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    SubjectRe: Direct io on block device has performance regression on 2.6.x kernel
    "Chen, Kenneth W" <kenneth.w.chen@intel.com> wrote:
    >
    > > Did you generate a kernel profile?
    >
    > Top 40 kernel hot functions, percentage is normalized to kernel utilization.
    >
    > _spin_unlock_irqrestore 23.54%
    > _spin_unlock_irq 19.27%

    Cripes.

    Is that with CONFIG_PREEMPT? If so, and if you disable CONFIG_PREEMPT,
    this cost should be accounting the the spin_unlock() caller and we can see
    who the culprit is. Perhaps dio->bio_lock.

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