On 11/19/2014 08:01 AM, Brian Monroe wrote:
On Thu Nov 13 2014 at 12:28:27 PM Jeff Sandys <jpsandys@xxxxxxxxx
<mailto:jpsandys@xxxxxxxxx>> 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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