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    SubjectRe: [PATCH 06/13] writeback: bdi write bandwidth estimation
    On Wed, Nov 24, 2010 at 06:58:22PM +0800, Peter Zijlstra wrote:
    > On Wed, 2010-11-17 at 12:27 +0800, Wu Fengguang wrote:
    >
    > > @@ -555,8 +592,10 @@ static void balance_dirty_pages(struct a
    > > pause = clamp_val(pause, 1, HZ/10);
    > >
    > > pause:
    > > + bdi_update_write_bandwidth(bdi, &bw_time, &bw_written);
    > > __set_current_state(TASK_INTERRUPTIBLE);
    > > io_schedule_timeout(pause);
    > > + bdi_update_write_bandwidth(bdi, &bw_time, &bw_written);
    > >
    > > /*
    > > * The bdi thresh is somehow "soft" limit derived from the
    >
    > So its really a two part bandwidth calculation, the first call is:
    >
    > bdi_get_bandwidth()
    >
    > and the second call is:
    >
    > bdi_update_bandwidth()
    >
    > Would it make sense to actually implement it with two functions instead
    > of overloading the functionality of the one function?

    Thanks, it's good suggestion indeed. However after looking around, I
    find it hard to split it up cleanly.. To make it clear, how about this
    comment update?

    Thanks,
    Fengguang
    ---

    --- linux-next.orig/mm/page-writeback.c 2010-11-24 19:05:01.000000000 +0800
    +++ linux-next/mm/page-writeback.c 2010-11-24 22:01:43.000000000 +0800
    @@ -554,6 +554,14 @@ out:
    return a;
    }

    +/*
    + * This can be repeatedly called inside a long run loop, eg. by wb_writeback().
    + *
    + * On first invocation it will find *bw_written=0 and take the initial snapshot.
    + * On follow up calls it will update the bandwidth if
    + * - at least 10ms data have been collected
    + * - the bandwidth for the time range has not been updated in parallel by others
    + */
    void bdi_update_write_bandwidth(struct backing_dev_info *bdi,
    unsigned long *bw_time,
    s64 *bw_written)
    @@ -575,9 +583,12 @@ void bdi_update_write_bandwidth(struct b
    * When there lots of tasks throttled in balance_dirty_pages(), they
    * will each try to update the bandwidth for the same period, making
    * the bandwidth drift much faster than the desired rate (as in the
    - * single dirtier case). So do some rate limiting.
    + * single dirtier case).
    + *
    + * If someone changed bdi->write_bandwidth_update_time, he has done
    + * overlapped estimation with us. So start the next round of estimation.
    */
    - if (jiffies - bdi->write_bandwidth_update_time < elapsed)
    + if (jiffies - bdi->write_bandwidth_update_time != elapsed)
    goto snapshot;

    written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]) - *bw_written;

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