What value HZ?

[Conrad Meyer]

Hi Constantine,

On Sat, Sep 7, 2019 at 3:05 PM Constantine A. Murenin
<cnst++ at freebsd.org> wrote:
>
> For the sake of discussion, I recall that there was a measurable power
> consumption difference between FreeBSD (HZ=1000) and OpenBSD (HZ=100) when
> I played with a Kill-A-Watt a number of years ago.

Unfortunately, this is an apples to oranges comparison, and perhaps
worse, vague and dated.  It would be more interesting to compare
HZ=1000 to HZ=100 with an otherwise identical CURRENT FreeBSD system.

As far as I can tell, FreeBSD grew "tickless" timer support in r212541
in 2010 (thanks mav@).  This is easily observed on my idle HZ=1000
amd64 system with 'vmstat -i': "cpu0:timer" is firing at an average
rate of 21 Hz — not 1000.  And that is the most frequent interrupt.

> Would it perhaps be prudent to try to qualify whether, and by how much,
> this change affects power consumption on MIPS/ARM/RISCV, not just whether
> each arch could cope with the change?

If someone has the time and inclination, then of course, it could be
an interesting test to run.

Best,
Conrad