Date: Mon, 5 Dec 2005 09:42:08 +0100 (CET) From: kama <kama@pvp.se> To: Peter Jeremy <PeterJeremy@optushome.com.au> Cc: freebsd-stable@freebsd.org Subject: Re: cpu-timer rate Message-ID: <20051205093804.D5758@ns1.as.pvp.se> In-Reply-To: <20051202190242.GL32006@cirb503493.alcatel.com.au> References: <20051202142731.H92866@ns1.as.pvp.se> <20051202190242.GL32006@cirb503493.alcatel.com.au>
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On Sat, 3 Dec 2005, Peter Jeremy wrote:
> 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.
Ehm, Im sorry, but did that even answer my question?
I appreciate that you took time to answer about the different clocks. But
that does not answer why vmstat -i shows a rate of 2000 when I have set
the hz to 1000.
/Bjorn
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