On Dec 19, 2011, at 9:17 PM, Jed Brown wrote: > On Mon, Dec 19, 2011 at 18:48, Mohamad M. Nasr-Azadani <mmnasr at gmail.com> > wrote: > I have been using BoomerAMG as a preconditioner joint with an iterative > solver, e.g. GMRES of BiCGs for regular 3D CFD problems. > On the top of my head, I can not remember if I had the strong scaling tests > done (I will look into it and let you know if you found any), but for the > weak-scaling case, I definitely saw some scaling issues. > As the size of the system increases, the number of iterations does also > increase (somewhat significantly for my test problem, i.e. incompressible N-S > with complex geometry) which ultimately deteriorates the weak-scaling > behaviors. > > It is very dangerous to use AMG directly on the indefinite problem produced > by most discretizations of incompressible flow. For example, for mixed finite > element methods, I have on multiple occasions observed BoomerAMG produce a > singular preconditioner (with huge null space), leading to the appearance of > convergence in the preconditioned norm, but no actual convergence. >
So please tell use how we SHOULD use AMG with those "indefinite problem produced by most discretizations of incompressible flow" dear teacher :-) Barry
