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    SubjectRe: [PATCH 2/5] writeback: dirty position control
    On Tue, Aug 23, 2011 at 12:01:00PM +0200, Peter Zijlstra wrote:
    > 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..

    Exactly. This is where it gets confusing and is bone of contention.

    >
    > So if you want completely separated feedback loops I would expect
    > something like:
    >
    > balance_rate_(i+1) = balance_rate_(i) * bw_ratio ; every 200ms
    >

    I agree. This makes sense. IOW.
    write_bw
    bdi->dirty_ratelimit_n = bdi->dirty_ratelimit_(n-1) * -------
    dirty_rate

    > 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.

    I think you meant.

    "if we throttled too much, the dirty_rate will have dropped and the bw_ratio
    will be >1 causing the balance_rate to increase hence 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.
    >

    We I thought that this is evident that.

    task_ratelimit = balanced_rate * pos_ratio

    What is not evident to me is following.

    balanced_rate_(i+1) = task_ratelimit_200ms * pos_ratio.

    Instead, like you, I also thought that following is more obivious.

    balanced_rate_(i+1) = balanced_rate_(i) * pos_ratio

    Thanks
    Vivek


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