[petsc-users] Running AMG libraries as standalone solvers

[Khurana, Parv]

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 here https://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

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