On Thu, Sep 17, 2015 at 7:07 AM, Erik Andreassen <[email protected]> wrote:
> I have played a bit with preconditioners for the different levels in PCMG, > and among others I have tried additive Schwartz (ASM), which does not seem > to work very well – especially with no overlap. I’m curious to hear if > anyone can explain why ASM with no overlap performs so much worse than SOR? > The point of a smoother is to wipe out the high frequencies in the error. SOR does this (provably for the Laplacian), but there is no reason ASM/ILU should do this. If this does not happen, then these parts of the solution remain uncorrected when you go to the coarse grid, and the convergence breaks down. Matt > Thanks, > > Erik > > > > PS: I had a previous question about setting levels in GAMG, and my first > impression is (in line with what Mark Adams wrote) that it is better to > leave it to figure it out automatically. Also when using it as the coarse > level preconditioner in a PCMG. > -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener
