On Fri, 2011-09-02 at 14:16 +0200, Peter Zijlstra wrote: > On Mon, 2011-08-29 at 21:37 +0800, Wu Fengguang wrote: > > > > > > Ok so this argument makes sense, is there some formalism to describe > > > such systems where such things are more evident? > > > > I find the most easy and clean way to describe it is, > > > > (1) the below formula > > write_bw > > bdi->dirty_ratelimit_(i+1) = bdi->dirty_ratelimit_i * --------- * pos_ratio > > dirty_bw > > is able to yield > > > > dirty_ratelimit_(i) ~= (write_bw / N) > > > > as long as > > > > - write_bw, dirty_bw and pos_ratio are not changing rapidly > > - dirty pages are not around @freerun or @limit > > > > Otherwise there will be larger estimation errors. > > > > (2) based on (1), we get > > > > task_ratelimit ~= (write_bw / N) * pos_ratio > > > > So the pos_ratio feedback is able to drive dirty count to the > > setpoint, where pos_ratio = 1. > > > > That interpretation based on _real values_ can neatly decouple the two > > feedback loops :) It makes full utilization of the fact "the > > dirty_ratelimit _value_ is independent on pos_ratio except for > > possible impacts on estimation errors". > > OK, so the 'problem' I have with this is that the whole control thing > really doesn't care about N. All it does is measure: > > - dirty rate > - writeback rate > > observe: > > - dirty count; with the independent input of its setpoint > > control: > > - ratelimit > > so I was looking for a way to describe the interaction between the two > feedback loops without involving the exact details of what they're > controlling, but that might just end up being an oxymoron. Hmm, so per Vivek's argument the system without pos_ratio in the feedback term isn't convergent. Therefore we should be able to argue from convergent/stability grounds that this term is indeed needed. Does the stability proof of a control system need the model of what its controlling? I guess I ought to go get a book on this or so. -- 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
