-ksp_type richardson If you use -ksp_typre preonly you effectively get one V-cycle (or whatever cycle type you may have selected) which in general, will give you a more or less garbage answer
> On Jul 5, 2023, at 11:28 AM, Khurana, Parv <[email protected]> wrote: > > Hello PETSc users, > > I am fairly inexperienced with PETSc as of now and am new to the mailing > list! Thanks for running this channel. > > I seek basic help regarding running AMG routines (BoomerAMG/ML/GAMG). I am > trying to compare the performance of solving a Poisson problem using a AMG > Preconditioned GMRES iterative solve vs using AMG as the solver. I use PETSc > options using the options database keys as of now, and it is connected to a > flow solver (Nektar++) I use for my research. > > I currently run the AMG Preconditioned GMRES iterative solve by setting > -ksp_type gmres and then specifying the preconditioner I want using, for e.g, > -pc_type hypre -pc_hypre_type boomeramg. If I want to use the AMG routine, I > am currently setting -ksp_type preonly and the same -pc_type. However, I am > not sure if this is correct way to go about it due to two reasons: a) my > solution using AMG as a solver with these options has a larger error than AMG > Preconditioned GMRES (which could still be acceptable), and b) I could not > find any clear documentation regarding how to use AMG directly as a solver. I > saw some hints in the examples herehttps://petsc.org/main/tutorials/handson/# > <https://petsc.org/main/tutorials/handson/>, but it hasn’t helped me. > > Any hints on how to use AMG directly as a solver? > > Best > Parv
