[petsc-users] Moving from KSPSetNullSpace to MatSetNullSpace

[Gil Forsyth]

On Tue, Sep 29, 2015 at 11:42 AM, Matthew Knepley <knepley at gmail.com> wrote:

> 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
>

Yes -- sorry, those are both in inline files and are called in the reverse
order that I wrote them out.

I've attached the output of

$PETIBM_DIR/petibm3.6/bin/petibm2d -directory . -poisson_pc_type gamg
-poisson_pc_gamg_type agg -poisson_gamg_agg_nsmooths 1 -poisson_ksp_view
-poisson_ksp_monitor_true_residual -poisson_ksp_converged_reason >
kspview.log



>
>
>> 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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