[petsc-users] VecGhost memory layout

[Barry Smith]

On Aug 8, 2013, at 2:56 PM, Mohammad Mirzadeh <mirzadeh at gmail.com> wrote:

> Hi guys,
> 
> I'm running into a bug that has made me question my understanding of
> memory layout in VecGhost.
> 
> First, I remember reading somewhere before (in the manual or mailing
> list which I cannot find now) that the way they are internally
> organized is all local values followed by all ghost values. In other
> words ghost values are ALWAYS padded to the end of the array. Is this
> correct?
> 

   Yes

> Second, when I want to access both local and ghosted values, I do the following,
> 
> VecGhostGetLocalForm(F, &F_loc);
> VecGetArray(F_loc, &F_loc_ptr);
> // do computation on F_loc_ptr
> VecRestoreArray(F_loc, &F_loc_ptr);
> VecGhostRestoreLocalForm(F, &F_loc);
> 
> here I assume that in accessing F_loc_ptr, all indecies from [0,
> numLocal) are local values and the rest are ghost values. Once I'm
> done, I need every processor to update its ghost region and I call
> 
   Good

> VecGhostUpdateBegin(F, INSERT_VALUES, SCATTER_FORWARD);
> VecGhostUpdateEnd(F, INSERT_VALUES, SCATTER_FORWARD);
> 
> Is there any flaw in what I'm doing?

   No

> Also, as a side question, if I
> call VecGetArray directly on F (and not F_loc) do I get junk values?

    No, they actually share the same array so you will get the same values.

   Barry

> 
> Thanks,
> M