On Fri, Jun 9, 2023 at 3:09 AM Nicolas Garcia Guzman <nicolas.gar...@tum.de> wrote:
> Hi, sorry I will attach the logs of a run using 17 restart iterations and > one using 19. Testing using other values like 100, the true residual norm > does diverge. Is there any recommendation for this case? The linear system > comes from a DGFEM discretization in FEniCS of a hyperbolic system of PDEs. > The system is non-SPD, but the equations have been non-dimensionalized. > 1) The problem is a breakdown in GMRES, indicated by DIVERGED_BREAKDOWN. We can get specifics of the breakdown by adding the flag -info ksp 2) Neither solver is actually solving this system. We recommend first using a (possibly slow) solver that solves the system, and then step-by-step backing off it until you are happy with the performance. Here I think we should first try accurate solves of the subdomain: -sub_pc_type lu Thanks, Matt > ------------------------------ > *De:* Matthew Knepley <knep...@gmail.com> > *Enviado:* viernes, 9 de junio de 2023 4:13:35 > *Para:* Nicolas Garcia Guzman > *Cc:* petsc-users@mcs.anl.gov > *Asunto:* Re: [petsc-users] Behavior of KSP iterations when using Restart > > On Thu, Jun 8, 2023 at 9:13 PM Nicolas Garcia Guzman < > nicolas.gar...@tum.de> wrote: > >> Hello, >> >> >> I am solving a linear system using petsc4py, with the following command: >> >> >> python main.py -ksp_type gmres -ksp_gmres_restart 16 -ksp_max_it 180000 >> -ksp_monitor -ksp_converged_reason -ksp_rtol 1e-15 -pc_type asm >> -sub_pc_type ilu -sub_pc_factor_levels 1 -sub_ksp_type preonly >> >> >> In the script all I do is import the libraries, load the linear system, >> set options and solve. >> >> >> However, simply changing the restart parameter to something like 20, will >> make the solution iterate to exactly 2 times the restart parameter, and >> then say the iteration diverged. This happens every time no matter the >> parameter chosen, unless it's 16 or less. Is this expected behavior or is >> the problem coming from my linear system? >> > > 1) Always send the complete output > > 2) I am guessing that the iteration stops because the restarted residual > is too large.This could be due to your linear system. What kind of system > are you solving? > > Thanks, > > Matt > > -- > What most experimenters take for granted before they begin their > experiments is infinitely more interesting than any results to which their > experiments lead. > -- Norbert Wiener > > https://www.cse.buffalo.edu/~knepley/ > <http://www.cse.buffalo.edu/~knepley/> > -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>