On Wed, 2011-08-24 at 08:12 +0800, Wu Fengguang wrote: > > You somehow directly jump to > > > > balanced_rate = task_ratelimit_200ms * write_bw / dirty_rate > > > > without explaining why following will not work. > > > > balanced_rate_(i+1) = balance_rate(i) * write_bw / dirty_rate > > Thanks for asking that, it's probably the root of confusions, so let > me answer it standalone. > > It's actually pretty simple to explain this equation: > > write_bw > balanced_rate = task_ratelimit_200ms * ---------- (1) > dirty_rate > > If there are N dd tasks, each task is throttled at task_ratelimit_200ms > for the past 200ms, we are going to measure the overall bdi dirty rate > > dirty_rate = N * task_ratelimit_200ms (2) > > put (2) into (1) we get > > balanced_rate = write_bw / N (3) > > So equation (1) is the right estimation to get the desired target (3). > > > As for > > write_bw > balanced_rate_(i+1) = balanced_rate_(i) * ---------- (4) > dirty_rate > > Let's compare it with the "expanded" form of (1): > > write_bw > balanced_rate_(i+1) = balanced_rate_(i) * pos_ratio * ---------- (5) > dirty_rate > > So the difference lies in pos_ratio. > > Believe it or not, it's exactly the seemingly use of pos_ratio that > makes (5) independent(*) of the position control. > > Why? Look at (4), assume the system is in a state > > - dirty rate is already balanced, ie. balanced_rate_(i) = write_bw / N > - dirty position is not balanced, for example pos_ratio = 0.5 > > balance_dirty_pages() will be rate limiting each tasks at half the > balanced dirty rate, yielding a measured > > dirty_rate = write_bw / 2 (6) > > Put (6) into (4), we get > > balanced_rate_(i+1) = balanced_rate_(i) * 2 > = (write_bw / N) * 2 > > That means, any position imbalance will lead to balanced_rate > estimation errors if we follow (4). Whereas if (1)/(5) is used, we > always get the right balanced dirty ratelimit value whether or not > (pos_ratio == 1.0), hence make the rate estimation independent(*) of > dirty position control. > > (*) independent as in real values, not the seemingly relations in equation The assumption here is that N is a constant.. in the above case pos_ratio would eventually end up at 1 and things would be good again. I see your argument about oscillations, but I think you can introduce similar effects by varying N. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href