option HZ=?

[cpghost]

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.

> Because all docs I found for HZ is in src/sys/conf/NOTES and it doesn't say 
> much.

-cpghost.

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