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. -- 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
