[petsc-users] different convergence behaviour

[Hoang Giang Bui]

Hi all

I'm a bit embarrassed that after careful investigation I found that I made
a wrong configuration in my problem settings. This makes the problem also
not converged with MUMPS at the first place. After fixing that problem, the
NR iterator converges normally with both MUMPS and Hypre. Although
ill-posed, the iterative solver achieves comparable tolerance comparing
with MUMPS solution.

Thanks again for all your helpful suggestions.

Giang

On Fri, Jul 15, 2016 at 6:32 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
>   Use -ksp_type gmres to get it to print the residuals. With preonly it
> doesn't compute or print them.
>
>
> > On Jul 15, 2016, at 11:28 AM, Hoang Giang Bui <hgbk2008 at gmail.com>
> wrote:
> >
> > I used
> >
> > -ksp_monitor_true_residual
> > -ksp_monitor_true_solution
> > -ksp_converged_reason
> >
> > with MUMPS but it does not compute the true residual. Should I compute
> that myself?
> >
> > Below is a sample for a full log of MUMPS
> > https://www.dropbox.com/s/fy5uknooxw77r19/log13Jun16_mumps?dl=0
> >
> >
> > Giang
> >
> > On Fri, Jul 15, 2016 at 2:52 AM, Mark Adams <mfadams at lbl.gov> wrote:
> >
> >
> > On Thu, Jul 14, 2016 at 7:29 PM, Matthew Knepley <knepley at gmail.com>
> wrote:
> > On Thu, Jul 14, 2016 at 6:27 PM, Mark Adams <mfadams at lbl.gov> wrote:
> >
> >
> > Notice that there are 7 orders of magnitude between the apparent
> residual (using the preconditioner), and the actual residual, Ax - b.
> > You are using Hypre, and this generally means the Hypre coarse grid
> operator is crap. Please
> >
> >
> > Huh?, this data looks fine, both the true and preconditioned residual
> stay separated by about 9 orders of magnitude. This just tells you that the
> norm of A (or is it A^-1) is 10^9.  Am I misunderstanding this?
> >
> > This is why Barry and I asked for a comparsion with MUMPS. If you are
> right, and its just the condition number,
> >
> > I said norm not condition number. I trust I'm missing something in this
> thread.
> >
> > the LU
> > will not be any more accurate.
> >
> >    Matt
> >
> > --
> > 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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