why did Ubuntu turn off kernel preemption in Edgy?

Phillip Susi psusi at cfl.rr.com
Tue Nov 28 17:02:48 GMT 2006

Karoliina Salminen wrote:
> behind what you are playing. The ear-finger interaction don't work and
> brains get confused and the playing gets so distracted that it is
> easier to play without hearing anything than hearing the notes coming
> several notes/bars behind.

While 500ms is too long, I thought that the brain could not notice sub 
100 ms of latency, certainly not 32 ms.

> The required 100% reliable length for the audio ring buffer drops to
> around 20 ms with the real time kernel. This is playable in my use
> with non-accurate timings in ambient style music. If I was playing

That sounds like what I would expect.  IIRC, the real time patches 
enable the kernel to switch to the audio task immediately in response to 
an event it was waiting on, rather than at the end of the current task's 
time quantum.  Is this correct Ben?  In that case a 100 Hz base timer 
should be able to manage a 20 ms audio ring buffer, and 250 Hz should 
have no trouble at all.

> some percussion instruments with high tempo, this would still be
> unacceptable. This is potentially a driver issue that it is not
> getting better than 20 ms. However, the drop from 100ms to 20 ms is
> certainly a kernel issue. For my use, however getting even to this
> level in Ubuntu would be great.

Have you tried a real time preemptable kernel with 1000 Hz?  I would 
think that would be able to handle a 5 ms ring buffer with ease.

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