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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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