[petsc-users] Roles of ksp_type and pc_type

Matthew Knepley knepley at gmail.com
Thu Nov 13 06:39:38 CST 2014

On Thu, Nov 13, 2014 at 6:24 AM, Massimiliano Leoni <
leoni.massimiliano1 at gmail.com> wrote:

> In data giovedì 13 novembre 2014 05:59:17, hai scritto:
> > On Thu, Nov 13, 2014 at 5:39 AM, Massimiliano Leoni <
> > We call Hypre directly to apply P^{-1}.
> >
> > It should be. Send -ksp_view and we can verify.
> It looks like that's what's happening! :D
> Here it is
> http://pastebin.com/6J3Rj4x1

This is not exactly right because here you are just using additive
fieldsplit. The mass matrix
is a good preconditioner for the Schur complement, not the zero matrix.
Take a look at these


at slide 115, or my talk from SIAM CS&E




> > You should use
> >   -pc_fieldsplit_schur_factorization_type diag
> Ok, This is not clear to me.
> According to what I studied, Schur decomposition is when I formally
> eliminate
> a variable [for Stokes, velocity u] and get a system in the others [sole
> pressure], which I then solve by some smart method and substitute back to
> find
> u. This is done on the full Stokes matrix A, not on the preconditioner...
> It should be not what I want to do, as I'm facing the whole system at once.
> What does PETSc mean by "Schur" here?
> >    Thanks,
> >
> >       Matt
> Thank you very much!
> Massimiliano

What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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