[petsc-users] KSP solver for single process

[Matthew Knepley]

On Tue, Aug 6, 2013 at 2:31 PM, Karl Rupp <rupp at mcs.anl.gov> wrote:

> 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.
>

Mudpack is geometric multigrid:
http://www2.cisl.ucar.edu/resources/legacy/mudpack
This should be faster.

   Matt


> 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
> >
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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