Increasing convergence rate

[jerome ho]

Thanks for your advice. Using the different boomeramg options helps to
trade off between speed and memory.

Right now, I've distribute the matrix to 2 processors. However, when
solving, the parallel version takes a longer time
with more iteration counts.
I enabled -ksp_monitor and they seems to converge at a different rate,
although using the same options.
Is there a reason for this?

The matrix formed by the serial and parallel are the same.

Jerome

On Thu, Jan 15, 2009 at 5:29 PM, Jed Brown <jed at 59a2.org> wrote:
> On Wed, Jan 14, 2009 at 20:04, jerome ho <jerome.snho at gmail.com> wrote:
>> boomeramg+minres: 388MB in 1min (8 iterations)
>> icc+cg: 165MB in 30min  (>5000 iterations)
>> bjacobi+cg: 201MB in 50min (>5000 iterations)
>
> Note that in serial, bjacobi is just whatever -sub_pc_type is (ilu by
> default).  In parallel, it's always worth trying -pc_type asm as an
> alternative to bjacobi.  You can frequently make the incomplete
> factorization stronger by using multiple levels (-pc_factor_levels N),
> but it will use more memory.  It looks like multigrid works well for
> your problem so it will likely be very hard for a traditional method
> to compete.
>
> To reduce memory usage in BoomerAMG, try these options
>
>  -pc_hypre_boomeramg_truncfactor <0>: Truncation factor for
> interpolation (0=no truncation) (None)
>  -pc_hypre_boomeramg_P_max <0>: Max elements per row for
> interpolation operator ( 0=unlimited ) (None)
>  -pc_hypre_boomeramg_agg_nl <0>: Number of levels of aggressive
> coarsening (None)
>  -pc_hypre_boomeramg_agg_num_paths <1>: Number of paths for
> aggressive coarsening (None)
>  -pc_hypre_boomeramg_strong_threshold <0.25>: Threshold for being
> strongly connected (None)
>
> For 3D problems, the manual suggests setting strong_threshold to 0.5.
>
> It's also worth trying ML, especially for vector problems.
>
> Jed
>