This is typical for weak preconditioners. Have you tried -pc_type gamg or -pc_type mg (algebraic and geometric multigrid, respectively)? On a structured grid with smooth coefficients, geometric multigrid is possible with low setup cost and convergence in a few iterations independent of problem size.
Maahi Talukder via petsc-users <petsc-users@mcs.anl.gov> writes: > Dear All, > > > I am solving Poisson grid generation equation using 9-point Finite > Difference. The type of convergence plot I am getting has been attached > herewith. As the plot shows, at the beginning, the convergence is very > fast, but later the convergence is slowing down. I am using GMRES with left > preconditioning? > > My question is, is it normal? Any ideas how to make the convergence faster? > > > > Thanks, > Maahi Talukder