Re: [PATCH 2/5] writeback: dirty position control

Peter Zijlstra
2011-08-26T08:56:34Z

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.

Also, I've yet to see what harm it does if you leave it out, all
feedback loops should stabilize just fine.