[petsc-dev] Multigrid is confusing

[Jed Brown]

On Tue, May 22, 2012 at 9:52 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>  How about a default nonlinear Krylov accelerator? fgrmes is not much more
> expensive than gmres under causal circumstances.


I could handle that. The extra storage concerns me more than time.

I'd eventually like to make PCMG linear by default, especially considering
that the convergence with GMRES(1) smoothers tends to be worse in my
experience (except when coarse level operators are poorly scaled/unstable)
and adds a ton of synchronization points.

Oh, FGMRES by default would necessarily switch from left to right
preconditioning. I'm not sure that's a good thing since it requires much
tighter tolerances for ill-conditioned systems to reach a given error
(especially penalty boundaries). There are advantages both ways and I keep
waffling. I expect that changing the default will cause some surprises.
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