linux-kernel - Re: CPU load

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Message-Id: <200702121712.50168.kernel@kolivas.org>
Date:	Mon, 12 Feb 2007 17:12:50 +1100
From:	Con Kolivas <kernel@...ivas.org>
To:	malc <av1474@...tv.ru>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: CPU load

On Monday 12 February 2007 16:54, malc wrote:
> On Mon, 12 Feb 2007, Con Kolivas wrote:
> > On 12/02/07, Vassili Karpov <av1474@...tv.ru> wrote:
>
> [..snip..]
>
> > The kernel looks at what is using cpu _only_ during the timer
> > interrupt. Which means if your HZ is 1000 it looks at what is running
> > at precisely the moment those 1000 timer ticks occur. It is
> > theoretically possible using this measurement system to use >99% cpu
> > and record 0 usage if you time your cpu usage properly. It gets even
> > more inaccurate at lower HZ values for the same reason.
>
> Thank you very much. This somewhat contradicts what i saw (and outlined
> in usnet article), namely the mplayer+/dev/rtc case. Unless ofcourse
> /dev/rtc interrupt is considered to be the same as the interrupt from
> PIT (on X86 that is)
>
> P.S. Perhaps it worth documenting this in the documentation? I caused
>       me, and perhaps quite a few other people, a great deal of pain and
>       frustration.

Lots of confusion comes from this, and often people think their pc suddenly 
uses a lot less cpu when they change from 1000HZ to 100HZ and use this as an 
argument/reason for changing to 100HZ when in fact the massive _reported_ 
difference is simply worse accounting. Of course there is more overhead going 
from 100 to 1000 but it doesn't suddenly make your apps use 10 times more 
cpu.

-- 
-ck
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