[petsc-users] Moving from KSPSetNullSpace to MatSetNullSpace

[Barry Smith]

  Olivier,

    In theory you do not need to change anything else. Are you using a different matrix object for the velocity_ksp object than the poisson_ksp object?

    The code change in PETSc is very little but we have a report from another CFD user who also had problems with the change so there may be some subtle bug that we can't figure out causing things to not behave properly.

   First run the 3.7.4 code with -poisson_ksp_view and verify that when it prints the matrix information it prints something like has attached null space if it does not print that it means that somehow the matrix is not properly getting the matrix attached.

    Though older versions had MatSetNullSpace() they didn't necessarily associate it with the KSP so it was not expected to work as a replacement for KSPSetNullSpace() with older versions.

    Because our other user had great difficulty trying to debug the issue feel free to send us at petsc-maint at mcs.anl.gov your code with instructions on building and running and we can try to track down the problem. Better than hours and hours spent with fruitless email. We will, of course, not distribute the code and will delete in when we are finished with it.

   Barry








> On Oct 25, 2016, at 4:38 PM, Olivier Mesnard <olivier.mesnard8 at gmail.com> wrote:
> 
> Hi all,
> 
> We develop a CFD code using the PETSc library that solves the Navier-Stokes equations using the fractional-step method from Perot (1993).
> At each time-step, we solve two systems: one for the velocity field, the other, a Poisson system, for the pressure field.
> One of our test-cases is a 2D lid-driven cavity flow (Re=100) on a 20x20 grid using 1 or 2 procs.
> For the Poisson system, we usually use CG preconditioned with GAMG.
> 
> So far, we have been using PETSc-3.5.4, and we would like to update the code with the latest release: 3.7.4.
> 
> As suggested in the changelog of 3.6, we replaced the routine `KSPSetNullSpace()` with `MatSetNullSpace()`.
> 
> Here is the list of options we use to configure the two solvers:
> * Velocity solver: prefix `-velocity_`
>   -velocity_ksp_type bcgs
>   -velocity_ksp_rtol 1.0E-08
>   -velocity_ksp_atol 0.0
>   -velocity_ksp_max_it 10000
>   -velocity_pc_type jacobi
>   -velocity_ksp_view
>   -velocity_ksp_monitor_true_residual
>   -velocity_ksp_converged_reason
> * Poisson solver: prefix `-poisson_`
>   -poisson_ksp_type cg
>   -poisson_ksp_rtol 1.0E-08
>   -poisson_ksp_atol 0.0
>   -poisson_ksp_max_it 20000
>   -poisson_pc_type gamg
>   -poisson_pc_gamg_type agg
>   -poisson_pc_gamg_agg_nsmooths 1
>   -poissonksp_view
>   -poisson_ksp_monitor_true_residual
>   -poisson_ksp_converged_reason
> 
> With 3.5.4, the case runs normally on 1 or 2 procs.
> With 3.7.4, the case runs normally on 1 proc but not on 2.
> Why? The Poisson solver diverges because of an indefinite preconditioner (only with 2 procs).
> 
> We also saw that the routine `MatSetNullSpace()` was already available in 3.5.4.
> With 3.5.4, replacing `KSPSetNullSpace()` with `MatSetNullSpace()` led to the Poisson solver diverging because of an indefinite matrix (on 1 and 2 procs).
> 
> Thus, we were wondering if we needed to update something else for the KSP, and not just modifying the name of the routine?
> 
> I have attached the output files from the different cases:
> * `run-petsc-3.5.4-n1.log` (3.5.4, `KSPSetNullSpace()`, n=1)
> * `run-petsc-3.5.4-n2.log`
> * `run-petsc-3.5.4-nsp-n1.log` (3.5.4, `MatSetNullSpace()`, n=1)
> * `run-petsc-3.5.4-nsp-n2.log`
> * `run-petsc-3.7.4-n1.log` (3.7.4, `MatSetNullSpace()`, n=1)
> * `run-petsc-3.7.4-n2.log`
> 
> Thank you for your help,
> Olivier
> <run-petsc-3.5.4-n1.log><run-petsc-3.5.4-n2.log><run-petsc-3.5.4-nsp-n1.log><run-petsc-3.5.4-nsp-n2.log><run-petsc-3.7.4-n1.log><run-petsc-3.7.4-n2.log>