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    Subject[PATCH 0/6] cpufreq: Add sampling window to enhance ondemand governor power efficiency
    Running a well-known power performance benchmark, current ondemand governor is
    not power efficiency. Even when workload is at 10%~20% of full capability, the
    CPU will also run much of time at highest frequency. In fact, in this situation,
    the lowest frequency often can meet user requirement. When running this
    benchmark on turbo mode enable machine, I compare the result of different
    governors, the results of ondemand and performance governors are the closest.
    There is no much power saving between ondemand and performance governor. If we
    can ignore the little power saving, the perfomance governor even better than
    ondemand governor, at leaset for better performance.

    One potential reason for ondemand governor is not power efficiency is that
    ondemand governor decide the next target frequency by instant requirement during
    sampling interval (10ms or possible a little longer for deferrable timer in idle
    tickless). The instant requirement can response quickly to workload change, but
    it does not usually reflect workload real CPU usage requirement in a small
    longer time and it possibly causes frequently change between highest and lowest

    This patchset add a sampling window for percpu ondemand thread. Each sampling
    window with max 150 record items which slide every sampling interval and use to
    track the workload requirement during latest sampling window timeframe.
    The average of workload during latest sample windows will be used to decide next
    target frequency. The sampling window targets to be more truly reflects workload
    requirement of CPU usage.

    The sampling window size can be set by user and default max sampling window
    is one second. When it is set to default sampling rate, the sampling window will
    roll back to original behaviour.

    The sampling window size also can be dynamicly changed in according to current
    system workload busy situation. The more idle, the smaller sampling window; the
    more busy, the larger sampling window. It will increase the respnose speed by
    decrease sampling window, while it will keep CPU working at high speed when busy
    by increase sampling window and also avoid unefficiently dangle between highest
    and lowest frequency in original ondemand.

    We set to up_threshold to 80 and down_differential to 20, so when workload reach
    80% of current frequency, it will increase to highest frequency. When workload
    decrease to below (up_threshold - down_differential)60% of current frequency
    capability, it will decrease the frequency, which ensure that CPU work above 60%
    of its current capability, otherwise lowest frequency will be used.

    The Turbo Mode (P0) will comsume much more power compare with second largest
    frequency (P1) and P1 frequency is often double, even more, with Pn lowest
    frequency; Current logic will increase sharply to highest frequency Turbo Mode
    when workload reach to up_threshold of current frequency capacity, even current
    frequency at lowest frequency. In this patchset, it will firstly evaluate P1 if
    it is enough to support current workload before directly enter into Turbo Mode.
    If P1 can meet workload requirement, it will save power compare of being Turbo

    On my test platform with two sockets Westmere-EP server and run the well-known
    power performance benchmark, when workload is low, the patched governor is
    power saving like powersave governor; while workload is high, the patched
    governor is as good as performance governor but the patched governor consume
    less power than performance governor. Along with other patches in this patchset,
    the patched governor power efficiey is improved about 10%, while the performance
    has no apparently decrease.
    Running other benchmarks in phoronix, kernel building save 5% power, while the
    performance without decrease. compress-7zip save power 2%, while the performance
    also does not apparently decrease. However, apache benchmark saves power but its
    performance decrease a lot.

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      Last update: 2010-12-23 07:23    [W:0.025 / U:12.104 seconds]
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