I'm experimenting with Kernel Preemption

[john bray]

On Wed, 2005-05-11 at 17:01 -0400, Dave Jones wrote:
> On Wed, May 11, 2005 at 02:17:25PM -0300, Juan Carlos Castro y Castro wrote:
>  > Not on Fedora's kernel sources, but with 2.6.11ac7. I based my config on 
>  > /boot/config-2.6.11-1.14_FC3 and deat with the additional options with 
>  > "make oldconfig". Then I browsed the configuration with "make 
>  > menuconfig", just for fun.
>  > 
>  > I saw kernel preemption was turned off, so I turned on. Afterwards, I 
>  > notice the system is noticeably faster. Bootup is faster. Shutdown is 
>  > faster. The Red Hat manu on GNOME pops up WAY faster. OpenOffice.org 
>  > loading is faster. I suspect other things are faster too, but I'd have 
>  > to time them.
> 
> Every time this comes up, theres no concrete numbers. Just
> 'it feels faster'. Given the best that preempt can do is
> lower the _average_ latency, rather than worse-case latencies
> which many folks believe, I find it hard to believe it makes
> a noticable difference. In a blind-test, given two kernels,
> I'd bet on you not being able to 'feel' which one had
> preempt enabled.
> 
>  > So my question is: why isn't preemption enabled in the FC3 packaged 
>  > kernel? Does it conflict with something I haven't encountered yet? maybe 
>  > some esoteric hardware combination? My hardware data is below.
> 
> It doesn't really buy anything worthwhile, and adds complexity and
 
> 
>  > Another thing: what crucial patch, if any, am I missing by using 
>  > 2.6.11ac7 instead of the FC3 packaged kernel?
> 
> Exec-shield is probably the biggest feature, but there are a bunch
> of other minor features (restricted /dev/mem, ipw wireless for eg).


hey dave!  thanks for that thoughtful reply.  i've been curious about
that for a long time, as well.  while i'm thinking of it, thanks for all
those thoughtful replies you've given.  i've appreciated the thought
that went into them on a number of occasions.

sure would be fun to try some timed tests with and without.  wish i had
the h/w to do it on.  :-(

dave, can you tell us a little more about the "...opportunity for
drivers to break...." comment?  i'd just like to understand a bit more
about the implications of it.

thanks!

john