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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:
>
> Andrew Morton wrote on Wednesday, March 09, 2005 2:45 PM
> > >
> > > > 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.
>
> CONFIG_PREEMPT is off.
>
> Sorry for all the confusion, I probably shouldn't post the first profile
> to confuse people. See 2nd profile that I posted earlier (copied here again).
>
> scsi_request_fn 7.54%
> finish_task_switch 6.25%
> __blockdev_direct_IO 4.97%
> __make_request 3.87%
> scsi_end_request 3.54%
> dio_bio_end_io 2.70%
> follow_hugetlb_page 2.39%
> __wake_up 2.37%
> aio_complete 1.82%

What are these percentages? Total CPU time? The direct-io stuff doesn't
look too bad. It's surprising that tweaking the direct-io submission code
makes much difference.

hm. __blockdev_direct_IO() doesn't actually do much. I assume your damn
compiler went and inlined direct_io_worker() on us.

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