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

Wu Fengguang
2011-08-16T08:55:32Z

Hi Vivek,

Sorry it made such a big confusion to you. I hope Peter's 3rd order
polynomial abstraction in v9 can clarify the concepts a lot.

As for the old global control line

origin - dirty
pos_ratio = -------------- (1)
origin - goal

where

origin = 4 * thresh (2)

effectively decides the slope of the line. The use of @limit in code

origin = max(4 * thresh, limit) (3)

is merely to safeguard the rare case that (2) might result in negative
pos_ratio in (1).

I have another patch to add a "brake" area immediately below @limit
that will scale pos_ratio down to 0. However that's no longer
necessary with the 3rd order polynomial solution.

Note that @limit will normally be equal to @thresh except in the rare
case that @thresh is suddenly knocked down and @limit is taking time
to follow it.

Thanks,
Fengguang

> Hi Fengguang,
>
> Ok, so just trying to understand this pos_ratio little better.
>
> You have following basic formula.
>
> origin - dirty
> pos_ratio = --------------
> origin - goal
>
> Terminology is very confusing and following is my understanding.
>
> - setpoint == goal
>
> setpoint is the point where we would like our number of dirty pages to
> be and at this point pos_ratio = 1. For global dirty this number seems
> to be (thresh - thresh / DIRTY_SCOPE)
>
> - thresh
> dirty page threshold calculated from dirty_ratio (Certain percentage of
> total memory).
>
> - Origin (seems to be equivalent of limit)
>
> This seems to be the reference point/limit we don't want to cross and
> distance from this limit basically decides the pos_ratio. Closer we
> are to limit, lower the pos_ratio and further we are higher the
> pos_ratio.
>
> So threshold is just a number which helps us determine goal and limit.
>
> goal = thresh - thresh / DIRTY_SCOPE
> limit = 4*thresh
>
> So goal is where we want to be and we start throttling the task more as
> we move away goal and approach limit. We keep the limit high enough
> so that (origin-dirty) does not become negative entity.
>
> So we do expect to cross "thresh" otherwise thresh itself could have
> served as limit?
>
> If my understanding is right, then can we get rid of terms "setpoint" and
> "origin". Would it be easier to understand the things if we just talk
> in terms of "goal" and "limit" and how these are derived from "thresh".
>
> thresh == soft limit
> limit == 4*thresh (hard limit)
> goal = thresh - thresh / DIRTY_SCOPE (where we want system to
> be in steady state).
> limit - dirty
> pos_ratio = --------------
> limit - goal
>
> Thanks
> Vivek