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    SubjectRe: tiobench read 50% regression with 2.6.30-rc1
    On Thu, Apr 09 2009, Zhang, Yanmin wrote:
    > Comparing with 2.6.29's result, tiobench (read) has about 50% regression
    > with 2.6.30-rc1 on all my machines. Bisect down to below patch.
    >
    > b029195dda0129b427c6e579a3bb3ae752da3a93 is first bad commit
    > commit b029195dda0129b427c6e579a3bb3ae752da3a93
    > Author: Jens Axboe <jens.axboe@oracle.com>
    > Date: Tue Apr 7 11:38:31 2009 +0200
    >
    > cfq-iosched: don't let idling interfere with plugging
    >
    > When CFQ is waiting for a new request from a process, currently it'll
    > immediately restart queuing when it sees such a request. This doesn't
    > work very well with streamed IO, since we then end up splitting IO
    > that would otherwise have been merged nicely. For a simple dd test,
    > this causes 10x as many requests to be issued as we should have.
    > Normally this goes unnoticed due to the low overhead of requests
    > at the device side, but some hardware is very sensitive to request
    > sizes and there it can cause big slow downs.
    >
    >
    >
    > Command to start the testing:
    > #tiotest -k0 -k1 -k3 -f 80 -t 32
    >
    > It's a multi-threaded program and starts 32 threads. Every thread does I/O
    > on its own 80MB file.

    It's not a huge surprise that we regressed there. I'll get this fixed up
    next week. Can you I talk you into trying to change the 'quantum' sysfs
    variable for the drive? It's in /sys/block/xxx/queue/iosched where xxx
    is your drive(s). It's set to 4, if you could try progressively larger
    settings and retest, that would help get things started.

    Thanks!

    --
    Jens Axboe



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