[petsc-dev] Multigrid is confusing

[Jed Brown]

It's easy to experiment, but I don't think it pays off.
On May 25, 2012 8:41 AM, "Mark F. Adams" <mark.adams at columbia.edu> wrote:

> I like to compartmetnalize: deal with the math and computer science
> separately.
>
> And you (or someone) decided to only do one comm for SSOR.  You could, and
> maybe should, communicate between SOR steps.
>
> On May 25, 2012, at 9:33 AM, Jed Brown wrote:
>
> 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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