PowerTOP for FreeBSD ?
Nate Lawson
nate at root.org
Wed May 16 07:01:00 UTC 2007
Wilkinson, Alex wrote:
> 0n Tue, May 15, 2007 at 10:12:14AM -0700, Eric Anholt wrote:
>
> Heya Eric!
>
> >Until we get tickless scheduling, it's not of much use.
>
> I keep hearing about this "tickless scheduling" but know SFA about it. I assume
> it means no ticks (HZ) therefore tones less interrupts therefore less power
> consumption? Anyone got a link that explains exactly what it is ?
Normally timers tick every 1 us on FreeBSD. Most interrupts are
serviced with no work to be performed, so it's just wasted computation.
The kernel knows when processes are set to wake up. It knows how long a
quantum a process deserves when it's first scheduled. From that, a
variable tick delay can be calculated.
Example: proc 1 is sleeping for 1 second, proc 2 is runnable (100 ms
quantum). Timer is set for 100 ms in advance and then proc 2 is
started. If timer fires, proc 2 was cpu bound. New computation occurs
and next tick scheduled.
Example: proc 1 is waiting for IO, proc 2 is sleeping for 1 second.
Timer is set for 1 sec in future and the kernel enters the idle halt
state (i.e. C1-3). If proc 1's IO completes, ATA irq fires and kernel
runs proc 1. If timer irq fires, proc 2 is run.
> >However, the power savings on desktop machines from going tickless and
> >fixing stupid apps is pretty amazing -- the keithp quote about
> >increasing battery life by over 50% was after only about two days of
> >fixing stupid apps, and we're looking at trying to do even better for
> >certain situations (let the cpu sleep for several frames at a time while
> >playing movies, for example). Most of the app fixes that intel people
> >playing with this tool have have developed (which has included at least
> >ff, evolution, xchat, gaim, network mangler, xf86-video-intel, and
> >gnome-terminal) have been pushed upstream already, so we should all be
> >seeing the wins soon if we can go tickless. Until then, we'll just get
> >the benefit of not blowing out caches and saving some context switches
> >from apps waking up in order to do nothing.
>
> What is meant by "stupid apps" ?
Things that poll.
--
Nate
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