All CPUs always running at profhz

[b. f.]

> I have systems running 7.4 and 8.2. The newer ones (in terms of age of
> hardware) are always running at profhz, i.e., when executing a "systat
> -vm 1" I see that all cores always get profhz interrupts per second.
>
> An older machine running 7.4 is running at hz (which for this machine I
> set to kern.hz=200 in loader.conf).
>
> Using ps -o ...,flags,... I see that no process has profiling turned on.
>
> Why is this so?
>
> Especially on laptops, I'd like to save power by lowering the interrupt
> rate.

I have not analyzed this problem.  However, by using 9-CURRENT, together with

http://people.freebsd.org/~mav/tm6292_idle.patch

, removing DEVICE_POLLING from my kernel, and reducing kern.hz to 100,
I was able to achieve reported interrupt rates of ~35-40 Hz on my UP
machines when idle, and substantial reductions when the machines were
loaded, as well.  When used with other power-saving measures, this was
a big help.

b.