default HZ value in 5.2.1

[Chuck Swiger]

Poul-Henning Kamp wrote:
[ ... ]
> Most of my systems run with HZ=1000 already, but that is hardly
> ground for changes to the default.  What we need is some pro et
> contra arguments, including benchmarks.

You're right.  Back around 1990, Avie Tenavian spent some brainpower figuring 
out the preemptive scheduling overhead for Mach, and determined that a 25MHz 
68040 machine took up to about 0.5 ms to handle a timer interrupt and run 
through the scheduler, which meant that the system lost about 5% overhead when 
using a 10ms scheduler quantum (or HZ=100, whatever).

While I think have some idea as to the time it takes a Pentium to do a context 
switch (300 clocks?), I don't know enough about the way the clock timer is 
managed under FreeBSD, nor do I know how much other stuff is glommed onto the 
periodic timer interrupt.  Mach used kernel threads and a messaging paradigm 
from day one, so it's scheduler was fairly simple-- less worrying about a 
queue of pending callbacks along the lines of libevent and kqueue.

Anyway, I suspect that the default scheduler quantum might be better chosen 
based on the scheduling overhead of each machine: set HZ as fast as the local 
system will deal with without exceeding a single-digit percent overhead...

-- 
-Chuck