Increasing convergence rate

[Jed Brown]

On Tue, Jan 20, 2009 at 21:52, jerome ho <jerome.snho at gmail.com> wrote:
> 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?

Most preconditioners are not the same in parallel, including these
implementations of AMG.  At a minimum, the smoother is using a block
Jacobi version of SOR or ILU.  As you add processes beyond 2, the
increase in iteration count is usually very minor.

If you are using multiple cores, the per-core floating point
performance will also be worse due to the memory bandwidth bottleneck.
 That may contribute to the poor parallel performance you are seeing.

Jed