Re: [PATCH 7/7] ondemand: Solve the big performance issue with ondemand during disk IO

[Arjan van de Ven <arjan@infradead.org>]

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.



-- 
Arjan van de Ven 	Intel Open Source Technology Centre
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