[petsc-users] KSP solver for single process

[Alan]

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