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

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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Received on Thu Oct 4 04:15:01 2007