On Thu, Aug 25, 2011 at 12:12:58AM +0800, Peter Zijlstra wrote: > 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. Yeah, it's very possible for N to change over time, in which case balanced_rate will adapt to new N in similar way. 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
