> > >> > The overwhleming cost of AMG is the Galerkin triple-product RAP. > > That is overstating it a bit. It can be if you have a hard 3D operator and coarsening slowly is best.
Rule of thumb is you spend 50% time is the solver and 50% in the setup, which is often mostly RAP (in 3D, 2D is much faster). That way you are within 2x of optimal and it often works out that way anyway. Mark
