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    SubjectRe: dramatic I/O slowdown after upgrading 2.6.38->3.0+
    On 04/10/2012 08:43 PM, Jan Kara wrote:
    > On Tue 10-04-12 10:00:38, Michael Tokarev wrote:
    >> On 10.04.2012 06:26, Dave Chinner wrote:
    >>
    >>> Barriers. Turn them off, and see if that fixes your problem.
    >>
    >> Thank you Dave for a hint. And nope, that's not it, not at all... ;)
    >> While turning off barriers helps a tiny bit, to gain a few %% from
    >> the huge slowdown, it does not cure the issue.
    >>
    >> Meanwhile, I observed the following:
    >>
    >> 1) the issue persists on more recent kernels too, I tried 3.3
    >> and it is also as slow as 3.0.
    >>
    >> 2) at least 2.6.38 kernel works fine, as fast as 2.6.32, I'll
    >> try 2.6.39 next.
    >>
    >> I updated $subject accordingly.
    >>
    >> 3) the most important thing I think: this is general I/O speed
    >> issue. Here's why:
    >>
    >> 2.6.38:
    >> # dd if=/dev/sdb of=/dev/null bs=1M iflag=direct count=100
    >> 100+0 records in
    >> 100+0 records out
    >> 104857600 bytes (105 MB) copied, 1.73126 s, 60.6 MB/s
    >>
    >> 3.0:
    >> # dd if=/dev/sdb of=/dev/null bs=1M iflag=direct count=100
    >> 100+0 records in
    >> 100+0 records out
    >> 104857600 bytes (105 MB) copied, 29.4508 s, 3.6 MB/s
    >>
    >> That's about 20 times difference on direct read from the
    >> same - idle - device!!
    > Huh, that's a huge difference for such a trivial load. So we can rule out
    > filesystems, writeback, mm. I also wouldn't think it's IO scheduler but
    > you can always check by comparing dd numbers after
    > echo none >/sys/block/sdb/queue/scheduler

    s/none/noop
    you meant noop, of course?


    Suresh


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