Re: [petsc-users] Accessing Vector's ghost values

23 Feb 2012

      On 2/23/2012 11:49 PM, Mohammad Mirzadeh wrote:
...
based on,
VecCreateGhost(PETSC_COMM_WORLD, n, PETSC_DECIDE, nghost, &ghosts[0], &x);
it seems to me that x is your actual ghosted vector. If this is true, 
then you need to get its "local" form via VecGhostGetLocalForm 
<http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecGhostGetLocalForm.html#VecGhostGetLocalForm>(). 
Once you have done, you should be able to access the ghosted nodes. 
Are you calling this function anywhere?
I tried that before.  I did:

Vec lx;
VecGhostGetLocalForm(x, &lx)

then I copied "lx" to my variable, like

for(Int i=1; i<N; i++)    /* N includes buffer cells */
   unk[i] = lx[i]

but ghost values were also zero.   I am thinking that PETSc somehow 
clears the ghost values after a call to KSP.  Is it the case?

     Kind regards,

     Bojan
...
On Thu, Feb 23, 2012 at 2:32 PM, Bojan Niceno <[email protected] 
<mailto:[email protected]>> wrote:
Dear Mohammad,
it doesn't help me, or I did not understand your explanation.
If I do this:
/* copy internal values (THIS WORKS, BUT COPIES NO BUFFER
    VALUES) */
for(Int i=0; i<n; i++) {
        Int gi = mesh.nodes[i].global_number;
        VecGetValues(x, 1, &gi, &unk[i]);
      }
/* copy ghost values (CREATES MANY WARNINGS */
for(Int i=n; i<N; i++) {
        VecGetValues(x, 1, &i, &unk[i]);
      }
I get arnings are like this.
[0]PETSC ERROR: --------------------- Error Message
    ------------------------------------
    [0]PETSC ERROR: Argument out of range!
    [0]PETSC ERROR: Can only get local values, trying 3518!
    [3]PETSC ERROR: --------------------- Error Message
    ------------------------------------
    [3]PETSC ERROR: Argument out of range!
    [3]PETSC ERROR: Can only get local values, trying 3511!
    [3]PETSC ERROR:
    ------------------------------------------------------------------------
What am I doing wrong here?
Cheers,
Bojan
On 2/23/2012 11:23 PM, Mohammad Mirzadeh wrote:
...
just index x with the local numberings. if you have 'n' local
    nodes and 'g' ghost nodes in x, ghost nodes indecies run from 'n'
    to 'n+g-1'
On Feb 23, 2012 1:16 PM, "Bojan Niceno" <[email protected]
    <mailto:[email protected]>> wrote:
Dear Jed,
thanks.
Now I have the following:
- Array unk, which should hold values inside the partition
        and in ghost cells.  It is big enough to hold both
        - Vec x, created by command VecCreateGhost, with proper
        padding for ghost cells
        - Successful call to linear solver in parallel.
But I need to copy ghost values from x to my array unk.
How can I do it?
Kind regards,
Bojan
On 2/23/2012 10:10 PM, Jed Brown wrote:
...
On Thu, Feb 23, 2012 at 15:05, Bojan Niceno
        <[email protected] <mailto:[email protected]>> wrote:
No, I use global.
The local form is just a local vector. It doesn't even know
        that a global problem exists. You can't index into it using
        global indices. (In general, there is no efficient way to
        look up information in the local vector (includes ghost
        points) using global indices.)
for(Int i=0; i<n; i++) {
                Int gi = mesh.nodes[i].global_number;
                VecGetValues(x, 1, &gi, &unk[i]);
              }
"n" is defined as the number of cells inside, i.e.
            without buffers.  "unk" is my external array.  If I try
            to access buffer values, I use:
for(Int i=0; i<N; i++) {
                Int gi = mesh.nodes[i].global_number;
                VecGetValues(x, 1, &gi, &unk[i]);
              }
But then I end up with tons of warnings, presumably
            because I am going beyond "n".  Vector x was created
            with VecCreateGhost.
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