[petsc-users] KSP solver for single process

[Karl Rupp]

Hi Alan,

please use -log_summary to get profiling information on the run. What is
the bottleneck? Is it the number of solver iterations increasing
significantly? If so, consider changing the preconditioner options (more
levels!). I don't expect a direct solver to be any faster in the 180k
case for a Poisson problem.

Best regards,
Karli


On 08/06/2013 02:22 PM, Alan wrote:
> Dear all,
> I hope you're having a nice day.
> I have a quick question on solving Poisson equation with KSP solvers
> (/src/ksp/ksp/example/tutorial/ex29.c). Currently, I run this solver with:
> -pc_type gamg -ksp_type cg -pc_gamg_agg_nsmooths 1 -mg_levels_ksp_max_it
> 1 -mg_levels_ksp_type richardson -ksp_rtol 1.0e-7
> It performs very well in parallel computation and scalability is fine.
> However, if I run it with a single process, the KSP solver is much
> slower than direct ones, i.e. Mudpack. Briefly, the speed difference
> between the KSP solver and the direct solver is negligible on dealing
> with small problems (i.e.36k DoFs ) but becomes very huge for moderate
> large problems (i.e. 180k DoFs). Although the direct solver inherently
> has better performance for moderate large problems in the single
> process, I wonder if any setup or approach can improve the performance
> of this KSP Poisson solver with the single process? or even make it
> obtain competitive speed (a little bit slower is fine) against direct
> solvers.
> 
> thanks in advance,
> Alan
>