[petsc-users] Difference between Block Jacobi and ILU?

[Satish Balay]

On Wed, 13 Jan 2016, Justin Chang wrote:

> Hi all,
> 
> What exactly is the difference between these two preconditioners? When I
> use them to solve a Galerkin finite element poisson problem, I get the
> exact same performance (iterations, wall-clock time, etc).

you mean - when you run sequentially?

With block jacobi - you decide the number of blocks. The default is 1-block/proc
i.e - for sequnetial run you have only 1block i.e  the whole matrix.

So the following are essentially the same:
-pc_type bjacobi -pc_bjacobi_blocks 1 [default] -sub_pc_type ilu [default]
-pc_type ilu

Satish

> Only thing is I can't seem to use ILU in parallel though.