[petsc-users] different convergence behaviour

[Hoang Giang Bui]

Hello

I encountered different convergence behaviour of Newton Raphson when using
different solver settings with PETSc

For the first solver configuration, I used direct solver
-ksp_type preonly
-pc_type lu
-pc_factor_mat_solver_package mumps
-mat_mumps_icntl_1 6
-mat_mumps_icntl_4 3
-mat_mumps_icntl_7 4
-mat_mumps_icntl_14 40
-mat_mumps_icntl_23 0

The simulation can run completely and the NR typically converged after 6/7
iterations. Of course, it's very slow. For the second solver configuration:
-ksp_type gmres
-ksp_max_it 300
-ksp_gmres_restart 300
-ksp_gmres_modifiedgramschmidt
-pc_view
-pc_fieldsplit_type multiplicative
-fieldsplit_u_pc_type hypre
-fieldsplit_u_pc_hypre_type boomeramg
-fieldsplit_u_pc_hypre_boomeramg_coarsen_type PMIS
-fieldsplit_u_pc_hypre_boomeramg_strong_threshold 0.6
-fieldsplit_u_pc_hypre_boomeramg_max_levels 25
-fieldsplit_wp_ksp_rtol 1.0e-8
-fieldsplit_wp_pc_type hypre
-fieldsplit_wp_pc_hypre_type boomeramg
-fieldsplit_wp_pc_hypre_boomeramg_coarsen_type PMIS
-fieldsplit_wp_pc_hypre_boomeramg_strong_threshold 0.6
-fieldsplit_wp_pc_hypre_boomeramg_max_levels 25

The solver runs much faster, but the NR does not converge in 30 iterations
after some time steps. I thought setting the solver tolerance -ksp_rtol
1.0e-12 but it doesn't help much because GMRES already terminate with
tolerance 1e-30 (see sample log file). Can we set the tolerance of the
sub-ksp of the Fieldsplit? I tried -fieldsplit_wp_ksp_rtol 1.0e-8 but it
doesn't work.

Sorry this problem is run with many time steps and is quite big so I cannot
reproduce in a simple test case.

Giang
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