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

[Peter Zijlstra <peterz@infradead.org>]

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.