[petsc-users] Moving from KSPSetNullSpace to MatSetNullSpace

[Matthew Knepley]

On Tue, Sep 29, 2015 at 10:28 AM, Gil Forsyth <gforsyth at gwu.edu> wrote:

> Hi all,
>
> I've been having some trouble with what should be a relatively simple
> update to an immersed boundary CFD solver from PETSc 3.5.4 to 3.6.1
>
> I'm getting indefinite PC errors for a simple lid-driven cavity test
> problem, 32x32 at Re 100
>
> Under PETSc 3.5.4 using KSPSetNullSpace we used the following to set the
> null space.  This is for a 2D Poisson system with no immersed boundary and
> so the null space is the constant vector.
>
>   MatNullSpace nsp;
>   ierr = MatNullSpaceCreate(PETSC_COMM_WORLD, PETSC_TRUE, 0, NULL, &nsp);
> CHKERRQ(ierr);
>   ierr = KSPSetNullSpace(ksp2, nsp); CHKERRQ(ierr);
>   ierr = MatNullSpaceDestroy(&nsp); CHKERRQ(ierr);
>
>
Clearly this has to happen in the reverse order, since ksp2 would not be
created yet.

For questions about solvers, we HAVE to see the complete output of
-ksp_view so we
know what we are dealing with. Its also nice to have
-ksp_monitor_true_residual -ksp_converged_reason

  Matt


> And then setup the KSP with
>
>   ierr = KSPCreate(PETSC_COMM_WORLD, &ksp2); CHKERRQ(ierr);
>   ierr = KSPSetOptionsPrefix(ksp2, "poisson_"); CHKERRQ(ierr);
>   ierr = KSPSetOperators(ksp2, QTBNQ, QTBNQ); CHKERRQ(ierr);
>   ierr = KSPSetInitialGuessNonzero(ksp2, PETSC_TRUE); CHKERRQ(ierr);
>   ierr = KSPSetType(ksp2, KSPCG); CHKERRQ(ierr);
>   ierr = KSPSetReusePreconditioner(ksp2, PETSC_TRUE); CHKERRQ(ierr);
>   ierr = KSPSetFromOptions(ksp2); CHKERRQ(ierr);
>
>
> The matrix QTBNQ does not change, only the rhs of the system is updated.
>
> We run this with `-pc_type gamg -pc_gamg_type agg -pc_gamg_agg_nsmooths 1`
> and everything seems to work as expected.
>
> Under PETSc 3.6.1, we change only the KSPSetNullSpace line, to
>
>   ierr = MatSetNullSpace(QTBNQ, nsp); CHKERRQ(ierr);
>
> and the same code diverges after 1 timestep and returns a -8
> KSP_DIVERGED_INDEFINITE_PC
>
> This is weird, especially because if we change nsmooths to 2, it runs for
> 264 timesteps and the returns the same error.  But we have explicitly set
> KSPSetReusePreconditioner so it should be using the same PC, right?
>
> Change nsmooths to 3 and it again diverges after 1 timestep.
>
> Change nsmooths to 4 and it runs to completion.
>
> It seems like either gamg's behavior has changed, or that KSPSetNullSpace
> was doing something implicitly that we now need to do explicitly in
> addition to MatSetNullSpace?
>
> Thanks,
> Gil Forsyth
>



-- 
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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