why did Ubuntu turn off kernel preemption in Edgy?

[Phillip Susi]

Karoliina Salminen wrote:
> Why the 1000 Hz couldn't be used?
> I could put it this way:
> CPU overhead raises from 0.1% to 1%. Who can notice that?

The reason why a high timer rate is bad is that it interferes with the 
cpu's low power states.  Entering and leaving the lower power states has 
a fairly high latency, so to make good use of them you need to enter the 
lower power state during an idle period, and the cpu must remain there 
until there is actual work to be done.  When it takes the cpu 250 us to 
enter/exit the lowest power state, and you are getting a timer interrupt 
every 1000 us, you can only spend half the time in the low power state 
at most.  Ideally if the system is actually idle the timer interrupt 
should be suspended when it switches to low power mode, or at least set 
to the longest delay possible given the current list of pending alarm 
events.  That way the cpu isn't constantly waking up just to increment 
the current time.

Oh, and please don't trim CC lists, use reply-to-all.