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    SubjectRe: CFQ read performance regression
    Hi Miklos,
    On Wed, Apr 21, 2010 at 6:05 PM, Miklos Szeredi <> wrote:
    > Jens, Corrado,
    > Here's a graph showing the number of issued but not yet completed
    > requests versus time for CFQ and NOOP schedulers running the tiobench
    > benchmark with 8 threads:
    > It shows pretty clearly the performance problem is because CFQ is not
    > issuing enough request to fill the bandwidth.
    > Is this the correct behavior of CFQ or is this a bug?
    This is the expected behavior from CFQ, even if it is not optimal,
    since we aren't able to identify multi-splindle disks yet. Can you
    post the result of "grep -r . ." in your /sys/block/*/queue directory,
    to see if we can find some parameter that can help identifying your
    hardware as a multi-spindle disk.
    > This is on a vanilla 2.6.34-rc4 kernel with two tunables modified:
    > read_ahead_kb=512
    > low_latency=0 (for CFQ)
    You should get much better throughput by setting
    /sys/block/_your_disk_/queue/iosched/slice_idle to 0, or
    /sys/block/_your_disk_/queue/rotational to 0.

    > Thanks,
    > Miklos

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