dolfin team mailing list archive

[Kent Andre <kent-and@xxxxxxxxx>]

On to., 2008-09-25 at 10:20 +0200, Jed Brown wrote:
> On Wed 2008-09-24 19:36, Nuno David Lopes wrote:
> > Also another doubt, (i'm guessing  that is my math ignorance only)
> > is it usual that  PETSc::gmres with hypre::amg preconditioner doesn't work.
> > (After reading that it is so powerfull...)
> > In a Stokes 566000^2  subsystem it simply blows up with all of the  RAM memory 
> > (16Gb).   
> > (I tried it with simpler problems but with bigger systems   and it worked 
> > perfectly, converging in <10 iterations.)
> 
> The Stokes problem is indefinite, hence normal preconditioners are not
> expected to work.  Reordering unknowns and diagonal shifts can help, but
> you really want a Schur complement preconditioner or Vanka smoothers for
> multigrid (not conveniently available).  The easiest thing is to
> assemble the matrix
> 
>   P1 = (A  B')         or         P2 = (A  0)
>        (0  M )                         (0  M)
> 
> (where M is the mass matrix) to precondition the Stokes system
> 
>   (A  B') (u)  =  (f)
>   (B  0 ) (p)     (0)
> 
> Unfortunately, Dolfin does not have the plumbing in place for separate
> preconditioning matrices so you will need to work with the KSP directly.
> 
> Jed

You can do it from Python toghether with PyTrilinos though. 
The code is under the sandbox
/dolfin/sandbox/la/trilinos/demo5.py 

Kent