Klaij, Christiaan wrote: > The velocity problem is segregated (I use BICG with Jacobi for the 3 > linear systems) but these need (much) less iterations than the pressure > problem. The pressure matrix changes at each solve.
It may change, but it might still make a good preconditioner for several time steps. How many dofs are in your pressure system? You mentioned a few million cells, but it makes a huge difference whether you are using tets vs. hexes, and what the pressure space is. If the pressure space is around 1M dofs, the system is relatively well-conditioned to converge in only 100 iterations with Jacobi which means that you stand a good chance of getting acceptable performance from a 1-level DD preconditioner (block jacobi or small-overlap additive Schwarz). So try Barry's suggestion of -pc_type sor and also -pc_type asm with a few choices of subdomain solver (-sub_pc_type). > Also, I did try ML and, like you say, it needs about two times more > iterations than boomerAMG. Overall, boomerAMG is a bit faster for my > cases than ML. To speed up Hypre, I've found these options to be especially useful. -pc_hypre_boomeramg_strong_threshold defaults to 0.25 which is good for 2D scalar problems, change to 0.5 or above for 3D problems -pc_hypre_boomeramg_agg_nl set this greater than 0 to use aggressive coarsening However, I almost always find ML to be faster. By default, it uses way more levels than you want (often making the coarse level have only 1 dof instead of around 1000) so try reducing -pc_ml_maxNlevels. Jed -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 260 bytes Desc: OpenPGP digital signature URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20090716/8f13f3c1/attachment.pgp>
