KSP/PC choice

[Matthew Knepley]

You could, of course, embed MUMPS as a preconditioner in a Krylov
iteration like GMRES and getthe effect that you want.

  Mat

On 7/27/07, Stephan Kramer <stephan.kramer at imperial.ac.uk> wrote:
> Hi Tim,
>
> As Guy said MUMPS is a direct solver, i.e. it constructs the inverse of
> the matrix (in some sense) and applies that to the rhs. This means first
> of all you need quite a lot of memory, as  you need to store a dense
> matrix, but also that there is no iterative process. You do not/should
> not supply an initial guess. ksp_type should really be preonly, anything
> else doesn't make sense. As there are no iterations, it also doesn't
> return a number of iterations and there is also no convergence to
> monitor,
>
> Hope this helps
>
> Cheers
> Stephan
>
> On Fri, 2007-07-27 at 09:21 +0200, Tim Kröger wrote:
> > Dear Matt
> >
> > On Wed, 25 Jul 2007, Matthew Knepley wrote:
> >
> > >> -ksp_type preonly
> > >> -pc_type lu
> > >> -mat_type aijmumps
> > >> -ksp_monitor
> > >>
> > > It appears that you never call MatSetFromOptions() on the matrix you
> > > create, which would prevent the option from changing the type.
> >
> > Thank you very much for your help.  Now, the MUMPS solver is working
> > for me, and it really seems to solve the problem that I was working on
> > for nearly a year now!
> >
> > However, I have one more question: I usually call
> > KSPGetIterationNumber() after the solve, and if it tells me that 0 KSP
> > iterations have been used, I assume that the initial guess was already
> > accurate enough.  In this case, I don't have to solve the system in
> > the next time steps anymore until some change of the setting takes
> > place (which happens from time to time in my application).  This saves
> > a lot of computational time.  However, with the MUMPS solver,
> > KSPGetIterationNumber() never supplies 0.  Is there any possibility to
> > find out whether the initial guess was accurate enough?
> >
> > (It might be noteworthy that I use "-ksptype richardson" instead of
> > "-ksptype preonly" since in the latter case I get an error message due
> > to my non-zero initial guess.)
> >
> > Also, I would like to know whether there is any possibility to monitor
> > the convergence of the MUMPS solver (analogously to -kspmonitor).
> >
> > Best Regards,
> >
> > Tim
> >
>
>


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