[petsc-users] ghost points, DMs, and vector operations

[Gideon Simpson]

I’ve got a simple problem where I use a DM to handle a representation of a 
vector complex numbers, storing the real and imaginary components at each 
lattice point. I also have ghost points at either end, i.e.:

DMDACreate1d (PETSC_COMM_WORLD, DM_BOUNDARY_GHOSTED, N, 2 , 1 , NULL, &dm ) ;

I have a few related questions:

1.  Is there a quick way to zero out the ghost points, other than to do 

DMGetLocalVector(dm,&local);    
DMGlobalToLocalBegin(dm,global,INSERT_VALUES,local);
DMGlobalToLocalEnd(dm,global,INSERT_VALUES,local);
DMDAVecGetArray(dm, local, &array);
DMDAGetLocalInfo(dm, &info);
  if(info.xs == 0){
    array[-1].u =0.0;
    array[-1].v =0.0;
  }
  if(info.xs + info.xm == info.mx){
    array[info.mx].u =0.0;
    array[info.mx].v =0.0;
  }
/* call restores after */

My point is, I don’t really need to interfere with any of the entries, just the 
values in those two ghost points.

2.  If I take a dot product of two such vectors (associated with the dm, as 
arrays of complex numbers in terms of real and imaginary parts), does the dot 
product operate on the ghost points too, or just the “real” values?  

3.  If I apply a VecCopy, VecAXPY, or any similar operation  to such vectors, 
do they also operate on the ghost points?

-gideon