power management, ahci link power management and DragonFly

Sepherosa Ziehau sepherosa at gmail.com
Mon Nov 8 23:50:55 PST 2010

On Mon, Nov 8, 2010 at 4:21 PM, Alex Hornung <ahornung at gmail.com> wrote:
> Hi,
> I've been playing around today to see the our power efficiency on my
> ThinkPad. I have been using acpiconf -i 0 to see the discharge rate, and
> it seems we can't get below 19W. On linux and Windows I can get as low
> as 10-12W.
> I've set the brightness to the lowest level. I didn't enable any wifi
> power management, but on linux I can't do that either since it has been
> marked as broken for my card on linux (iwl3945).
> I've set the P-State to be fixed at 1GHz (the lowest), since powerd
> doesn't seem to work correctly; it didn't seem to scale down from 2GHz
> anymore after a while on an idle system. FWIW, estd seems to do a better
> job, though. In any case discussing the sense or lack thereof behind
> powerd is not the subject of this mail.
> I've also set the lowest C-state to C3 and according to
> hw.acpi.cpu*.cx_usage, it is actually spending most of the idle time in
> C3 (around 75%).

I don't know how well C/P-state could work together on Intel CPUs.
But using C-state > C1, has at least one drawback: lapic timer will
not be used, instead i8254 is used, in which case the clock interrupt
rate could be double on each CPU (at least it's the case last time I

I don't know whether your Linux kernel has enabled the "tickless"
stuff (hasso mentioned it to me almost one year ago), it is quite
interesting thing.  Since our clock interrupt rate could be changed on
the fly, I think "tickless" is doable in dragonfly (I did take a look
at how to implement it in dragonfly, but it seems to affects some
process bookkeeping and scheduling stuffs, which are completely out of
my gasp).

Best Regards,

Tomorrow Will Never Die

More information about the Kernel mailing list