[petsc-users] SAME_PRECONDITIONER vs. DIFFERENT_NONZERO_PATTERN

[Matthew Knepley]

On Wed, May 30, 2012 at 3:16 PM, John Fettig <john.fettig at gmail.com> wrote:

> On Wed, May 30, 2012 at 3:04 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
> > On Wed, May 30, 2012 at 1:58 PM, John Fettig <john.fettig at gmail.com>
> wrote:
> >>
> >> I have a question about the difference between SAME_PRECONDITIONER and
> >> DIFFERENT_NONZERO_PATTERN.  If you solve multiple RHS's (with
> >> different initial guesses) but the matrix does not change, should you
> >> expect to get identical results with SAME_PRECONDITIONER and
> >> DIFFERENT_NONZERO_PATTERN?
> >
> >
> > Why are you even calling KSPSetOperators? Just call KSPSolve() for each
> > right hand side.
>
> What I mean is if I run the code twice, once with SAME_PRECONDITIONER
> and once with DIFFERENT_NONZERO_PATTERN, the solution for the second
> solve in the code is different.  I am sure the matrix does not change.
>  There are a couple of calls to MatZeroRows but I have verified that
> the diagonal is set to the exact same value both time.  The difference
> in the solution is not insignificant (it is probably below the solver
> tolerance but still in the 3rd significant digit).


Is the initial residual the same. It would be helpful to see the
-ksp_monitor_true_residual
output

   Matt



>
> John
>



-- 
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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