[petsc-dev] Multigrid is confusing

[Jed Brown]

Fair point for large subdomains, but two additive have two comm steps
instead of one.
On May 25, 2012 8:24 AM, "Mark F. Adams" <mark.adams at columbia.edu> wrote:

>
> 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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