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SubjectRe: [PATCH 2/5] writeback: dirty position control
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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) ?


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