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    SubjectRe: [PATCH 02/18] writeback: dirty position control
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    On Sun, 2011-09-04 at 09:53 +0800, Wu Fengguang wrote:
    > + * (o) bdi control lines
    > + *
    > + * The control lines for the global/bdi setpoints both stretch up to @limit.
    > + * The below figure illustrates the main bdi control line with an auxiliary
    > + * line extending it to @limit.
    > + *
    > + * o
    > + * o
    > + * o [o] main control line
    > + * o [*] auxiliary control line
    > + * o
    > + * o
    > + * o
    > + * o
    > + * o
    > + * o
    > + * o--------------------- balance point, rate scale = 1
    > + * | o
    > + * | o
    > + * | o
    > + * | o
    > + * | o
    > + * | o
    > + * | o------- connect point, rate scale = 1/2
    > + * | .*
    > + * | . *
    > + * | . *
    > + * | . *
    > + * | . *
    > + * | . *
    > + * | . *
    > + * [--------------------+-----------------------------.--------------------*]
    > + * 0 bdi_setpoint x_intercept limit
    > + *
    > + * The auxiliary control line allows smoothly throttling bdi_dirty down to
    > + * normal if it starts high in situations like
    > + * - start writing to a slow SD card and a fast disk at the same time. The SD
    > + * card's bdi_dirty may rush to many times higher than bdi_setpoint.
    > + * - the bdi dirty thresh drops quickly due to change of JBOD workload

    In light of the global control thing already having a hard stop at
    limit, what's the point of the auxiliary line? Why not simply run the
    bdi control between [0.5, 1.5] and leave it at that?


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