[petsc-users] Convergence of AMG

[Jed Brown]

Manav Bhatia <bhatiamanav at gmail.com> writes:

> I also observed that the number of KSP iterations increases with an increase in the levels of AMG. Is this true, in general, for all/most applications? 

Adding levels replaces a direct solve with a deeper V-cycle.  When MG is
behaving well, this makes little difference because the coarse
approximations are no worse than the approximations made in the first
coarsening (from fine level to first coarsened level).  But there are
many stubborn problems for which it does hurt, as you have seen.