[petsc-users] Multi-DOF DMDA Vec

[Anush Krishnan]

Thanks, Jed!


On 13 June 2014 14:43, Jed Brown <jed at jedbrown.org> wrote:

> Anush Krishnan <anush at bu.edu> writes:
>
> > Hello petsc-users,
> >
> > I created a vector using DMDACreate with 3 degrees of freedom. Is it
> > possible for me to access each vector corresponding to a degree of
> freedom?
> > Seeing that I need to access the array as [k][j][i][dof], does it mean
> that
> > the values of each component are not contiguous?
>
> The values are interlaced.  This is generally better for memory
> performance (cache reuse).  See, for example, the PETSc-FUN3D papers or
> the various discretization frameworks that rediscover this every once in
> a while.
>
> > Also, what is the difference between DMDAVecGetArray and
> DMDAVecGetArrayDOF?
>
> With DMDAVecGetArray for multi-component problems, you usually write
>
>   typedef struct {PetscScalar u,v,w;} Field;
>   Field ***x;
>   DMDAVecGetArray(dm,X,&x);
>   ...
>   x[k][j][i].u = 1;
>
> With DMDAVecGetArrayDOF, you use an extra set of indices instead of the
> struct.
>
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