why did Ubuntu turn off kernel preemption in Edgy?

Phillip Susi psusi at cfl.rr.com
Mon Nov 27 20:32:02 GMT 2006

Ulrik Mikaelsson wrote:
> to. Windows, for instance use a default interrupt timer of 100hz [2],
> so I would guess if you got 2-3ms latency out of you system in
> windows, it definately wasn't thanks to the interrupt rate.

Windows dynamically increases the timer interrupt rate to meet the 
requested needs of the multimedia timers.

> * Virtualization becomes difficult with higher interrupt rates, since
> several OS:s running on the same hardware will further increase the
> interrupt yield, causing the system to ultimately start loosing
> interrupts. [3]

Having the host hardware/kernel using a high interrupt rate allows it to 
provide the virtual environment with more reliable virtual interrupts. 
The guest OS in the virtual machine would do better to use a lower 
interrupt rate than the host OS.

> Based on this, I would say that any powerful machine with hard latency
> requirements, such as a hard-core gaming and audio/video-production
> would benefit from a high interrupt rate, while laptops, servers and
> "weaker" machines where battery life or throughput is more important
> than latency, lower interrupt rates is highly advised. (Maybe even as
> low as 100hz?)

I don't even see the higher rates as a benefit to hard core gaming and 
audio/video applications.  As you showed, a modest 250 Hz timer can 
easily provide 16ms latency which should be more than sufficient for 
these applications.  The audio hardware should be operating out of a 
ring buffer that holds at least 32 ms of sound, and video frame rates 
are typically 29 fps ( for ntsc video ) so a 100 Hz timer is plenty to 
synchronize the frame rate to.  For games, they synchronize to the video 
card's vertical sync interrupt anyhow.

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