rt kernel

[Scott Bohon]

On Sun, 2011-04-03 at 14:42 -0500, Scott Lavender wrote:
> 
> 
> 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!
>         
>         
>         
>         
>         --
>         Ubuntu-Studio-users mailing list
>         Ubuntu-Studio-users at lists.ubuntu.com
>         Modify settings or unsubscribe at:
>         https://lists.ubuntu.com/mailman/listinfo/ubuntu-studio-users
> 
> 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.

Thank you Scott and all. This is very interesting, if I can find some
more time I would like to delve into some of the more technical issues,
however, since I am a musician...
> 
> 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.

...this does really help. So I now need to get my Tascam US-144mkII
working!! How do I disable ehci-hcd (USB 2.0)?? Can I do this by port
address? (I will start a new thread!)
> 
> 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
> 
>