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    SubjectRe: [PATCH 7/7] ondemand: Solve the big performance issue with ondemand during disk IO
    On Fri, 23 Apr 2010 07:38:58 +0200
    Willy Tarreau <w@1wt.eu> wrote:

    > On Fri, Apr 23, 2010 at 07:24:39AM +0200, Pavel Machek wrote:
    > > Hi!
    > >
    > > > The ondemand cpufreq governor uses CPU busy time (e.g. not-idle
    > > > time) as a measure for scaling the CPU frequency up or down.
    > > > If the CPU is busy, the CPU frequency scales up, if it's idle,
    > > > the CPU frequency scales down. Effectively, it uses the CPU busy
    > > > time as proxy variable for the more nebulous "how critical is
    > > > performance right now" question.
    > > >
    > > > This algorithm falls flat on its face in the light of workloads
    > > > where you're alternatingly disk and CPU bound, such as the ever
    > > > popular "git grep", but also things like startup of programs and
    > > > maildir using email clients... much to the chagarin of Andrew
    > > > Morton.
    > > >
    > > > This patch changes the ondemand algorithm to count iowait time as
    > > > busy, not idle, time. As shown in the breakdown cases above,
    > > > iowait is performance critical often, and by counting iowait, the
    > > > proxy variable becomes a more accurate representation of the "how
    > > > critical is performance" question.
    > >
    > > Well, and now, if you do something like cat /dev/<your usb1.1 hdd> >
    > > /dev/null, you'll keep cpu on max frequency. Not a problem for new
    > > core i7, but probably big deal for athlon 64.
    >
    > So that also means that my notebook's CPU fan will spin like mad as
    > soon as I access a USB key ?

    unlikely. your notebook CPU will stop its clocks, if not drop its
    voltage, during idle. So during that time the frequency is 0; the only
    difference is in how much leakage you get from a higher voltage
    for those CPUs that do not powergate or drop their cpu in idle.
    Most CPUs that I know of do either or both of that.



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    Arjan van de Ven Intel Open Source Technology Centre
    For development, discussion and tips for power savings,
    visit http://www.lesswatts.org


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