[petsc-dev] Separately configuring pre and post smooth KSPs in PCMG

[Lawrence Mitchell]

Dear petsc-dev,

I'd like to try out doing SOR-like smoothing for a problem with MG
where on the pre-smooth I run forwards, and on the post-smooth backwards.

I can do this by setting up the PC and then spinning over the up and
down smoothers separately.  I wonder if it would be possible to extend
the setup to allow it to be done from options.

I had a look, but I got very lost in where/how the setup of the up
smoother separate from the down smoother works.  It seems to be
triggered on whether you select a different number of smoothing steps
up and down, but I don't necessarily want that.

I think I just want to select a different options prefix in
PCMGGetSmootherUp, but I can't obviously see how to ensure that's
called appropriately.

Cheers,

Lawrence