On Mon 28-11-11 21:53:39, Wu Fengguang wrote: > Add an upper limit to balanced_rate according to the below inequality. > This filters out some rare but huge singular points, which at least > enables more readable gnuplot figures. > > When there are N dd dirtiers, > > balanced_dirty_ratelimit = write_bw / N > > So it holds that > > balanced_dirty_ratelimit <= write_bw > > The singular points originate from dirty_rate in the below formular: > > balanced_dirty_ratelimit = task_ratelimit * write_bw / dirty_rate > where > dirty_rate = (number of page dirties in the past 200ms) / 200ms > > In the extreme case, if all dd tasks suddenly get blocked on something > else and hence no pages are dirtied at all, dirty_rate will be 0 and > balanced_dirty_ratelimit will be inf. This could happen in reality. > > Note that these huge singular points are not a real threat, since they > are _guaranteed_ to be filtered out by the > min(balanced_dirty_ratelimit, task_ratelimit) > line in bdi_update_dirty_ratelimit(). task_ratelimit is based on the > number of dirty pages, which will never _suddenly_ fly away like > balanced_dirty_ratelimit. So any weirdly large balanced_dirty_ratelimit > will be cut down to the level of task_ratelimit. > > There won't be tiny singular points though, as long as the dirty pages > lie inside the dirty throttling region (above the freerun region). > Because there the dd tasks will be throttled by balanced_dirty_pages() > and won't be able to suddenly dirty much more pages than average. > > Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> After your explanation I agree as well. So you can add Acked-by: Jan Kara <jack@xxxxxxx> Honza > --- > mm/page-writeback.c | 5 +++++ > 1 file changed, 5 insertions(+) > > --- linux-next.orig/mm/page-writeback.c 2011-11-17 20:18:03.000000000 +0800 > +++ linux-next/mm/page-writeback.c 2011-11-17 20:18:23.000000000 +0800 > @@ -804,6 +804,11 @@ static void bdi_update_dirty_ratelimit(s > */ > balanced_dirty_ratelimit = div_u64((u64)task_ratelimit * write_bw, > dirty_rate | 1); > + /* > + * balanced_dirty_ratelimit ~= (write_bw / N) <= write_bw > + */ > + if (unlikely(balanced_dirty_ratelimit > write_bw)) > + balanced_dirty_ratelimit = write_bw; > > /* > * We could safely do this and return immediately: > > -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- 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
