Date: Wed, 17 Jan 2001 19:59:49 +0100 From: Francesco Casadei <fcasadei@inwind.it> To: Shashank <shashank@evl.uic.edu> Cc: freebsd-questions@freebsd.org Subject: Re: Kern.Clockrate. Message-ID: <20010117195949.A1206@goku.kasby> In-Reply-To: <Pine.SGI.3.95.1010116121639.1937001Z-100000@laurel.evl.uic.edu>; from shashank@evl.uic.edu on Tue, Jan 16, 2001 at 12:22:36PM -0600 References: <Pine.SGI.3.95.1010116121639.1937001Z-100000@laurel.evl.uic.edu>
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On Tue, Jan 16, 2001 at 12:22:36PM -0600, Shashank wrote:
> Hi ,
> The defualt value for this parameter in the kernel is 10msec (HZ = 100).
> Suppose if i change it to 1 msec (HZ = 1000) , then what side effects will
> I have.
>
> Also I am a bit lost regarding the usage of this parameter.
> Are the TCP fast (500ms) and slow (200 ms) timers affected when the timing
> granularity is changed from 10msec to 1 msec.. If yes , then in what way??
>
> Any refernces regarding this will be greatly appreciated.
>
> Regards and thanks
>
> Shashank
>
>
>
>
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> end of the original message
# sysctl -a | grep hz
kern.clockrate: { hz = 100, tick = 10000, tickadj = 5, profhz = 100, stathz =
100 }
The system clock interrupts at regular interval. Each interrupt is referred to
as a tick. Here hz = 100 is the number of ticks per second (i.e. the system
clock ticks 100 times per second, one each 10ms). Each clock interrupt is
handled by hardclock() routine. This routine is executed at high interrupt
priority, higher than the priority of the network protocol processing. The
more hardclock() is called the more network controllers miss packets (this
is not desiderable).
hardclock() also mantains the time of day for the system, so it must finish
its job before another clock interrupt happens, otherwise the system will
lose time.
The clockrate values depend on the underlying hardware of the system (i.e.
the programmable clocks), I don't think you can change them.
DISCLAIMER: I'm not a system developer (though I'd like to) so I may be
completely wrong here. This information is taken from section 3.4 - Clock
Interrupts of The Design and Implementation of the 4.4BSD Operating System.
Francesco Casadei
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