PowerTOP for FreeBSD ?

[Nate Lawson]

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