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    SubjectRe: Drive performance bottleneck
    Ian Godin <Ian.Godin@lowrydigital.com> wrote:
    >
    >
    > I am trying to get very fast disk drive performance and I am seeing
    > some interesting bottlenecks. We are trying to get 800 MB/sec or more
    > (yes, that is megabytes per second). We are currently using
    > PCI-Express with a 16 drive raid card (SATA drives). We have achieved
    > that speed, but only through the SG (SCSI generic) driver. This is
    > running the stock 2.6.10 kernel. And the device is not mounted as a
    > file system. I also set the read ahead size on the device to 16KB
    > (which speeds things up a lot):
    > ...
    > samples % symbol name
    > 848185 8.3510 __copy_to_user_ll
    > 772172 7.6026 do_anonymous_page
    > 701579 6.9076 _spin_lock_irq
    > 579024 5.7009 __copy_user_intel
    > 361634 3.5606 _spin_lock
    > 343018 3.3773 _spin_lock_irqsave
    > 307462 3.0272 kmap_atomic
    > 193327 1.9035 page_fault

    Something funny is happening here - it looks like there's plenty of CPU
    capacity left over.

    It's odd that you're getting a lot of pagefaults in this test but not with
    the sg_dd test, too. I wonder why dd is getting so many pagefaults? (I
    recall that sg_dd did something cheaty, but I forget what it was).

    Could you monitor the CPU load during the various tests? If the `dd'
    workload isn't pegging the CPU then it could be that there's something
    wrong with the I/O submission patterns.
    -
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