[Fedora-music-list] Low Latency vs. Real Time Kernel - actual latencies ?

[Brian Monroe]

There's no such thing as a hard kernel. [sarcasm]But that means Ubuntu was
wrong, or they lied! I'm shocked! Everything on the internets is
true![/sarcasm]

Thanks for straitening that out for me. I had emailed the maintainer at
Ubuntu but never got a response. (maybe my emails give offensive order
because I haven't heard from anyone at RedHat lately either)  Sigh...

I'm bummed to hear that progress on this may be halting. I wish there was
someone to bully to get some traction or funding for this.

Sigh...

On Tue Dec 30 2014 at 10:07:47 PM Fernando Lopez-Lezcano <
nando at ccrma.stanford.edu> wrote:

> On 11/19/2014 08:01 AM, Brian Monroe wrote:
> > On Thu Nov 13 2014 at 12:28:27 PM Jeff Sandys <jpsandys at gmail.com
> > <mailto:jpsandys at gmail.com>> wrote:
> >
> >     Real Time Kernels are available from PlanetCCRMA:
> >     http://ccrma.stanford.edu/planetccrma/mirror/fedora/
> linux/planetcore/20/x86_64/repoview/kernel-rt.html
> >
> >     There are concerns about the implementation of Real Time Kernel as
> >     expressed in the Musician's Guide:
> >     http://docs.fedoraproject.org/en-US/Fedora/15/html/
> Musicians_Guide/sect-Musicians_Guide-CCRMA_Security_and_Stability.html
> >
> >     I am not a systems programmer so I can't speak to these concerns.
> >     Some of the names of the real time kernel developers listed on the
> >     PlanetCCRMA kernel-rt page are Red Hat employees.
> >
> >     I would like to see Fedora be the premier linux distribution for
> >     music.  But until we can overcome the concerns listed in the
> >     Musician's Guide we will probably not have a real time kernel in the
> >     Fedora repositories.  Maybe a Fedora "Re-Mix", or Fedora.Next and
> >     Workstation with the works with Fedora software library may break
> >     the ice.
> >
> > Well, I think those concerns are why Ubuntu moved to their low-latency
> > kernel instead of their kernel-rt. According to their documentation
> > (wiki) the lowlatency kernel is a "soft" preempt kernel and the rt
> > kernel is "hard' but there's no docs on what make them different, and
> > people in channel don't seem to know.
>
> (sorry for the very late response to this thread - busy busy busy, I
> think it is still worth to comment on this)
>
> Brian, there is no "hard realtime" kernel for Linux. There are only
> degrees of latency you can achieve with different combinations of
> compile-time options, patching and runtime optimization. All of them are
> "soft". You cannot guarantee deadlines on Linux - but you can get pretty
> close with a fully optimized system and the RT patch, and if you have
> the right hardware.
>
> I don't know what options Ubunutu uses to build their "low latency"
> kernel, I imagine they just enable full preemption and irq threading.
> This gives you better latency than a "non-preempt" kernel, but in my
> experience that is not enough for realtime audio work with small buffers
> (by that I mean jack running at 128x2 or 64x2). YMMV.
>
> To get really low latencies you need the RT patch (which has stability
> issues as has been mentioned in the thread - there are as many testers
> as for mainline). To fix those we would need more testers and proper bug
> reports that are followed up. But of course that needs kernel gurus that
> can respond.
>
> If you read this article you will see that the future of the RT patch is
> not good at this point:
>
> http://lwn.net/Articles/617140/
>
> Let's hope this gets better (it is not the first time that the rt patch
> has lagged way behind mainline, I think a major rewrite happened in the
> Fedora 1 or 2 timeframe and for a while there was nothing new). And BTW,
> this is why we are currently stuck at 3.14.x in the Planet CCRMA
> kernels, there is nothing newer...
>
> Sigh...
> -- Fernando
>
>
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