-ksp_type fgmres -pc_type ksp -ksp_ksp_type gmres -ksp_pc_type none
But this is just nesting unpreconditioned GMRES inside FGMRES. It is trivial
to do but I doubt that it buys you anything over just
-ksp_type gmres -pc_type none
Barry
On Jan 16, 2011, at 7:05 PM, Darcoux Christine wrote:
> 2011/1/16 Barry Smith <bsmith at mcs.anl.gov>
>
> On Jan 16, 2011, at 5:59 PM, Darcoux Christine wrote:
>
> > I am new to PETSc and I am interested to use the nonlinear solver in a CFD
> > code for low speed compressible fluid.
> >
> > According to some papers, it seems that GMRES (or any other Krylov method)
> > could be used to precondition FGMRES in a way that is completely
> > matrix-free. Is it something possible with the fgmres implementation
> > provided by PETSc ?
>
> Yes, but like anything with "matrix-free" is the question how you provide a
> decent preconditioner without forming any matrices. If you can do that then
> you are all set.
>
> KSPSetOperators(ksp,A,B,...) or SNESSetJacobian(snes,A,B, ....) where A
> is a a MATSHELL that does matrix vector products or use MatCreateMFFD()
>
> -ksp_type fgmres -pc_type ksp -ksp_ksp_type gmres -ksp_view
>
> If B is some approximate representation of A then B will be used to
> construct the preconditioner, if you never have a matrix-representation but
> have a function/subroutine that is supposedly a good preconditioner then you
> would use PCSHELL to provide it. So there are several possibilities depending
> on what you have and what you want to do.
>
> Barry
>
> > Christine
> >
>
>
> Thank you for the detailled explanations. My idea is to simply precondition
> fGMRES with GMRES to avoid creating a B that is an approximate representation
> of A. If A is represented by a mat-vec routine, I think that the
> preconditionner M (approximation of the inverse of A) could be defined by the
> action of GMRES. Here's a code pseudo-code showing how I would like to define
> such an M.
>
> 1. Define matrix A as a mat-vec operation (matrix free), right-hand-side b
> and x0 a zero vector
> 2. The tricky part is now to define M by a mat-vec function like this
>
> function precon_matvec(x) {
> //Basically, GMRES approximates inverse(A) with a polynomial
> // P(A) so that x = x0 + P(A)*r, where r is the residual. Thus in the
> // call to GMRES, x0 is a zero vector and the right-hand-side is x.
> return gmres(A, x, x0, tolerance=gmres_tol, maxiter=10)
> }
>
> M = CreateMatrixFreePreconditionne
> r (precon_matvec)
>
> 3. Now call fGMRES with the above M preconditionner
>
> Is it possible to acheive something like this with PETSc ?
>
> Cheers,
>
> Christine
>
