[petsc-dev] Multigrid is confusing

[Mark F. Adams]

On May 25, 2012, at 8:59 AM, Jed Brown wrote:

> On Fri, May 25, 2012 at 1:07 AM, Mark F. Adams <mark.adams at columbia.edu> wrote:
> And, I've never seen Gauss-Siedel used with Cheby because G-S has the correct damping properties, as is, for the Laplacian.
> 
> The point is to have something adequate for things that are not Laplacians. I tried running SOR without Cheby, but it was far less robust.
> 
> So I know it looks funny, but I don't have a similarly robust alternative. If we are living in a world where local work is cheap, we might as well do local SOR instead of pbjacobi. (Note that Cheby+pbjacobi is nearly as good as Cheby+SOR in some cases, but much worse in others.)
> 
> 
> Note, G-S is not symmetric and Cheby for unsymmetric is a different can of worms.  So if A is symmetric then maybe try SSOR.
> 
> The default SOR is local_symmetric.

So it does a forward and backward pass.  So it is really two smoothing steps.  One should compare it with two additive smoothers.

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