Re: Gurus, a silly question for preemptive behavior

[Con Kolivas <kernel@kolivas.org>]

jesse writes:

>  
> As i know, in linux, user space application is
> preemptive at any time. however, linux kernel is NOT
> preemptive, that means, even some event is finished,
> Linux kernel scheduler itself still can't have
> opportunity to interrupt the current user application
> and switch it out. it is called scheduler latency.

The kernel is preemptible if you enable the preempt option in the 
configuration. There are some code paths that are not preemptible despite 
this, but they are gradually being improved over time.

> normally , the latency is about 88us in mean , maximum
> : 200ms. Thus, the short latency shouldn't impact user
> applications too much and is not a problem. It is an
> issue in those embedded voice processing systems by
> introducing jitters, thus smart people came up with
> two kernel schedule patch: preemptive patch and low
> latency patch. 

You're thinking about the 2.4 kernel. 2.6 is effectively both of those 
patches inclusive.

> my system: 
> [root@sa-c2-7 proc]# uname  -a 
> Linux sa-c2-7 2.4.21-15.ELsmp #1 SMP Thu Apr 22
> 00:18:24 EDT 2004 i686 i686 i386 GNU/Linux 

If you want lower latency on a 2.4 kernel you need further patches. You are 
most likely to benefit from a move to a 2.6 kernel and enabling preempt.

Cheers,
Con