why did Ubuntu turn off kernel preemption in Edgy?

Matthew Garrett mjg59 at srcf.ucam.org
Wed Nov 29 17:43:14 GMT 2006

On Wed, Nov 29, 2006 at 12:27:25PM -0500, Phillip Susi wrote:

> No, this is the beat again.  You are being shown images that show the 
> wheel more than 180 degrees out of phase with the previous and next 
> images, which is the same as if it were going backwards at a slower 
> rate.  If the wheel is spinning at 23 rpm and the film is 24 fps, then 
> you get a 1 Hz beat.

If the human eye is unable to distinguish movement at faster than the 
equivalent of 24 frames per second, then you'd see exactly the same 
thing in real life. You don't, which indicates that faster movement is 
still perceivable. In computer games, the difference between 30 frames 
per second and 60 frames per second is quite easy to distinguish.

In reality, there is no concept of "frames per second" in the human 
visual system. The eye receptors are sufficiently sensitive that a 
single photon will produce a visible response[1]. The fact that we 
perceive 16 frames per second as smooth motion is an artifact of 
higher-level processing, not an indication that we're unable to perceive 
flicker above 16Hz - there's no well defined lower bound on the minimum 
period of time a stimulus has to appear for before it reaches higher 
levels of consciousness, and it varies between individuals. Even with 
continuous flickering (a worst case scenario for determining whether 
a stimulous is continuous or not), some people are able to perceive 
120Hz flicker.

> What are you talking about?  The light source in the projector is always 
> on and 24 film frames pass in front of it each second.  Where is the 
> double illumination?

If that were the case, you'd see the frames moving vertically. You 
don't. There's a shutter between the light and the film, as described in 
http://en.wikipedia.org/wiki/Movie_projector . On modern projectors, the 
shutter opens more than once per frame.

[1] Sadly the spontaneous firing rate is higher than this, so a single 
photon is somewhere below the noise threshold
Matthew Garrett | mjg59 at srcf.ucam.org

More information about the ubuntu-devel mailing list