[petsc-users] help with snes

[Jed Brown]

On Mon, Apr 4, 2011 at 07:59, Kontsantinos Kontzialis <ckontzialis at lycos.com
> wrote:

> I use snes to apply an implicit Runge-Kutta method. I see that the snes and
> ksp solvers converge very slowly. Here is the runtime options I use:
>
> mpiexec -np 4 ./hoac cylinder -snes_mf_operator -pc_type hypre
> -pc_hypre_type euclid -pc_hypre_euclid_levels 2 -ksp_type gmres -dt 1.0e-1
> -n_out 10 -file_out cylinder.txt -mat_inode_limit 5 -snes_monitor -end_time
> 1.0e+1 -roe_flux -snes_converged_reason -snes_max_fail 50 -u_mom 0.2
> -implicit -implicit_type 2 -snes_atol 1.0e-6 -snes_ksp_ew_conv
> -ksp_gmres_cgs_refinement_type REFINE_IFNEEDED
> -ksp_gmres_classicalgramschmidt
>
> Also, I use coloring to compute the jacobian of the system. Any
> suggestions?
>

This is not enough information to do more than guess. What equations are you
solving, what methods have you tried, and how do they perform (show
convergence history, "very slowly" means very different things to different
people)?

To speed up Newton, you can (a) more accurate linear solve, (b) better
initial guess, e.g. provided by stable extrapolation or grid sequencing, (c)
more exotic things like nonlinear Schwarz. For the linear solve, you usually
have to improve the preconditioner (unless something is being done "wrong"
like failing to acknowledge a low-dimensional null space).
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20110404/0d97aab6/attachment.htm>