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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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