[petsc-users] boomerAmg scalability

[Barry Smith]

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