[petsc-users] Behaviour of Newton Methods with Direct Solvers.

[Matthew Knepley]

On Fri, Aug 30, 2013 at 9:35 PM, subramanya sadasiva <potaman at outlook.com>wrote:

> Hi Barry,
> I have attached output from the solver with the options that you
> suggested. As you can see the direct solver causes the nonlinear solver to
> diverge - but the code converges easily with gmres
>

Your Jacobian is wrong. The iterative solver does not solve it very
accurately, and thus is able to
get into the basin of convergence.

You can try and verify this by using -snes_fd on a small problem (since its
so slow).

  Thanks,

      Matt


> Thanks,
> Subramanya
>
>
> > Subject: Re: [petsc-users] Behaviour of Newton Methods with Direct
> Solvers.
> > From: bsmith at mcs.anl.gov
> > Date: Fri, 30 Aug 2013 21:16:33 -0500
> > CC: petsc-users at mcs.anl.gov
> > To: potaman at outlook.com
> >
> >
> > On Aug 30, 2013, at 9:14 PM, subramanya sadasiva <potaman at outlook.com>
> wrote:
> >
> > > Hi Barry,
> > > The linear solvers converge in 1 iterations. But the SNES solver fails
> when I use the direct solver.
> >
> > Hmm. I don't have an explanation for this. Could you send some output
> when running with -snes_monitor -ksp_monitor_true_residual for both the
> iterative case and the direct solver case?
> >
> > Barry
> >
> > > Thanks,
> > > Subramanya
> > >
> > > > Subject: Re: [petsc-users] Behaviour of Newton Methods with Direct
> Solvers.
> > > > From: bsmith at mcs.anl.gov
> > > > Date: Fri, 30 Aug 2013 21:11:36 -0500
> > > > CC: petsc-users at mcs.anl.gov
> > > > To: potaman at outlook.com
> > > >
> > > >
> > > > I would not expect this. Are you sure that direct solvers are
> actually working well? You can run -ksp_type gmres -pc_type lu to
> "accelerate" the direct solver. Under normal circumstances the direct
> solver will converge in one iteration but if, for example, the problem is
> singular or nearly singular several iterations might solve the linear
> system but one may not.
> > > >
> > > > Barry
> > > >
> > > > On Aug 30, 2013, at 9:08 PM, subramanya sadasiva <
> potaman at outlook.com> wrote:
> > > >
> > > > > Hi,
> > > > > Is there a reason that the convergence of the newton methods in
> SNES is much better with iterative solvers instead of with direct solvers?
> This is particularly in reference with the VI solvers. I get nearly
> quadratic convergence with the iterative solvers, but the code just
> diverges or converges linearly when i run with -ksp_type preonly .
> > > > > Thanks,
> > > > > Subramanya
> > > >
> >
>



-- 
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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