Re: difference between realtime-kernel and low-latency-kernel?

["porl sheean" <porl42@xxxxxxxxx>]

i have actually had no end of trouble with ubuntustudio's (and now ubuntu's) rt kernel. on an amd 6000+ system with 1gig ram and a rme9652 soundcard i can't get reliable performance under 40 or so ms. i even tried a vanilla kernel with the rt patches and had the same trouble. the 64studio kernel worked fine, however. i'm currently running at 5ms with it and have had no problems. this is even with compiz fusion running and spinning the cube whilst playing back audio from an 18 channel ardour project. what patches would cause such a difference in performance? it isn't any options selected in 'make menuconfig' - i loaded the 64studio's ones in and used them. still no luck. i can only assume they have added more patches to do with realtime performance than just the -rt patchset. any ideas?

porl

On 05/10/2007, thomas fisher <studio1@xxxxxxxxxxxxx> 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. 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.
Tom
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
>
>
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