<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div><br></div> BTW: in my experience<div><br></div><div>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).</div><div><br></div><div>Algebraic multigrid is usually run with a Krylov solver and running without a Krylov solver, as far as I am aware, generally performs poorly.<br><div><br><div><br><blockquote type="cite"><div>On Jul 5, 2023, at 11:46 AM, Barry Smith <bsmith@petsc.dev> wrote:</div><br class="Apple-interchange-newline"><div><meta http-equiv="content-type" content="text/html; charset=utf-8"><div style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div><br></div> -ksp_type richardson<div><br></div><div> 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</div><div><br></div><div><br><div><br><blockquote type="cite"><div>On Jul 5, 2023, at 11:28 AM, Khurana, Parv <p.khurana22@imperial.ac.uk> wrote:</div><br class="Apple-interchange-newline"><div><meta charset="UTF-8"><div class="WordSection1" style="page: WordSection1; caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 18px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;">Hello PETSc users,<o:p></o:p></div><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;"><o:p> </o:p></div><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;">I am fairly inexperienced with PETSc as of now and am new to the mailing list! Thanks for running this channel.<o:p></o:p></div><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;"><o:p> </o:p></div><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;">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.<o:p></o:p></div><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;"><o:p> </o:p></div><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;">I currently run the AMG Preconditioned GMRES iterative solve by setting<span class="Apple-converted-space"> </span><span style="font-family: "Courier New";">-ksp_type gmres</span><span class="Apple-converted-space"> </span>and then specifying the preconditioner I want using, for e.g, -<span style="font-family: "Courier New";">pc_type hypre -pc_hypre_type boomeramg</span>. If I want to use the AMG routine, I am currently setting<span class="Apple-converted-space"> </span><span style="font-family: "Courier New";">-ksp_type preonly</span><span class="Apple-converted-space"> </span>and the same<span class="Apple-converted-space"> </span><span style="font-family: "Courier New";">-pc_type</span>. 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<a href="https://petsc.org/main/tutorials/handson/" style="color: rgb(5, 99, 193); text-decoration: underline;">https://petsc.org/main/tutorials/handson/#</a>, but it hasn’t helped me.<o:p></o:p></div><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;"><o:p> </o:p></div><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;">Any hints on how to use AMG directly as a solver?<o:p></o:p></div><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;"><o:p> </o:p></div><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;">Best<o:p></o:p></div><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;">Parv</div></div></div></blockquote></div><br></div></div></div></blockquote></div><br></div></div></body></html>