[petsc-users] VecGhost memory layout

[Mohammad Mirzadeh]

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?

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

VecGhostUpdateBegin(F, INSERT_VALUES, SCATTER_FORWARD);
VecGhostUpdateEnd(F, INSERT_VALUES, SCATTER_FORWARD);

Is there any flaw in what I'm doing? Also, as a side question, if I
call VecGetArray directly on F (and not F_loc) do I get junk values?

Thanks,
M