You can use DMDASetBlockFills() or DMDASetBlockFillsSparse().
If you need any finer scale control then they offer you need to copy the
PETSc source that does the preallocation for DMDA generated matrices and
customize it exactly for your problem.
Barry
> On Sep 17, 2018, at 3:31 PM, Oleksandr Koshkarov <[email protected]> wrote:
>
> Hello All,
>
> I have a question about preallocation of DMDA matrix. As I understand, it
> preallocates the number of nonzeros using the stencil width info. However, it
> seems it will not be efficient for me: I have a large dof (around 2000),
> where only some of those dof's are coupled, so those dof*dof blocks inside
> the DMMatrix would be also sparse. I also use this DMDA matrix as
> preconditioner, which has lower coupling than original DMDA array, so stencil
> is effectively smaller (I precondition higher order finite volume method with
> lower order FV method). Can I manually specify how many zeros needs to be
> preallocated for the DMDA matrix?
>
> p.s. I do not want o use normal matrix, as I relay on dmda indexing
> (MatSetValuesStencil).
>
> Thank you and best regards,
>
> Oleksandr.
>
