On Thu, 16 Aug 2007, Matthew Knepley wrote:
> On 8/16/07, Barry Smith <bsmith at mcs.anl.gov> wrote: > > > > > > > > On Sun, 12 Aug 2007, Zhu Liang wrote: > > > > > Yes, you are right, I am trying to solve Oseen equation. > > > > > > I tried using preconditioner PCILU(1) which works well. > > > But for PCILU(0) it did not work, as well as PCBJACOBI. > > > I have tried to insert zero on the diagonal, but it still does not work. > > > > > > In your opinion, what is the best parallel solver and preconditioner > > > for the Oseen equation ? > > > > Black box? I wish I did! > > > > Matt, when are we going to add a "general purpose" Stokes solver to > > PETSc? > > Does that question even make sense? > > It makes sense if we know more about the discretization and weak form. There > are a few good ways to solve Stokes: block smoothers on an element-by-element > basis, block triangular preconditioning by field, efficient projection onto > the > null space of B. > These all require that we can slice up the unknowns correctly. Please be specific here; do you mean more than just saying which values are in the lower block and which are not? > Once people buy into my discretization framework, which will happen shortly > after I finish coding it, we can start to get this going. I have a > Stokes example > now, without any of these nifty solvers :) They will get done this fall. > > Matt > > > Barry > > > > > > > > Thanks for your reply. > > > Liang > > > > > > On 8/12/07, Barry Smith <bsmith at mcs.anl.gov> wrote: > > > > > > > > > > > > It is not really the BiCGstab Krylov solver that it complaining; > > > > it is the default ILU or block Jacobi with ILU on the blocks that is > > > > complaining. > > > > > > > > You first need to make sure you insert 0 entries on the all the > > > > diagonal > > > > entries that are 0. BUT given the structure of your matrix you may want > > > > to think about solvers that specifically handle that type of matrix. > > > > > > > > Barry > > > > > > > > Stokes type solvers? > > > > > > > > > > > > On Mon, 13 Aug 2007, Zhu Liang wrote: > > > > > > > > > By the way, the block of my matrix is like : > > > > > > > > > > U , a > > > > > A^T , 0 > > > > > > > > > > > > > > > > > > > > ---------- Forwarded message ---------- > > > > > From: Zhu Liang <liangcanada at gmail.com> > > > > > Date: Aug 13, 2007 5:46 AM > > > > > Subject: How can I use BICGStab on a matrix with zero entries on the > > > > > diagonal > > > > > To: petsc-users at mcs.anl.gov > > > > > > > > > > > > > > > Dear petsc-users > > > > > > > > > > When I try to solve a linear equation Ax=b with BicgStab > > > > > preconditioner, > > > > I > > > > > got > > > > > an error "Matrix is missing diagonal number". That is because I have > > > > zeros > > > > > on > > > > > the diagonal of the matrix. > > > > > > > > > > I am wondering if there is some simple method to avoid that? > > > > > > > > > > Best, > > > > > Liang > > > > > > > > > > > > > > > > > > > > > > >
