option HZ=?

Ivan Voras ivoras at freebsd.org
Wed Jan 30 06:55:52 PST 2008

cpghost wrote:
> On Wed, Jan 30, 2008 at 02:55:30PM +0200, Stefan Lambrev wrote:
>> Greetings,
>> I want to know what is the bad effect of increasing HZ too much?
>> And when is too much? What problems can I expect when HZ>2000?
>> Can I change this value without pre-compiling the kernel?
> You can change HZ by adding a line to /boot/loader.conf like this:
> kern.hz="100"
> (it works on RELENG_6 and RELENG_7 and I'm using this conservative
> setting on all my boxes, since I don't need faster context switching)
> If you set HZ too high, the kernel will spend too much overhead
> on unnecessary context switching, and it may even reach a point
> (with very high values of HZ) where interrupt service routines
> get interrupted way too often by clock ticks; i.e. interrupts
> would eventually come in faster than the kernel can service them.

Isn't there also something bad about TCP timestamp overflow?

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