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    SubjectRe: [PATCH 2/5] writeback: dirty position control
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    On Fri, 2011-08-12 at 22:20 +0800, Wu Fengguang wrote:
    > On Fri, Aug 12, 2011 at 09:04:19PM +0800, Peter Zijlstra wrote:
    > > On Tue, 2011-08-09 at 19:20 +0200, Peter Zijlstra wrote:
    >
    > To start with,
    >
    > write_bw
    > ref_bw = task_ratelimit_in_past_200ms * --------
    > dirty_bw
    >
    > where
    > task_ratelimit_in_past_200ms ~= dirty_ratelimit * pos_ratio
    >
    > > > Now all of the above would seem to suggest:
    > > >
    > > > dirty_ratelimit := ref_bw
    >
    > Right, ideally ref_bw is the balanced dirty ratelimit. I actually
    > started with exactly the above equation when I got choked by pure
    > pos_bw based feedback control (as mentioned in the reply to Jan's
    > email) and introduced the ref_bw estimation as the way out.
    >
    > But there are some imperfections in ref_bw, too. Which makes it not
    > suitable for direct use:
    >
    > 1) large fluctuations

    OK, understood.

    > 2) due to truncates and fs redirties, the (write_bw <=> dirty_bw)
    > becomes unbalanced match, which leads to large systematical errors
    > in ref_bw. The truncates, due to its possibly bumpy nature, can hardly
    > be compensated smoothly.

    OK.

    > 3) since we ultimately want to
    >
    > - keep the dirty pages around the setpoint as long time as possible
    > - keep the fluctuations of task ratelimit as small as possible

    Fair enough ;-)

    > the update policy used for (2) also serves the above goals nicely:
    > if for some reason the dirty pages are high (pos_bw < dirty_ratelimit),
    > and dirty_ratelimit is low (dirty_ratelimit < ref_bw), there is no
    > point to bring up dirty_ratelimit in a hurry and to hurt both the
    > above two goals.

    Right, so still I feel somewhat befuddled, so we have:

    dirty_ratelimit - rate at which we throttle dirtiers as
    estimated upto 200ms ago.

    pos_ratio - ratio adjusting the dirty_ratelimit
    for variance in dirty pages around its target

    bw_ratio - ratio adjusting the dirty_ratelimit
    for variance in input/output bandwidth

    and we need to basically do:

    dirty_ratelimit *= pos_ratio * bw_ratio

    to update the dirty_ratelimit to reflect the current state. However per
    1) and 2) bw_ratio is crappy and hard to fix.

    So you propose to update dirty_ratelimit only if both pos_ratio and
    bw_ratio point in the same direction, however that would result in:

    if (pos_ratio < UNIT && bw_ratio < UNIT ||
    pos_ratio > UNIT && bw_ratio > UNIT) {
    dirty_ratelimit = (dirty_ratelimit * pos_ratio) / UNIT;
    dirty_ratelimit = (dirty_ratelimit * bw_ratio) / UNIT;
    }

    > > > 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);
    >
    > The above are merely constraints to the dirty_ratelimit update.
    > It serves to
    >
    > 1) stop adjusting the rate when it's against the position control
    > target (the adjusted rate will slow down the progress of dirty
    > pages going back to setpoint).

    Not strictly speaking, suppose pos_ratio = 0.5 and bw_ratio = 1.1, then
    they point in different directions however:

    0.5 < 1 && 0.5 * 1.1 < 1

    so your code will in fact update the dirty_ratelimit, even though the
    two factors point in opposite directions.

    > 2) limit the step size. pos_bw is changing values step by step,
    > leaving a consistent trace comparing to the randomly jumping
    > ref_bw. pos_bw also has smaller errors in stable state and normally
    > have larger errors when there are big errors in rate. So it's a
    > pretty good limiting factor for the step size of dirty_ratelimit.

    OK, so that's the min/max stuff, however it only works because you use
    pos_bw and ref_bw instead of the fully separated factors.

    > Hope the above elaboration helps :)

    A little..


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