We call simple iteration in PETSc Richardson, while preonly is defined as "apply the preconditioner once" so -ksp_type richardson -pc_type gamg is what you want to try, there is no "tweaks" for -ksp_type preonly to turn it into simple iteration since -ksp_type richardson is exactly for using simple iteration.
> On Jul 5, 2023, at 12:09 PM, Khurana, Parv <p.khuran...@imperial.ac.uk> wrote: > > Thank you Mark, Barry for the advice. > > -ksp_type richardson may be what I am looking for. I wanted a simple > iterative scheme that allows AMG to be the main work horse – this allows that. > On using -ksp_type preonly, I observed only 1 KSP iteration and it does not > drop the residual norm to my desired level (-ksp_rtol 1e-7). Whereas when I > use gmres or Richardson I see 6-10 ksp iterations and the desired residual > drop. Is it possible to increase (and fix) the number of KSP iterations with > preonly? You mentioned below that it gives only one cycle so I presume it is > not possible. > > Geometric mg is not a feasible option for my application as I will be > ultimately dealing with 3D, unstructured meshes for hp-finite element > methods. Some clever tweaks may allow it (like having sub-structured meshes) > but not as of now. > > Best > Parv > > > > From: Barry Smith <bsm...@petsc.dev <mailto:bsm...@petsc.dev>> > Sent: 05 July 2023 16:51 > To: Khurana, Parv <p.khuran...@imperial.ac.uk > <mailto:p.khuran...@imperial.ac.uk>> > Cc: petsc-users@mcs.anl.gov <mailto:petsc-users@mcs.anl.gov> > Subject: Re: [petsc-users] Running AMG libraries as standalone solvers > > This email from bsm...@petsc.dev <mailto:bsm...@petsc.dev> originates from > outside Imperial. Do not click on links and attachments unless you recognise > the sender. If you trust the sender, add them to your safe senders list > <https://spam.ic.ac.uk/SpamConsole/Senders.aspx> to disable email stamping > for this address. > > > > 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 <bsm...@petsc.dev > <mailto:bsm...@petsc.dev>> 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 <p.khuran...@imperial.ac.uk > <mailto:p.khuran...@imperial.ac.uk>> 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
