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    SubjectRe: [RFC v1][PATCH]page_fault retry with NOPAGE_RETRY
    On Thu, 2008-11-27 at 01:28 -0800, Mike Waychison wrote:

    > Correct. I don't recall the numbers from the pathelogical cases we were
    > seeing, but iirc, it was on the order of 10s of seconds, likely
    > exascerbated by slower than usual disks. I've been digging through my
    > inbox to find numbers without much success -- we've been using a variant
    > of this patch since 2.6.11.

    > We generally try to avoid such things, but sometimes it a) can't be
    > easily avoided (third party libraries for instance) and b) when it hits
    > us, it affects the overall health of the machine/cluster (the monitoring
    > daemons get blocked, which isn't very healthy).

    If its only monitoring, there might be another solution. If you can keep
    the required data in a separate (approximate) copy so that you don't
    need mmap_sem at all to show them.

    If your mmap_sem is so contended your latencies are unacceptable, adding
    more users to it - even statistics gathering, just isn't going to cure
    the situation.

    Furthermore, /proc code usually isn't written with performance in mind,
    so its usually simple and robust code. Adding it to a 'hot'-path like
    you're doing doesn't seem advisable.

    Also, releasing and re-acquiring mmap_sem can significantly add to the
    cacheline bouncing that thing already has.

     \ /
      Last update: 2008-11-27 11:03    [W:0.019 / U:109.404 seconds]
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