[petsc-dev] PCFieldSplitSetFields() API change

[Jed Brown]

But what does using B as 00 block actually mean? It seems to me that you
pivot, producing P' P J where P J has B in the 00 location.
On Mar 23, 2012 6:13 PM, "Barry Smith" <bsmith at mcs.anl.gov> wrote:

>
> On Mar 23, 2012, at 6:00 PM, Jed Brown wrote:
>
> > On Fri, Mar 23, 2012 at 17:48, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >    I'm confused. I am NOT injecting it into the original definition, I
> am injecting it into the PCFieldSplit in the same way that your
> "permutation" would inject it just into the PCFieldSplit, it is just (in my
> opinion) a more user friendly way of the user providing this permutation.
> Note that this "two fields" business is NOT going in the DM (and hence into
> the original definition of the problem) and would normally only be used
> when the DM definition was "bad" (due to zero block for example) just like
> your permutation would only be used when things were bad.
> >
> > 1. The PCFieldSplitSetIS() interface does not have this.
> >
> > 2. The reason for pivoting is not that the DM ordering was "bad", it's
> that a symmetric ordering without the 0 block does not exist (factorization
> does not work with _any_ symmetric ordering). So you pivot for
> factorization and restore.
>
>   I don't agree. Consider   [A  B ;  C D] where all four matrices are
> square and non-singular. It may be that using B as the 00 block is just
> better for convergence etc
>
> >
> > 3. You now have one name for a set of rows and columns that may not "go
> together".
>
>   That name is only for setting fieldsplit options etc. It is not
> fieldnames in the way that DM fieldnames are names of fields.
>
> >
> > Somehow I think this is just more confusing and less run-time-friendly,
> but maybe that's my fault. In any case, the change has to be consistent
> (PCFieldSplitSetIS) and we have to decide how the DM can specify when
> pivoting is needed.
>
>   You may be right. Spec out your alternative Functions I would call and
> command line options I would use to get the same effect as the affect
> Jungho and I were trying to achieve.  I was bothered by the separation in
> your original proposal between the
> selection of the blocks the "pivoting" of your model. I don't really see
> using a different block in 00 as necessarily a "pivoting" that is why you
> proposal turned me off.
>
>
>   Barry
>
>
>
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