-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 27/04/15 18:58, Tobin Isaac wrote: > On Mon, Apr 27, 2015 at 04:06:30PM +0100, Lawrence Mitchell wrote: >>> Dear all, >>> >>> we recently noticed a slowdown when using GAMG that I'm trying >>> to track down in a little more detail. I'm solving an Hdiv-L2 >>> "helmholtz" pressure correction using a schur complement. I >>> precondition the schur complement with 'selfp', which morally >>> looks like a normal Helmholtz operator (except in the DG >>> space). The domain is very anisotropic (a thin atmospheric >>> shell), so getting round to trying Toby's column-based >>> coarsening plugin is on the horizon but I haven't done it yet. >>> >>> I don't have a good feel for exactly when things go worse, but >>> here are two data points: >>> >>> A recentish master (e4b003c), and master from 26th Feb >>> (30ab49e4). I notice in the former that MatPtAP takes >>> significantly longer (full logs below), different coarsening >>> maybe? As a point of comparison, the PCSetup for Hypre takes >>> ballpark half a second on the same operator. >>> >>> I test with KSP ex6 (with a constant RHS): >>> >>> Any ideas? > While there may be other changes that have affected your > performance, I see two things in your logs: > > - The coarse matrix is much smaller (3 vs. 592). The default > coarse equations limit was recently changed from 800 to 50. You > can recover the old behavior with `-pc_gamg_coarse_eq_limit 800`. - > GAMG now uses the square of the adjacency graph only on the finest > level. This means that matrices on the coarser levels will be > larger and have more entries, which probably explains the extra > PtAP time. Maybe Mark can explain the decision to make this > change.
Yes, thanks, this is the change. If I switch back to using the square of the adjacency graph on all levels then I recover the previous behaviour. Mark, any comments? Cheers, Lawrence -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQEcBAEBAgAGBQJVP1ipAAoJECOc1kQ8PEYve1QH/3SChuuIZhDOoOOfxYnezCuC beXRvqrjem6SWMBIft67kvmAr9htGsiGg/vE+NRP9EwNu20kD+fYocJ+mBBcxFit 2FdUperqA8F9gys2wJ+TijEQx58ieBNJJyPmQRa1DOmKHmD/WWcCIBV5q+yML9kt B3z59ujUH1kJrmmpYmUk6aMUDAG0qU2KlVHX+p6zL1kV7ooGdYcKbiAbYdYrKO1l 6vNw65+51Oiyi2X/oAsnxeJeAvMN2ljaSYLZJ1RZgAndSLi6bcZPQZvtzwXfOLs9 C0qSPfXbd5O2DoenYnKzCS7t/kLnl0J3QimYex4tOSQxNQ7MtMG4CMNDmAIepUs= =OY/s -----END PGP SIGNATURE-----
