[petsc-users] DMDA questions

[Barry Smith]

On Feb 25, 2014, at 1:08 PM, Xiangdong <epscodes at gmail.com> wrote:

> Okay. The local vector is equal to the local portion of global vector + ghost values. Both of them use the same PETSc ordering.
> 
> The reason I got confused before is I use VecView(vglobal, PETSC_VIEWER_STDOUT_WORLD) to view the global vector. In the output, it has process[0] 1 2 3 4 process[1] 5 6 7 8 .... They are actually not the values stored in processor 0 and 1. They are different from the values printed through VecGetArray. 

   Correct. VecView() on the global vector automatically converts the vector to the natural ordering since that is much easier to understand and reload from files.

    Barry

  I admit it can be confusing that VecView() on these vectors actually reorders them on the fly.

> 
> Thanks.
> 
> Xiangdong
> 
> 
> On Tue, Feb 25, 2014 at 12:48 PM, 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.
>