[petsc-users] Accessing Vector's ghost values

[Bojan Niceno]

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 <bojan.niceno at psi.ch 
> <mailto:bojan.niceno at psi.ch>> 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" <bojan.niceno at psi.ch
>>     <mailto:bojan.niceno at psi.ch>> 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
>>>         <bojan.niceno at psi.ch <mailto:bojan.niceno at psi.ch>> 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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