[petsc-dev] Preconditioner for Schur complement (pcfieldsplit)

[Thomas Witkowski]

Jed Brown wrote:
> On Thu, Apr 7, 2011 at 09:42, Thomas Witkowski 
> <thomas.witkowski at tu-dresden.de 
> <mailto:thomas.witkowski at tu-dresden.de>> wrote:
>
>     The problem is that the data field (vec->data) of the nested
>     vectors is empty. I created the nested vectors exactly in the same
>     way as in /src/ksp/ksp/examples/tests/ex22.c. When I would make a
>     VecGetArray(myvec, &vecptr) on a nested vector myvec, than vecptr
>     is not a vailde pointer.
>
>
> Indeed, this is why this test uses no preconditioner. I do not 
> recommend VecNest for general use, there are many places where it will 
> be problematic. So go ahead and use the MatNest, but just create a 
> normal Vec.
Great, now it works fine!

Thomas
>
> Eventually, I want to remove the explicit scatters in the PCFieldSplit 
> implementation in favor of VecGetSubVector(). When that is done, 
> VecNest will provide a way to eliminate some copies (at the expense of 
> memory layout so it will still not always be faster), but its use 
> should still remain hidden from user code. Don't bother with it now.