Re: [PATCH 2/5] writeback: dirty position control

[Peter Zijlstra <peterz@infradead.org>]

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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