kern.hz="100" stops high-pitched whine

cpghost cpghost at
Mon Jul 2 12:11:42 UTC 2007

Ian Smith wrote:
> On Sun, 01 Jul 2007 13:12:28 +0200 cpghost <cpghost at> wrote:
>  > I'm using fan-less Soekris (266 MHz) and EPIA (500 MHz)
>  > boards with -STABLE. Those boards have been emitting a
>  > very high pitched annoying whine, similar to a dog
>  > whistle. Not everyone I asked could hear it, but I was,
>  > and it was driving me crazy for months.
>  > 
>  > This whine stopped entirely the moment I put
>  > kern.hz="100" in /boot/loader.conf and rebooted.
>  > Quite relieving!
> Possibly a beat frequency between the 1kHz default slicing and some
> faster oscillator on board?  You could play around with kern.clockrate,
> sometimes quite small frequency changes make a difference, especially if
> it's a harmonic.  Default HZ jumped from 100 to 1000 probably around the
> time when more servers were seen running at nearer 3000MHz than 300 :)

Interesting! I've booted the EPIA with Hz=840 and Hz=1160,
and the whine was *considerably* quieter in both cases. It really
seemed to be some kind of harmonic oscillation at Hz=1000.
Now I'm back to HZ=100, which seems to be ideal.

>  > Are there any reasons NOT changing kern.hz from the
>  > default 1000 back to 100? With my typical mix of
>  > desktop apps (EPIA) and networking / server (Soekris),
>  > everything seems to be running just as smoothly with
>  > 100 Hz than with 1000 Hz (testing now for two weeks
>  > without problems). Even playing videos with mplayer
>  > on the EPIA doesn't look different in any way.
> No problem then, and a lighter interrupt load; probably enough to notice
> in top.  If you want to run dummynet or something else? wanting quicker
> ticking you could try HZ=200 - 500.  200Hz on my Celeron 300 works well.
>  > Is it okay to stay with 100 Hz with this type of
>  > low-speed CPU/boards? Or are there some compelling
>  > reasons not to?
> If it's happy, smooth and keeping up, it's fine.
> Cheers, Ian

I'll stay with 100 Hz for now. Should I need dummynet, I'll
crank it up slowly.


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