[petsc-users] How to speed up geometric multigrid

[Jed Brown]

"Mark F. Adams" <mfadams at lbl.gov> writes:
> run3.txt uses:
>
> -ksp_type richardson
>
> This is bad and I doubt anyone recommended it intentionally.  

I would have expected FGMRES, but Barry likes Krylov smoothers and
Richardson is one of a few methods that can tolerate nonlinear
preconditioners.

> You also have, in this file,
>
> -mg_levels_ksp_type gmres
>
> did you or the recommenders mean
>
> -mg_levels_ksp_type richardson  ???
>
> you are using gmres here, which forces you to use fgmres in the outer solver.  This is a safe thing to use you if you apply your BCa symmetrically with a low order discretization then
>
> -ksp_type cg
> -mg_levels_ksp_type richardson 
> -mg_levels_pc_type sor
>
> is what I'd recommend.  

I thought that was tried in an earlier round.

I don't understand why SOR preconditioning in the Krylov smoother is so
drastically more expensive than BJacobi/ILU and why SOR is called so
many more times even though the number of outer iterations 

bjacobi: PCApply              322 1.0 4.1021e+01 1.0 6.44e+09 1.0 3.0e+07 1.6e+03 4.5e+04 74 86 98 88 92 28160064317351226 20106
bjacobi: KSPSolve              46 1.0 4.6268e+01 1.0 7.52e+09 1.0 3.0e+07 1.8e+03 4.8e+04 83100100 99 99 31670065158291309 20800

sor:     PCApply             1132 1.0 1.5532e+02 1.0 2.30e+10 1.0 1.0e+08 1.6e+03 1.6e+05 69 88 99 88 93 21871774317301274 18987
sor:     KSPSolve             201 1.0 1.7101e+02 1.0 2.63e+10 1.0 1.1e+08 1.8e+03 1.7e+05 75100100 99 98 24081775248221352 19652
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 835 bytes
Desc: not available
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20131002/2c61bdc1/attachment.pgp>