Date: Sat, 3 Dec 2005 06:02:43 +1100 From: Peter Jeremy <PeterJeremy@optushome.com.au> To: kama <kama@pvp.se> Cc: freebsd-stable@freebsd.org Subject: Re: cpu-timer rate Message-ID: <20051202190242.GL32006@cirb503493.alcatel.com.au> In-Reply-To: <20051202142731.H92866@ns1.as.pvp.se> References: <20051202142731.H92866@ns1.as.pvp.se>
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On Fri, 2005-Dec-02 14:32:58 +0100, kama wrote:
>I am just wondering why the cpu-timer is doubled from what I set in
>kern.hz?
>
># vmstat -i
>interrupt total rate
...
>cpu0: timer 14314031 1999
>Total 14750922 2060
>
># sysctl -a | grep hz
>kern.clockrate: { hz = 1000, tick = 1000, profhz = 666, stathz = 133 }
There's only a single timer but FreeBSD needs two independent clocks.
The 'tick' clock is used to update the TOD counters and decide when to
reschedule processes. The 'stathz' is used to collect statistics on
CPU utilisation ('profhz' is used instead if any process is using
profiling). Since processes tend to synchronize to 'tick' the
statistics clock needs to be independent to ensure that a CPU utilisation
is correctly allocated.
In order to simulate two clocks, FreeBSD runs the hardware clock at a
high rate and uses two different divisors for the soft clocks (/2 for
tick, /3 for profhz and /15 for stathz). Larger divisors are better
for utilisation statistics but increase clock interrupt overheads.
--
Peter Jeremy
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