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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2011-08-23 at 11:40 +0800, Wu Fengguang wrote:
> - not a factor at all for updating balanced_rate (whether or not we do (2))
>   well, in this concept: the balanced_rate formula inherently does not
>   derive the balanced_rate_(i+1) from balanced_rate_i. Rather it's
>   based on the ratelimit executed for the past 200ms:
> 
>           balanced_rate_(i+1) = task_ratelimit_200ms * bw_ratio

Ok, this is where it all goes funny..

So if you want completely separated feedback loops I would expect
something like:

	balance_rate_(i+1) = balance_rate_(i) * bw_ratio   ; every 200ms

The former is a complete feedback loop, expressing the new value in the
old value (*) with bw_ratio as feedback parameter; if we throttled too
much, the dirty_rate will have dropped and the bw_ratio will be <1
causing the balance_rate to drop increasing the dirty_rate, and vice
versa.

(*) which is the form I expected and why I thought your primary feedback
loop looked like: rate_(i+1) = rate_(i) * pos_ratio * bw_ratio

With the above balance_rate is an independent variable that tracks the
write bandwidth. Now possibly you'd want a low-pass filter on that since
your bw_ratio is a bit funny in the head, but that's another story.

Then when you use the balance_rate to actually throttle tasks you apply
your secondary control steering the dirty page count, yielding:

	task_rate = balance_rate * pos_ratio

>   and task_ratelimit_200ms happen to can be estimated from
> 
>           task_ratelimit_200ms ~= balanced_rate_i * pos_ratio

>   We may alternatively record every task_ratelimit executed in the
>   past 200ms and average them all to get task_ratelimit_200ms. In this
>   way we take the "superfluous" pos_ratio out of sight :) 

Right, so I'm not at all sure that makes sense, its not immediately
evident that <task_ratelimit> ~= balance_rate * pos_ratio. Nor is it
clear to me why your primary feedback loop uses task_ratelimit_200ms at
all. 

>   There is fundamentally no dependency between balanced_rate_(i+1) and
>   balanced_rate_i/task_ratelimit_200ms: the balanced_rate estimation
>   only asks for _whatever_ CONSTANT task ratelimit to be executed for
>   200ms, then it get the balanced rate from the dirty_rate feedback.

How can there not be a relation between balance_rate_(i+1) and
balance_rate_(i) ?