On Fri, Aug 26, 2011 at 04:56:11PM +0800, Peter Zijlstra wrote: > On Fri, 2011-08-26 at 09:56 +0800, Wu Fengguang wrote: > > /* > > * A linear estimation of the "balanced" throttle rate. The theory is, > > * if there are N dd tasks, each throttled at task_ratelimit, the bdi's > > * dirty_rate will be measured to be (N * task_ratelimit). So the below > > * formula will yield the balanced rate limit (write_bw / N). > > * > > * Note that the expanded form is not a pure rate feedback: > > * rate_(i+1) = rate_(i) * (write_bw / dirty_rate) (1) > > * but also takes pos_ratio into account: > > * rate_(i+1) = rate_(i) * (write_bw / dirty_rate) * pos_ratio (2) > > * > > * (1) is not realistic because pos_ratio also takes part in balancing > > * the dirty rate. Consider the state > > * pos_ratio = 0.5 (3) > > * rate = 2 * (write_bw / N) (4) > > * If (1) is used, it will stuck in that state! Because each dd will be > > * throttled at > > * task_ratelimit = pos_ratio * rate = (write_bw / N) (5) > > * yielding > > * dirty_rate = N * task_ratelimit = write_bw (6) > > * put (6) into (1) we get > > * rate_(i+1) = rate_(i) (7) > > * > > * So we end up using (2) to always keep > > * rate_(i+1) ~= (write_bw / N) (8) > > * regardless of the value of pos_ratio. As long as (8) is satisfied, > > * pos_ratio is able to drive itself to 1.0, which is not only where > > * the dirty count meet the setpoint, but also where the slope of > > * pos_ratio is most flat and hence task_ratelimit is least fluctuated. > > */ > > I'm still not buying this, it has the massive assumption N is a > constant, without that assumption you get the same kind of thing you get > from not adding pos_ratio to the feedback term. The reasoning between (3)-(7) actually assumes both N and write_bw to be some constant. It's documenting some stuck state.. > Also, I've yet to see what harm it does if you leave it out, all > feedback loops should stabilize just fine. That's a good question. It should be trivial to try out equation (1) and see how it work out in practice. Let me collect some figures.. 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
