[petsc-users] sor vs. asm + sor

[Tobin Isaac]

What's the difference between "-pc_type sor -pc_sor_local_symmetric"
and "-pc_type asm -sub_pc_type sor -sub_pc_sor_local_symmetric"?

Specifically, this converges in 30 iterations:

./ex49 -mx 100 -my 100 -elas_ksp_view -elas_ksp_monitor -elas_ksp_type
cg -elas_pc_type gamg -elas_pc_gamg_verbose 10 -elas_pc_gamg_threshold
0. -elas_mg_coarse_pc_type cholesky -elas_pc_mg_smoothup 1
-elas_pc_mg_smoothdown 1 -elas_mg_levels_ksp_type richardson
-elas_mg_levels_pc_type asm -elas_ml_levels_1_sub_pc_type sor

While this iterates forever:

./ex49 -mx 100 -my 100 -elas_ksp_view -elas_ksp_monitor -elas_ksp_type
cg -elas_pc_type gamg -elas_pc_gamg_verbose 10 -elas_pc_gamg_threshold
0. -elas_mg_coarse_pc_type cholesky -elas_pc_mg_smoothup 1
-elas_pc_mg_smoothdown 1 -elas_mg_levels_ksp_type richardson
-elas_mg_levels_pc_type sor

Thanks,
Toby
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