[petsc-users] boomerAmg scalability

Barry Smith bsmith at mcs.anl.gov
Mon Dec 19 22:57:39 CST 2011

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 :-)


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