[LAU] Re: difference between realtime-kernel and low-latency-kernel?

[Florian Schmidt]

On Thursday 04 October 2007, Kjetil S. Matheussen wrote:

> > For a truly reliable system (where you can count on no audio period being
> > missed because you forgot to disable the damn updatedb cronjob) you need
> > a system patched with ingo's realtime preemption patches and have it
> > properly configured.
>
> Hasn't this been fixed a long time ago? Its true that you were required to
> have a realtime kernel when using linux 2.4 to avoid dropouts for cronjobs
> etc., and in practice you probably also couldn't get reliable realtime
> performance with old versions of 2.6. But I thought it shouldn't be like
> that anymore? At least I haven't had any dropouts with my vanilla 2.6
> kernel as long as I've used it.

Well, with a vanilla kernel you simply don't get the fine grained control over 
what code gets the cpu at what times as with a realtime-preemption kernel..

It is true that for many people a vanilla kernel with CONFIG_PREEMPT and 
CONFIG_HZ=1000 delivers great performance, probably even better than 
a "lowlatency" 2.4.x kernel. But basically one badly behaving kernel driver 
might cause delay, so for differing people the results differ. With a -rt 
kernel you would just give this device a nice and low prio, so it doesn't 
even get a chance to disturb the soundcard/jack..

This goes as far that if one sees an xrun while running a properly 
setup -rt-kernel one knows it's an application bug or a soundcard driver 
bug ;) The kernel itself or any userland processes (X, cronjobs, whatever) as 
a source of timing problems are pretty much eliminated.

Regards,
Flo


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