[petsc-users] Variable ordering in PC FieldSplit

[Subramanya G]

Hi Barry,
Thanks for clearing it up. It will be useful if this info was added to the
 documentation for SetFields.
Thanks again
Subramanya

Subramanya G Sadasiva,

Graduate Research Assistant,
Hierarchical Design and Characterization Laboratory,
School of Mechanical Engineering,
Purdue University.

"The art of structure is where to put the holes"
Robert Le Ricolais, 1894-1977


On Tue, Jul 9, 2013 at 10:05 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> On Jul 9, 2013, at 8:57 PM, Subramanya G <subramanya.g at gmail.com> wrote:
>
> > Hi Barry,
> > Thanks. I've been looking at KSP/ex43.C and from what I understand the
> following are the main steps.
> >      PCFieldSplitSetBlockSize(pc,3);  // Total number of fields
> >      PCFieldSplitSetFields(pc,"u"
> > ,2,ufields,ufields); // split into groups  (u and v)
> >
> > PCFieldSplitSetFields(pc,"p",1,pfields,pfields); // p
> >
> > I am unable to see where petsc  expects to know whether the values in
> node major or variable major order.
>
>    Yes, this is why I wrote "the code is easier to use with node major"
> :-).   The SetFields() interface is for simple node major order. To use any
> other ordering you need to use the PCFieldSplitSetIS() interface to set the
> fields. Most of the examples use node major. Sorry for the confusion.
>
>    Barry
>
> >
> > Thanks,
> > Subramanya
> >
> >
> > Subramanya G Sadasiva,
> >
> > Graduate Research Assistant,
> > Hierarchical Design and Characterization Laboratory,
> > School of Mechanical Engineering,
> > Purdue University.
> >
> > "The art of structure is where to put the holes"
> > Robert Le Ricolais, 1894-1977
> >
> >
> > On Tue, Jul 9, 2013 at 9:49 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >
> >    PCFieldsplit is officially neutral as to the arrangement. One can
> define IS's to select any arbitrary subset of variables for each field.
> >
> >    Unofficially we generally recommend node major order, it generally is
> more computationally efficient and the code is a bit easier to use with
> this default.
> >
> >     Barry
> >
> > On Jul 9, 2013, at 8:41 PM, subramanya sadasiva <subramanya.g at gmail.com>
> wrote:
> >
> > > I have a small question about the use of PC field split, does it
> expect variables to be arranged in a particular fashion? Should it be
> variable major (all degrees of freedom corresponding to the first followed
> by all degrees of freedom corresponding to the next) or node major ( all
> degrees of freedom corresponding to each node - with a sub block structure
> . Is there some way of switching between the two ?
> > > Thanks,
> > > Subramanya Sadasiva
> >
> >
>
>
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