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

From: Jan Kara
Date: Wed Aug 10 2011 - 18:34:45 EST

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...

> pos_ratio seems to be the feedback on the deviation of the dirty pages
> around its setpoint. 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.
>
> From that we derive the condition that:
>
> pos_ratio(setpoint) := 1
>
> 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.
>
>
> Now all of the above would seem to suggest:
>
> dirty_ratelimit := ref_bw
>
> 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);
>
> You have:
>
> pos_bw = dirty_ratelimit * pos_ratio
>
> Which is ref_bw without the write_bw/dirty_bw factor, this confuses me..
> why are you ignoring the shift in output vs input rate there?

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