[petsc-dev] How to use fieldsplit for a block system with sizes change dynamically in TS

[Matthew Knepley]

On Tue, Sep 22, 2020 at 12:20 PM Mebratu Wakeni <
Mebratu.Wakeni at glasgow.ac.uk> wrote:

> Hello,
>
>
> I have the following issue regarding TS and field split.
>
>
> In a TS setting, I have a linear problem with the IJacobian of the
> IFunction with a block form
>
>
>
> | K   0 |
>
> | 0   D |
>
>
>
> Where K is sparse, while D is a diagonal. The sizes of the square matrices
> K and D are dynamic (varying between time steps).
>

How are you doing this in TS? Are you using just TSStep and calling
TSReset() after each step?


> The fact that, K and D are completely decoupled, and D is just a diagonal
> motivated us to use fieldsplit techniques rather than a monolithic one.
> However, this needs to recreate a fieldsplit preconditioner at each time
> step using the indices of K. To achieve this, I use the TSSetPreStep(ts,
> fun) function to perform the fun(TS ts) routine which does following
>
>
>
> 1.     Get indices of K,
>
>
>
> PetscInt *idx_k; // size_of_k
>
>
>
> 2.     Create an IS corresponding to K (IS is_k) using
>
>
>
>  ISCreatGeneral(mpiComm, size_of_k, idx_k, PETSC_COPY_VALUES, &is_k);
>
>
>
> 3.     Then set a fieldsplit preconditioner PC pc using is_k through
>
>
>
> PCFieldSplitSetIS(pc, NULL, is_k);
>
>
>
> 4.     Finally the is_k is properly destroyed through
>
>
>
> ISDestroy(&is_lo);
>

That all looks fine.


> With the above setup, I have also used the following runtime options
>
>
>
> -ts_monitor
>
> -ts_exact_final_time stepover
>
> -ts_dt 0.1
>
> -ts_max_time 1
>
> -ts_adapt_type none
>
>
>
> -ksp_type gmres
>
> -pc_type fieldsplit
>
> -pc_fieldsplit_diag_use_amat
>

You do not need this unless you have a separate preconditioner matrix.


> -pc_fieldsplit_type additive
>
> -fieldsplit_0_ksp_type preonly
>
> -fieldsplit_0_pc_type lu
>
> -fieldsplit_0_pc_factor_mat_solver_type mumps
>
> -fieldsplit_1_ksp_type preonly
>
> -fieldsplit_1_pc_type jacobi
>
>
>
> -snes_atol 1e-8
>
> -snes_rtol 1e-12
>
> -ksp_monitor
>
> -snes_monitor
>
> -snes_lag_jacobian 1
>

You should not need this since your SNES should converge in 1 iterate with
an exact linear solver and a linear problem.


> Result: with the setup described above and the options, I expect to get
> ksp convergence in a single step (at every time step), however, what I am
> getting in actuality is that the fieldsplit works as expected only for the
> first timestep, after that it either take too many ksp steps to converge or
> diverge completely. I have a suspicion that, the way the fieldsplit set up
> at each time using the TSSetPreStep() somehow adds new split over previous
> splits. For this, I have also tried using TSSetPostStep() function to reset
> both the ksp and pc as follows
>

First check -snes_view to see if you have the solver you think you do, and
also

  -snes_monitor -ksp_monitor_true_residual -ksp_converged_reason
-snes_converged_reason

It sounds to me like something is not being recreated correctly.

  Thanks,

     Matt


> KSPRest(ksp);
>
> PCReset(pc);
>
>
> But these don’t help either.
>


-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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