[petsc-users] neumann failure in my version of snes ex12

[Matthew Knepley]

On Fri, Nov 22, 2013 at 6:25 PM, Geoffrey Irving <irving at naml.us> wrote:

> On Fri, Nov 22, 2013 at 4:17 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
> > Geoffrey Irving <irving at naml.us> writes:
> >> Here are results with -ksp_monitor_true_residual -pc_type none:
> >>
> >>     http://naml.us/random/laplace-rtol.txt # with -ksp_rtol 1e-5
> >>     http://naml.us/random/laplace-atol.txt # with -ksp_atol 1e-5
> >
> > Looks like the preconditioner is singular.
>
> The preconditioner is none: -pc_type none.
>
> >> Both versions converge in 3 iterations for the first SNES iteration,
> >> but the relative one starts to churn after that since the residual
> >> starts very small.  The true residual goes down to 4/3 and stagnates.
> >> Is there a convenient way to print out the RHS to see whether it has a
> >> component in the nullspace (which seems likely given the true residual
> >> stagnation)?
> >
> > -snes_monitor_residual will plot the solution.  (This option should be
> >  upgraded to support arbitrary output formats.)  You can also call
> >  VecView from your residual function.
> >
> >> I suppose I already do print the result of SNESComputeFunction on the
> >> zero vec, which is
> >>
> >>     RHS = [  6.66666667e-01   1.33333333e+00   6.66666667e-01
> 1.11022302e-16]
> >
> > Looks like you need to project out the null space.
>
> Yep, I was hallucinating that analytically consistent implies
> discretely consistent.
>
> Is there a standard way to do this in the context of an SNES, where
> I'm not computing the residual directly myself?  Should I write a
> wrapper around DMPlexComputeResidualFEM and pass the wrapper to
> DMSNESSetFunctionLocal, or is there a way to tell SNES about the
> nullspace directly?  Is such a projection happening somewhere in snes
> ex12?
>

SNES will do it automatically. You can just call MatNullSpaceRemove().

   Matt


> Thanks,
> Geoffrey
>



-- 
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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