Messages in this thread | | | Date | Fri, 12 Aug 2011 21:19:54 +0800 | From | Wu Fengguang <> | Subject | Re: [PATCH 2/5] writeback: dirty position control |
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On Wed, Aug 10, 2011 at 01:20:27AM +0800, Peter Zijlstra wrote: > On Tue, 2011-08-09 at 12:32 +0200, Peter Zijlstra wrote: > > > origin - dirty > > > pos_ratio = -------------- > > > origin - goal > > > > > which comes from the below [*] control line, so that when (dirty == goal), > > > pos_ratio == 1.0: > > > > OK, so basically you want a linear function for which: > > > > f(goal) = 1 and has a root somewhere > goal. > > > > (that one line is much more informative than all your graphs put > > together, one can start from there and derive your function) > > > > That does indeed get you the above function, now what does it mean? > > So going by: > > write_bw > ref_bw = dirty_ratelimit * pos_ratio * -------- > dirty_bw > > pos_ratio seems to be the feedback on the deviation of the dirty pages > around its setpoint.
Yes.
> So we adjust the reference bw (or rather ratelimit) > to take account of the shift in output vs input capacity as well as the > shift in dirty pages around its setpoint.
However the above function should better be interpreted as
write_bw ref_bw = task_ratelimit_in_past_200ms * -------- dirty_bw
where task_ratelimit_in_past_200ms ~= dirty_ratelimit * pos_ratio
It would be highly confusing if trying to find the direct "logical" relationships between ref_bw and pos_ratio in the above equation.
> From that we derive the condition that: > > pos_ratio(setpoint) := 1
Right.
> Now in order to create a linear function we need one more condition. We > get one from the fact that once we hit the limit we should hard throttle > our writers. We get that by setting the ratelimit to 0, because, after > all, pause = nr_dirtied / ratelimit would yield inf. in that case. Thus: > > pos_ratio(limit) := 0 > > Using these two conditions we can solve the equations and get your: > > limit - dirty > pos_ratio(dirty) = ---------------- > limit - setpoint > > Now, for some reason you chose not to use limit, but something like > min(limit, 4*thresh) something to do with the slope affecting the rate > of adjustment. This wants a comment someplace.
Thanks to your reasoning that lead to the more elegant
setpoint - dirty 3 pos_ratio(dirty) := 1 + (----------------) limit - setpoint
Thanks, Fengguang
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