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    SubjectRe: [PATCHv4 17/17] writeback: lessen sync_supers wakeup count
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    On Fri, 2010-05-28 at 13:29 -0700, Andrew Morton wrote:
    > void mark_sb_dirty(struct super_block *sb)
    > {
    > sb->s_dirty = 1;
    >
    > if (!supers_timer_armed) {
    > spin_lock(&supers_timer_lock);
    > if (!supers_timer_armed) {
    > bdi_arm_supers_timer();
    > supers_timer_armed = 1;
    > }
    > } else if (supers_timer_armed == -1)
    > spin_lock(&supers_timer_lock);
    > if (supers_timer_armed == -1)
    > supers_timer_armed = 1;
    > spin_unlock(&supers_timer_lock);
    > }
    > }
    >
    > I didn't try very hard there, but you get the idea: examine the state
    > before taking that expensive global spinlock, so we only end up taking
    > the lock once per five seconds, rather than once per possible
    > superblock dirtying. That's like a six-orders-of-magnitude reduction
    > in locking frequency, which is worth putting some effort into.

    Andrew, thanks for review!

    I just did not consider spinlock to be expensive because I thought that
    marking superblock as dirty is a relatively rare operation. And my small
    experiments kind of confirmed that.

    But Nick suggested a good locking scheme which uses only smp_mb() in
    this thread, which I am going to stick with.

    --
    Best Regards,
    Artem Bityutskiy (Артём Битюцкий)

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