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    SubjectRe: udevd is killing file write performance.
    Andrew Morton wrote:

    >Robin Holt <holt@sgi.com> wrote:
    >
    >
    >>Let me reiterate, I know _VERY_ little about filesystems. Can the
    >> dentry->d_lock be changed to a read/write lock?
    >>
    >>
    >
    >Well, it could, but I suspect that won't help - the hold times in there
    >will be very short so the problem is more likely acquisition frequency.
    >
    >However it's a bit strange that this function is the bottleneck. If their
    >workload is doing large numbers of reads or writes from large numbers of
    >processes against the same file then they should be hitting heavy
    >contention on other locks, such as i_sem and/or tree_lock and/or lru_lock
    >and others.
    >
    >Can you tell us more about the kernel-visible behaviour of this app?
    >
    >

    I have also seen this problem, and it's hard to reproduce. What you will
    see is udev getting spawned
    multiple times as reported by top. I have found its related to
    intermittent failures of the hard drive and
    the hotpluger for some reason invoking udev multiple times in response
    to this. I saw it on a Compaq
    laptop right before my hard drive croaked, and it seems BIOS specific as
    well, since I have never seen
    it or been able to reproduce it reliably.

    Jeff

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