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

Ian Smith smithi at
Sun Jul 1 14:16:19 UTC 2007

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 :)

 > 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

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