Messages in this thread | | | Subject | Re: [PATCH 2/5] writeback: dirty position control | From | Peter Zijlstra <> | Date | Mon, 22 Aug 2011 17:38:07 +0200 |
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On Fri, 2011-08-12 at 22:20 +0800, Wu Fengguang wrote: > On Fri, Aug 12, 2011 at 09:04:19PM +0800, Peter Zijlstra wrote: > > On Tue, 2011-08-09 at 19:20 +0200, Peter Zijlstra wrote: > > To start with, > > write_bw > ref_bw = task_ratelimit_in_past_200ms * -------- > dirty_bw > > where > task_ratelimit_in_past_200ms ~= dirty_ratelimit * pos_ratio > > > > Now all of the above would seem to suggest: > > > > > > dirty_ratelimit := ref_bw > > Right, ideally ref_bw is the balanced dirty ratelimit. I actually > started with exactly the above equation when I got choked by pure > pos_bw based feedback control (as mentioned in the reply to Jan's > email) and introduced the ref_bw estimation as the way out. > > But there are some imperfections in ref_bw, too. Which makes it not > suitable for direct use: > > 1) large fluctuations
OK, understood.
> 2) due to truncates and fs redirties, the (write_bw <=> dirty_bw) > becomes unbalanced match, which leads to large systematical errors > in ref_bw. The truncates, due to its possibly bumpy nature, can hardly > be compensated smoothly.
OK.
> 3) since we ultimately want to > > - keep the dirty pages around the setpoint as long time as possible > - keep the fluctuations of task ratelimit as small as possible
Fair enough ;-)
> the update policy used for (2) also serves the above goals nicely: > if for some reason the dirty pages are high (pos_bw < dirty_ratelimit), > and dirty_ratelimit is low (dirty_ratelimit < ref_bw), there is no > point to bring up dirty_ratelimit in a hurry and to hurt both the > above two goals.
Right, so still I feel somewhat befuddled, so we have:
dirty_ratelimit - rate at which we throttle dirtiers as estimated upto 200ms ago.
pos_ratio - ratio adjusting the dirty_ratelimit for variance in dirty pages around its target
bw_ratio - ratio adjusting the dirty_ratelimit for variance in input/output bandwidth
and we need to basically do:
dirty_ratelimit *= pos_ratio * bw_ratio
to update the dirty_ratelimit to reflect the current state. However per 1) and 2) bw_ratio is crappy and hard to fix.
So you propose to update dirty_ratelimit only if both pos_ratio and bw_ratio point in the same direction, however that would result in:
if (pos_ratio < UNIT && bw_ratio < UNIT || pos_ratio > UNIT && bw_ratio > UNIT) { dirty_ratelimit = (dirty_ratelimit * pos_ratio) / UNIT; dirty_ratelimit = (dirty_ratelimit * bw_ratio) / UNIT; }
> > > However for that you use: > > > > > > if (pos_bw < dirty_ratelimit && ref_bw < dirty_ratelimit) > > > dirty_ratelimit = max(ref_bw, pos_bw); > > > > > > if (pos_bw > dirty_ratelimit && ref_bw > dirty_ratelimit) > > > dirty_ratelimit = min(ref_bw, pos_bw); > > The above are merely constraints to the dirty_ratelimit update. > It serves to > > 1) stop adjusting the rate when it's against the position control > target (the adjusted rate will slow down the progress of dirty > pages going back to setpoint).
Not strictly speaking, suppose pos_ratio = 0.5 and bw_ratio = 1.1, then they point in different directions however:
0.5 < 1 && 0.5 * 1.1 < 1
so your code will in fact update the dirty_ratelimit, even though the two factors point in opposite directions.
> 2) limit the step size. pos_bw is changing values step by step, > leaving a consistent trace comparing to the randomly jumping > ref_bw. pos_bw also has smaller errors in stable state and normally > have larger errors when there are big errors in rate. So it's a > pretty good limiting factor for the step size of dirty_ratelimit.
OK, so that's the min/max stuff, however it only works because you use pos_bw and ref_bw instead of the fully separated factors.
> Hope the above elaboration helps :)
A little..
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