[ubuntu-studio-users] Real-time - Was: ubuntu-studio-users Digest, Vol 79, Issue 27

Ralf Mardorf ralf.mardorf at alice-dsl.net
Thu Nov 28 08:07:59 UTC 2013

On Wed, 2013-11-27 at 19:53 -0500, Mike Holstein wrote:
> On Wed, Nov 27, 2013 at 2:57 PM, Alex Armani
> <alex.armani at rocketmail.com> wrote:
>         What are the advantages and disadantages of using the low
>         latency kernel? How is this different from the real time
>         kernel? I use the Gnome 3.8 desquetop 64 bit studio.
> the realtime kernel is basically discontinued. you can add it via ppa,
> or build it yourself. if you need the realtime kernel, you'll know,
> and seek it out. what are its advantages? lowlatency. disadvantages?
> the things that are compromised to provide lowlatency and audio
> priority can (and will) change performance of other things, such as
> power management. to address these differences between a normal, well
> supported, standard generic kernel, and the realtime kernel, a
> compromise was introduced. the lowlatency kernel is supposed to
> address these concerns, and provide an appropriate kernel that can do
> realtime at lower latency settings without the compromise in
> performance in other areas. if you are considering a realtime kernel
> via PPA, keep in mind those sources are not officially supported.
> there is no reason to remove any kernels. you can always keep the
> lowlatency and the generic, as well as a realtime kernel from a ppa
> installed, and choose between them at boot time from the grub menu.
> its likely you dont need lowlatency at all. if you are doing realtime
> effects or software synths as an instrument, you will want lower
> latency, but really anything around or under 8ms is plenty. enjoy!

There are different levels of real-time for Linux and non of the levels
does provide hard real-time, but nowadays it's that close to good old
hard real-time, that it doesn't matter.

Latency shouldn't be an issue, nowadays many amateur engineers expect
lower latencies for virtual stuff, then real musical instruments do
provide. Important is, that there is no jitter.

If you do MIDI work with external MIDI gear, than the lowlatency kernel
is not a very good choice. To get rid of MIDI jitter, you need the
hardest real-time the rt-patch does provide and very good hardware.

Preemption Model
  1. No Forced Preemption (Server) (PREEMPT_NONE)
  2. Voluntary Kernel Preemption (Desktop) (PREEMPT_VOLUNTARY)
  3. Preemptible Kernel (Low-Latency Desktop) (PREEMPT__LL)
  4. Preemptible Kernel (Basic RT) (PREEMPT_RTB)
  5. Fully Preemptible Kernel (RT) (PREEMPT_RT_FULL)



Real-time at the moment IMO is a serious issue


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