[petsc-users] Using nonzero -pc_hypre_boomeramg_restriction_type in field split

[Barry Smith]

  It would be useful to see the convergences inside the linear solve so perhaps start with 

-ksp_monitor_true_residual 

-fieldsplit_u_ksp_type richardson    (this is to allow the monitor below to work)
-fieldsplit_u_ksp_max_its 1 
-fieldsplit_u_ksp_monitor

Perhaps others, Matt/Jed/Pierre/Stefano likely know better off the cuff than me.

We should have a convenience option like -pc_fieldsplit_schur_monitor similar to the -pc_fieldsplit_gkb_monitor



> On Apr 13, 2023, at 4:33 PM, Alexander Lindsay <alexlindsay239 at gmail.com> wrote:
> 
> Hi, I'm trying to solve steady Navier-Stokes for different Reynolds numbers. My options table
> 
> -dm_moose_fieldsplit_names u,p
> -dm_moose_nfieldsplits 2
> -fieldsplit_p_dm_moose_vars pressure
> -fieldsplit_p_ksp_type preonly
> -fieldsplit_p_pc_type jacobi
> -fieldsplit_u_dm_moose_vars vel_x,vel_y
> -fieldsplit_u_ksp_type preonly
> -fieldsplit_u_pc_hypre_type boomeramg
> -fieldsplit_u_pc_type hypre
> -pc_fieldsplit_schur_fact_type full
> -pc_fieldsplit_schur_precondition selfp
> -pc_fieldsplit_type schur
> -pc_type fieldsplit
> 
> works wonderfully for a low Reynolds number of 2.2. The solver performance crushes LU as I scale up the problem. However, not surprisingly this options table struggles when I bump the Reynolds number to 220. I've read that use of AIR (approximate ideal restriction) can improve performance for advection dominated problems. I've tried setting -pc_hypre_boomeramg_restriction_type 1 for a simple diffusion problem and the option works fine. However, when applying it to my field-split preconditioned Navier-Stokes system, I get immediate non-convergence:
> 
>  0 Nonlinear |R| = 1.033077e+03
>       0 Linear |R| = 1.033077e+03
>   Linear solve did not converge due to DIVERGED_NANORINF iterations 0
> Nonlinear solve did not converge due to DIVERGED_LINEAR_SOLVE iterations 0
> 
> Does anyone have an idea as to why this might be happening? If not, I'd take a suggestion on where to set a breakpoint to start my own investigation. Alternatively, I welcome other preconditioning suggestions for an advection dominated problem.
> 
> Alex

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