[petsc-users] Multi-DOF DMDA Vec

[Jed Brown]

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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