Re: HZ, preferably as small as possible

[george anzinger <george@mvista.com>]

"Grover, Andrew" wrote:
> 
> I'd like to see HZ closer to 100 than 1000, for CPU power reasons. Processor
> power states like C3 may take 100 microseconds+ to enter/leave - time when
> both the CPU isn't doing any work, but still drawing power as if it was. We
> pop out of C3 whenever there is an interrupt, so reducing timer interrupts
> is good from a power standpoint by amortizing the transition penalty over a
> longer period of power savings.
> 
> But on the other hand, increasing HZ has perf/latency benefits, yes? Have
> these been quantified? I'd either like to see a HZ that has balanced
> power/performance, or could we perhaps detect we are on a system that cares
> about power (aka a laptop) and tweak its value at runtime?

HZ is used in a LOT of places.  I suspect "tweaking" at run
time would be a bit difficult.  

The high-res-timers patch give high resolution timers with
out changing HZ.  Interrupts are scheculed as needed,
between the 1/HZ ticks, so a quite system will have few (if
any) interrupts between the ticks.

-- 
George Anzinger   george@mvista.com
High-res-timers: 
http://sourceforge.net/projects/high-res-timers/
Real time sched:  http://sourceforge.net/projects/rtsched/
Preemption patch:
http://www.kernel.org/pub/linux/kernel/people/rml