[petsc-users] DMDA questions

[Matthew Knepley]

On Feb 25, 2014 11:49 AM, "Jed Brown" <jed at jedbrown.org> wrote:
>
> Xiangdong <epscodes at gmail.com> writes:
> > For example, if the values on the 4-by-4 grid are [1,2,3,4;  5,6,7,8;
> >  9,10,11,12; 13,14,15,16]. If I use 4 processors and set m=2, n=2 (or
use
> > petsc_decide), then on processor zero, the local portion of the global
> > vector is 1,2,3,4
>
> No, PETSc global ordering is different from natural.  There is a
> detailed picture of this in the users manual and in most PETSc
> tutorials.  Please read that.
>
> > while the local vector has value 1,2,5,6. On processor one, the local
> > portion of the global vector is 5,6,7,8; and the local vector is
> > 3,4,7,8. It looks like the global is natural order, while local vector
> > is petsc order.
>
> No.
>
> >
> >
> >
> >>
> >> > 2) DOF. In each cell, I have two unknowns, say ux and uy. One way is
to
> >> > store them using one global vector with dof=2. The other way is to
create
> >> > two global vectors for each ux and uy with dof=1. Is one approach
better
> >> > than the other?
> >>
> >> The former is better for memory streaming unless your operations
> >> traverse the grid using only one at a time (and then, it would be
better
> >> to rephrase to traverse fewer times, using both values each time).
> >>
> >
> > Any examples in petsc tutorials demonstrating the case dof>1? I found
most
> > of them are dof=1. For dof>1, are the values stored in a interleaved
> > manner?
>
> src/snes/examples/tutorials/ex48.c uses dof=2 and
MatSetValuesBlockedStencil.

SNES ex19 uses 4 dof and is much simpler.

   Matt
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