BTW: in my experience Geometric multigrid generally works great as a standalone solver, that is accelerating it with a Krylov method is not needed, and while it may improve the convergence rate slightly, it may end up taking a bit more time then not using the Krylov solver (due to the extra Krylov solver overhead).
Algebraic multigrid is usually run with a Krylov solver and running without a Krylov solver, as far as I am aware, generally performs poorly. > On Jul 5, 2023, at 11:46 AM, Barry Smith <[email protected]> wrote: > > > -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 >
