[petsc-users] Moving from KSPSetNullSpace to MatSetNullSpace

[Barry Smith]

   This can't work. You can't use a GMRES inside a CG.   Try changing to -poisson_mg_coarse_ksp_type preonly

KSP Object:(poisson_) 1 MPI processes
  type: cg

KSP Object:    (poisson_mg_coarse_)     1 MPI processes
      type: gmres
        GMRES: restart=30, using Classical (unmodified) Gram-Schmidt Orthogonalization with no iterative refinement
        GMRES: happy breakdown tolerance 1e-30
      maximum iterations=1, initial guess is zero


> On Sep 29, 2015, at 10:53 AM, Gil Forsyth <gforsyth at gwu.edu> wrote:
> 
> 
> 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
> 
> <kspview.log>