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

[Fernando Lopez-Lezcano]

On Thu, 2007-10-04 at 12:15 -0700, thomas fisher wrote:
> I can supply no quantifications for the 32 bit 2.6.20-16-realtime kernel  in 
> ubuntustudio other than no xruns have been observed. With the low latency 
> kernel, xruns were observed. 

Do you remember by any chance which version of the rt patched kernel was
giving you trouble? There was a mainline kernel bug which created a lot
of problems from 2.6.18 onwards and was fixed in January 2007 (I could
not update to that one, or later versions till the problem was
fixed[*]). 

> Jack is the only app that has a -20 priority 
> assigned. The general workstation has been running without fault. The general 
> Debian / Ubuntu philosophy tends towards system stability.  

If you found a setup that works by all means stick to it! The list below
is only indicative, I have found the rt patched kernel to be better but
I don't have the exact same setup you have. There are many things that
have to be just right for everything to work fine, and of course it also
depends on how low are the latencies used (I routinely work at 2x128 and
also 64x2 - the default Jack 1024x2 is less problematic of course). 

-- Fernando

[*] http://lkml.org/lkml/2007/1/5/291


> On Wednesday 03 October 2007 14:54:32 Fernando Lopez-Lezcano wrote:
> > On Wed, 2007-10-03 at 18:39 +0200, Frank Barknecht wrote:
> > > Hallo,
> > >
> > > Matthias Schönborn hat gesagt: // Matthias Schönborn wrote:
> > > > I've just read that there's a difference between a realtime-kernel and
> > > > the low-latency-kernel provided by ubuntustudio. The text in the german
> > > > wiki on ubuntuusers.de said, that a realtime-kernel is slightly better
> > > > than the lowlatencykernel (http://wiki.ubuntuusers.de/Echtzeitkernel) -
> > > > then why isn't it used in ubuntustudio? Or do I just mix something up?
> > >
> > > I think, this wiki and maybe Ubuntustudio as well are using a very
> > > confusing terminology.
> > >
> > > Generally we have two kinds of kernels: The "vanilla" kernel as
> > > downloadable on kernel.org and the same kernel, but patched with Ingo
> > > Molnars RT-patches. The vanilla kernel, if configured properly with
> > > CONFIG_PREEMPT etc., already gives very good performance in the low
> > > latency department, enough for many users, even audio users. I run one
> > > of these.
> > >
> > > If you want more, then you can install a RT-patched kernel, as is
> > > provided in the linux-rt or linux-realtime packages.  I would call the
> > > Ingo-Molnar-patched kernels Realtime-Kernels or Low-Latency-Kernels.
> >
> > To further clarify (or confuse?) the issue, how "low latency" the kernel
> > is also depends on how you configure the kernel build options before or
> > after patching the kernel with Ingo's patch. For Ingo's patch
> > CONFIG_PREEMPT_RT is the best option in terms of latency but there are
> > others (CONFIG_PREEMPT_DESKTOP) that have a more conservative approach
> > but have (relatively speaking) higher latencies. So from worst to best
> > it would be something like:
> >
> >   vanilla linuz + CONFIG_PREEMPT_NONE
> >   vanilla + CONFIG_PREEMPT_VOLUNTARY (used by the stock Fedora kernel)
> >   vanilla + Ingo + CONFIG_PREEMPT_DESKTOP
> >   vanilla + Ingo + CONFIG_PREEMPT_RT (the one I use for Planet CCRMA)
> >
> > (there's more granularity and options in the CONFIG_PREEMPT* world but
> > those are the ones that have the biggest impact as far as I can
> > remember)
> >
> > -- Fernando
> >
> >
> > _______________________________________________
> > Linux-audio-user mailing list
> > Linux-audio-user at lists.linuxaudio.org
> > http://lists.linuxaudio.org/mailman/listinfo.cgi/linux-audio-user
> 
> 
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