[petsc-users] Accessing Vector's ghost values

[Mohammad Mirzadeh]

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?

On Thu, Feb 23, 2012 at 2:32 PM, Bojan Niceno <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> 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> 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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