why did Ubuntu turn off kernel preemption in Edgy?

[Rocco Stanzione]

Thus spake Adam Dingle:
> In the Edgy kernel we've turned off CONFIG_PREEMPT, turning on
> CONFIG_PREEMPT_VOLUNTARY instead (see the changelog excerpt below). This
> change may not be great for multimedia users (especially those
> editing audio) since it can increase latency. Out of curiosity, what was
> the motivation for this?

I asked the same thing on #ubuntu-kernel on freenode.  After first being told 
that the kernel *was* preemptible, I eventually confirmed that we are only 
preempting the Big Kernel Lock, which isn't the same thing, and it was 
explained that it was decided that since kernel preemption may not be 
appropriate for everyone, it was disabled.  I suffer significantly from the 
change, as my desktop gets mighty laggy during heavy i/o, of which there is 
more than I would expect when I install packages.

As far as I can tell, CONFIG_PREEMPT is pretty well always appropriate on a 
desktop machine.  If not, I'm interested to know more, but I would very much 
like a preemptible kernel.

Rocco