On Wed, Nov 24, 2010 at 07:15:41PM +0800, Peter Zijlstra wrote: > On Wed, 2010-11-17 at 12:27 +0800, Wu Fengguang wrote: > > plain text document attachment > > (writeback-speedup-per-bdi-threshold-ramp-up.patch) > > Reduce the dampening for the control system, yielding faster > > convergence. > > > > Currently it converges at a snail's pace for slow devices (in order of > > minutes). For really fast storage, the convergence speed should be fine. > > > > It makes sense to make it reasonably fast for typical desktops. > > > > After patch, it converges in ~10 seconds for 60MB/s writes and 4GB mem. > > So expect ~1s for a fast 600MB/s storage under 4GB mem, or ~4s under > > 16GB mem, which seems reasonable. > > > > $ while true; do grep BdiDirtyThresh /debug/bdi/8:0/stats; sleep 1; done > > BdiDirtyThresh: 0 kB > > BdiDirtyThresh: 118748 kB > > BdiDirtyThresh: 214280 kB > > BdiDirtyThresh: 303868 kB > > BdiDirtyThresh: 376528 kB > > BdiDirtyThresh: 411180 kB > > BdiDirtyThresh: 448636 kB > > BdiDirtyThresh: 472260 kB > > BdiDirtyThresh: 490924 kB > > BdiDirtyThresh: 499596 kB > > BdiDirtyThresh: 507068 kB > > ... > > DirtyThresh: 530392 kB > > > > CC: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> > > CC: Richard Kennedy <richard@xxxxxxxxxxxxxxx> > > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> > > --- > > mm/page-writeback.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > --- linux-next.orig/mm/page-writeback.c 2010-11-15 13:08:16.000000000 +0800 > > +++ linux-next/mm/page-writeback.c 2010-11-15 13:08:28.000000000 +0800 > > @@ -125,7 +125,7 @@ static int calc_period_shift(void) > > else > > dirty_total = (vm_dirty_ratio * determine_dirtyable_memory()) / > > 100; > > - return 2 + ilog2(dirty_total - 1); > > + return ilog2(dirty_total - 1) - 1; > > } > > > > /* > > You could actually improve upon this now that you have per-bdi bandwidth > estimations, simply set the period to (seconds * bandwidth) to get > convergence in @seconds. I'd like to, but there is the global vs. bdi discrepancy to be addressed first :) How about doing this simple fix first, and then revisit doing per-bdi vm_dirties after the bandwidth estimation goes upstream? Thanks, Fengguang -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
