[petsc-users] VecGhost memory layout

[Mohammad Mirzadeh]

Awesome. Thanks Barry for the quick response.

On Thu, Aug 8, 2013 at 1:42 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
> 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
>