Re: RFC: powerd Patch & proposed future changes

> Date: Mon, 06 Jul 2009 15:04:51 +0200
> From: Aragon Gouveia <aragon_at_phat.za.net>
> Sender: owner-freebsd-current_at_freebsd.org
> 
> Hi,
> 
> Patrick Lamaiziere wrote:
> >>> I would like an option to set the minimum allowed CPU speed, instead
> >>> the use of the sysctl debug.cpufreq.lowest.
> >> This is so that powerd doesn't take so long to spin up power in the
> >> adaptive modes?
> > 
> > If the speed is too low, my machine is not very interactive here
> > (running KDE and all...).
> 
> Have you tried disabling throttling (P4TCC)?  Add these to your loader.conf:
> 
> hint.p4tcc.0.disabled=1
> hint.acpi_throttle.0.disabled=1
> 
> When you reboot you'll notice a lot of frequencies missing from 
> dev.cpu.0.freq_levels.  The remaining ones are those affected by EIST only.
> 
> The difference?  EIST controls CPU frequency *and* voltage.  Throttling 
> controls only frequency, and is much less effective at saving power 
> (while being very effective at slowing your system down).  If you 
> disable throttling and use just EIST, your system will be much more 
> responsive at a very small power cost.

It's really even worse than this. Throttling/TCC don't actually change
clock speed. Thy simply "skip" N of every 8 clock cycles. The result is
that power and performance under load are reduced by exactly the same
amount and the effect of throttling drops with CPU load and is zero on
an idle system. EST and C3 (or higher when available) are a much bigger
wins. I see little value in the use of TCC or throttling.

> The power cost above can be made up by setting C-state to C2 or even C3.
> 
> If you're running FreeBSD 8, another way of saving a lot of power on a 
> notebook is to power down USB devices.  Most notebooks' webcams and 
> fingerprint readers are internally connected to the USB bus.

Note that even having the USB drivers loaded runs up battery use and
blocks C3 even if no devices are connected to the USB. This is fixed in
current by the new USB stack, but in v7, I build the kernel on my laptop
without USB and load as needed.
> 
> Alexander Motin did a lot of testing a while ago.  Take a look at this 
> thread:
> 
> http://lists.freebsd.org/pipermail/freebsd-current/2009-May/006436.html
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman_at_es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751