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

[Willy Tarreau]

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 ? It will not help the key go faster...

Probably that iowait should only change the time required to go back to
low frequency. That way, if there is no CPU nor I/O activity, we can
switch back to a low frequency quickly, but if we see I/O activity, we
could decide to wait for 1 second (for instance) for CPU idle before
switching back to low frequency.

> Maybe modern cpus can and should simply react faster?

It's above all that we should not try to switch too fast if we know the
CPU can't follow.

Willy

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