rt kernel

[Scott Lavender]

On Sat, Apr 2, 2011 at 9:21 PM, Scott Bohon <scott.bohon at gmail.com> wrote:

> I am a newbie to Ubuntu and Linux. I upgraded from Ubuntu 10.10 to
> Studio, but I got a message during the upgrade that essentially said
> linux-rt could not be found. Is linux-rt loaded with the audio package?
> Do I need -rt? How do I check if -rt is loaded? If I still need it, how
> do I get it?
>
> Thank you for your help!
>
> Scott Bohon
> cellist and new techno-musician!
>
>
>
>
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Hi Scott,

A lot of the other posts in this thread are pretty intensive and involved,
and since you are a professed "newbie" I thought I would add some more
explanation.

In this context, the package name "linux-rt" refers to a special type of
kernel, the "real-time" kernel.  The "real-time" kernel can help with
recording audio because it affords users to experience more stable
performance at lower latencies (the amount of time from when a sound is made
to when it is heard).

And keep in mind there are two types of considerations, the "stable
performance" and "lower latencies" in what I mentioned.  By "stable
performance" I mean the avoidance of xruns, which are skips or ugly digital
artefacts in recorded audio due to overruns or underruns in the audio
buffer.  And "lower latencies" mean that the sound is routed with due
consideration so that the sound made is recorded in a timely manner.

Obviously, it doesn't do much good to set the latency extremely low but have
many xruns so that the sound recorded has unwanted noise introduced in it.
Conversely, it doesn't do much good to avoid xruns but have a latency so
high that it appears that you are playing against your own echo in the Grand
Canyon, which throws off timing.

Again, while the linux-generic kernel provides a moderate amount of
performance, it is expected that the linux-rt kernel can provide a better
performance.  But this also considers in other factors such as your computer
and your audio interface.

Lower powered computers (say a netbook) or a USB2 audio interface would
provide a lower baseline performance in contrast with a quad-core, 6 gigs of
memory desktop machine with an M-Audio Delta 66 PCI card audio interface.

It seems to resolve to a matter of compromise which what you have (hardware)
and what you can get (stable performance and latencies).

It would seem that the -rt kernel is a very important thing.  And it is.
Unfortunately it isn't always available.

The linux kernel, in general, is used by everyone and therefore sees much
attention.  In contrast, the real-time kernel (which is made by applying a
patch to the -generic kernel) is only used by a very niche group and is
supported by only a very small group.  In times past, the patch was made and
maintained by a single person.  I think the group now consists of three
people, but I cannot say how involved each is.  I'm probably wrong on the
quantity, though.

But the result of this is that the real-time patch that is created by a very
small group of people that provides functionality to a niche group (but very
small percentage) of users is not available for every kernel release.
Therefore, we cannot have a real-time kernel for every kernel version
released. Unfortunately, this also means that the kernel version chosen for
Ubuntu (and therefore Ubuntu Studio) may not align with the available
current real-time patch and Ubuntu (and Ubuntu Studio) will not have a
real-time kernel for that release.

I think this was a consideration, among with others, that caused the
linux-rt kernel to be pulled out of the repositories.  Well, this isn't the
most accurate description.  The linux-rt package was kept in the
repositories for the extant releases but new linux-rt packages would not be
created for future release of Ubuntu.  I think Lucid (10.04) is the last
Ubuntu (and Ubuntu Studio) release that had the linux-rt package available
from the official repositories.

So, having said all that, to answer your questions:

No the linux-rt package is not loaded with the audio package.  it is a
separate package that contains the "real-time" kernel, but is not available
for Ubuntu (or Ubuntu Studio) 10.10 from the official repository.

You may need the linux-rt package if you have tried the -generic kernel,
adjusted your settings for jack, and still find that you have unacceptable
performance.  Likewise, if you have a firewire device you may be suffering
from irq conflicts and may need the linux-rt kernel to help you resolve
those conflicts.  But again, it all matters as to what hardware you have and
what your current performance is.  Without more specifics I doubt anyone can
say with certainly whether you need it or not.  But it can help in some
cases.

To check if it is loaded, you can start a terminal and type 'uname -a' and
enter.  This will give you a description of your currently running kernel.
This is different than going into synaptic and seeing which kernels are
available on your computer, this will give you the kernel that is currently
running.  For example:
scott at lucid-studio:~$ uname -a
Linux lucid-studio 2.6.33-29-realtime #1-Ubuntu SMP PREEMPT RT Wed Aug 4
17:22:37 UTC 2010 x86_64 GNU/Linux

I am running the "real-time" kernel currently.

If you find that you cannot achieve acceptable performance and you wish to
try the real-time kernel on Ubuntu 10.10 or later releases you will most
likely need to use an individual's personal package archive, colloquially
known as a PPA.  This are like the official repositories for software but
are "maintained" by individuals.  I say maintained because you neither know
the quality of their work nor how well they keep up with it.  I would stick
with ones that many people recommend.  I think someone recommended Falktx's
PPA, I would recommend his too.  As to the mechanics of PPA's, rather than
assume you need this information and post it, I will wait until you identify
that you need the real-time kernel and also that you need to understand how
PPA's work.

I hope this expansive email helps you.

If anyone notices where I misspoke or was simply wrong, please do not
hesitate to correct me.  I imagine it will help Scott and others greatly to
avoid misinformation.

ScottL
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