Re: Scheduler degradation since 2.5.66

[Nick Piggin <piggin@cyberone.com.au>]

Guillaume Foliard wrote:

>Hello,
>
>I have been playing with kernel 2.5/2.6 for around 6 months now. I was quite 
>pleased with 2.5.65 to see that the soft real-time behaviour was much better 
>than 2.4.x. Since then I tried most of the 2.5/2.6 versions. But recently 
>someone warned me about some degradations with 2.6.0-test6. To show the 
>degradation since 2.5.66 I have run a simple test program on most of the 
>versions. This simple program is measuring the time it takes to a process to 
>be woken up after a call to nanosleep.
>As the results are plots, please visit this small website for more 
>information : http://perso.wanadoo.fr/kayakgabon/linux
>I'm ready to perform more tests or provide more information if necessary.
>

This isn't a problem with the scheduler, its a problem with sys_nanosleep.
jiffies_to_timespec( {1000000us} ) returns 2 jiffies, and nanosleep adds
an extra one and asks to sleep for that long (ie. 3ms).

The more erratic timings could be due to interactivity changes as you say,
but you probably aren't running without RT priority