On Fri, 17 Aug 2007, Ben Tay wrote:
> Hi, > > I tried to test ml using "-pc_type ml" on my problem. It was working ok using > LU or hypre/boomeramg. However I got the warning: > > Gen_Prolongator warning : max eigen <= 0.0 > Gen_Prolongator warning : max eigen <= 0.0 > Gen_Prolongator warning : max eigen <= 0.0 > Gen_Prolongator warning : max eigen <= 0.0 > Gen_Prolongator warning : max eigen <= 0.0 > > When I use hypre, I used "-pc_type hypre -pc_hypre_type boomeramg which gives > good and fast result for solving my poisson eqn. Is there a recommendation for > ml as well? > > Moreover, although I got answers from ml, my answer using ml differs from LU > and hypre from the 3 sig. fig. When I run the example ex2f, I get different > norms of error: > > LU: Norm of error 0.1192E-05 iterations 4 Huh? Why is this not 1.e-14, since LU is a direct solver, do you mean ILU? > > hypre: Norm of error < 1.e-12,iterations 1 > > ml: Norm of error 0.2098E-03 iterations 2 You may need to play around with the -ksp_rtol <rtol> value or options in ml to (run with -pc_type ml -help to see ML options). Run with -ksp_monitor_true_residual and send us the results, We don't have a lot of experience with ML on real problems. Barry > > It seems that norm of error for ml is a bit too high. How can I make it lower? > > Thank you. > > > > Hong Zhang wrote: > > On Thu, 16 Aug 2007, Ben Tay wrote: > > > > > > > Btw, is the file to download > > > ftp://ftp.mcs.anl.gov/pub/petsc/externalpackages/ml-5.0.tar.gz? > > > > > > > Yes. You can simply use '--download-ml=1' during configuration. > > > > Run your program with '-help' to see all options for using ml. > > See ~petsc/src/ksp/ksp/examples/tests/ex26.c on how to use ML. > > > > Hong > > > > > > > Thanks > > > > > > Matthew Knepley wrote: > > > > > > > The multigrid PC needs information about the grid and discretization. If > > > > you > > > > cannot provide this, use AMG through Hypre or ML. Or if you have a > > > > structured grid, consider using DMMG. > > > > > > > > Thanks, > > > > > > > > Matt > > > > > > > > On 8/15/07, Ben Tay <zonexo at gmail.com> wrote: > > > > > > > > > > > > > Hi, > > > > > > > > > > I found that there's a multigrid option in the manual which can be > > > > > used > > > > > by adding -pctype mg -pcmgtype PC_MG_MULTIPLICATIVE. > > > > > > > > > > However, it seems that I am actually just using LU since with or > > > > > without > > > > > this option, I got exactly the same answer. Am I using multigrid the > > > > > wrong way? > > > > > > > > > > Thanks > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >
