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    SubjectRe: CFQ read performance regression
    On Thu 22-04-10 12:23:29, Miklos Szeredi wrote:
    > > >
    > > > 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.
    >
    > slice_idle=0 definitely helps. rotational=0 seems to help on 2.6.34-rc
    > but not on 2.6.32.
    >
    > As far as I understand setting slice_idle to zero is just a workaround
    > to make cfq look at all the other queues instead of serving one
    > exclusively for a long time.
    Yes, basically it disables idling (i.e., waiting whether a thread sends
    more IO so that we can get better IO locality).

    > I have very little understanding of I/O scheduling but my idea of what's
    > really needed here is to realize that one queue is not able to saturate
    > the device and there's a large backlog of requests on other queues that
    > are waiting to be served. Is something like that implementable?
    I see a problem with defining "saturate the device" - but maybe we could
    measure something like "completed requests / sec" and try autotuning
    slice_idle to maximize this value (hopefully the utility function should
    be concave so we can just use "local optimization").

    Honza
    --
    Jan Kara <jack@suse.cz>
    SUSE Labs, CR


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