`On Tue, Aug 23, 2011 at 12:01:00PM +0200, Peter Zijlstra wrote:> On Tue, 2011-08-23 at 11:40 +0800, Wu Fengguang wrote:> > - not a factor at all for updating balanced_rate (whether or not we do (2))> >   well, in this concept: the balanced_rate formula inherently does not> >   derive the balanced_rate_(i+1) from balanced_rate_i. Rather it's> >   based on the ratelimit executed for the past 200ms:> > > >           balanced_rate_(i+1) = task_ratelimit_200ms * bw_ratio> > Ok, this is where it all goes funny..Exactly. This is where it gets confusing and is bone of contention.> > So if you want completely separated feedback loops I would expect> something like:> > 	balance_rate_(i+1) = balance_rate_(i) * bw_ratio   ; every 200ms> I agree. This makes sense. IOW.						      write_bwbdi->dirty_ratelimit_n = bdi->dirty_ratelimit_(n-1) * -------						      dirty_rate> The former is a complete feedback loop, expressing the new value in the> old value (*) with bw_ratio as feedback parameter; if we throttled too> much, the dirty_rate will have dropped and the bw_ratio will be <1> causing the balance_rate to drop increasing the dirty_rate, and vice> versa.I think you meant."if we throttled too much, the dirty_rate will have dropped and the bw_ratio will be >1 causing the balance_rate to increase hence increasing the dirty_rate, and vice versa."> > (*) which is the form I expected and why I thought your primary feedback> loop looked like: rate_(i+1) = rate_(i) * pos_ratio * bw_ratio> > With the above balance_rate is an independent variable that tracks the> write bandwidth. Now possibly you'd want a low-pass filter on that since> your bw_ratio is a bit funny in the head, but that's another story.> > Then when you use the balance_rate to actually throttle tasks you apply> your secondary control steering the dirty page count, yielding:> > 	task_rate = balance_rate * pos_ratio> > >   and task_ratelimit_200ms happen to can be estimated from> > > >           task_ratelimit_200ms ~= balanced_rate_i * pos_ratio> > >   We may alternatively record every task_ratelimit executed in the> >   past 200ms and average them all to get task_ratelimit_200ms. In this> >   way we take the "superfluous" pos_ratio out of sight :) > > Right, so I'm not at all sure that makes sense, its not immediately> evident that <task_ratelimit> ~= balance_rate * pos_ratio. Nor is it> clear to me why your primary feedback loop uses task_ratelimit_200ms at> all. > We I thought that this is evident that.task_ratelimit = balanced_rate * pos_ratioWhat is not evident to me is following.balanced_rate_(i+1) = task_ratelimit_200ms * pos_ratio.Instead, like you, I also thought that following is more obivious.balanced_rate_(i+1) = balanced_rate_(i) * pos_ratioThanksVivek`