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

From: Jan Kara
Date: Thu Aug 11 2011 - 07:14:44 EST

On Thu 11-08-11 10:29:52, Wu Fengguang wrote:
> On Thu, Aug 11, 2011 at 06:34:27AM +0800, Jan Kara wrote:
> > On Tue 09-08-11 19:20:27, 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.
> > > >
> > > > 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
> >
> > Actually, thinking about these formulas, why do we even bother with
> > computing all these factors like write_bw, dirty_bw, pos_ratio, ...
> > Couldn't we just have a feedback loop (probably similar to the one
> > computing pos_ratio) which will maintain single value - ratelimit? When we
> > are getting close to dirty limit, we will scale ratelimit down, when we
> > will be getting significantly below dirty limit, we will scale the
> > ratelimit up. Because looking at the formulas it seems to me that the net
> > effect is the same - pos_ratio basically overrules everything...
>
> Good question. That is actually one of the early approaches I tried.
> It somehow worked, however the resulted ratelimit is not only slow
> responding, but also oscillating all the time.
Yes, I think I vaguely remember that.

> This is due to the imperfections
>
> 1) pos_ratio at best only provides a "direction" for adjusting the
> ratelimit. There is only vague clues that if pos_ratio is small,
> the errors in ratelimit should be small.
>
> 2) Due to time-lag, the assumptions in (1) about "direction" and
> "error size" can be wrong. The ratelimit may already be
> over-adjusted when the dirty pages take time to approach the
> setpoint. The larger memory, the more time lag, the easier to
> overshoot and oscillate.
>
> 3) dirty pages are constantly fluctuating around the setpoint,
> so is pos_ratio.
>
> With (1) and (2), it's a control system very susceptible to disturbs.
> With (3) we get constant disturbs. Well I had very hard time and
> played dirty tricks (which you may never want to know ;-) trying to
> tradeoff between response time and stableness..
Yes, I can see especially 2) is a problem. But I don't understand why
your current formula would be that much different. As Peter decoded from
write_bw
ref_bw = dirty_ratelimit * pos_ratio * --------
dirty_bw

while previously it was essentially:
ref_bw = dirty_ratelimit * pos_ratio

So what is so magical about computing write_bw and dirty_bw separately? Is
it because previously you did not use derivation of distance from the goal
for updating pos_ratio? Because in your current formula write_bw/dirty_bw
is a derivation of position...

Honza
--
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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