 at 4:39 PM, Michele Rosso <mrosso at uci.edu> wrote:
> Thank you, > > I will try to use option 2 as you suggested. > I'd prefer to implement the multigrid preconditioner directly inside the > code > rather then using he command line options. > Could you point me to an example where this (or something similar) is done? > You can use -help and grep to find the functions that provide the functionality in command line options. We recommend using command line options to explore which methods work. Once you have a configuration that you really like, you can put it in the code or (easier) just put it in an options file. > > Thank you, > > Michele > > > On 07/30/2012 02:18 PM, Jed Brown wrote: > > 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/20120730/65de1e8e/attachment-0001.html>
