[PlanetCCRMA] What about the real time kernel

Fernando Lopez-Lezcano nando at ccrma.Stanford.EDU
Thu Nov 11 12:20:18 PST 2021

On 11/11/21 4:08 AM, Yann Collette wrote:
> Hello,
> I still compile the RT kernel for Fedora 33, 34 and 35.

Hi Yann,
Thanks so much for doing this!

I have been too busy and there has not been demand (till now) for an rt 
patched kernel. I will probably skip building Planet CCRMA's rt kernels 
as there is an option for those that need it.

As to what I use for music...

I have started using the "normal" Fedora kernel. Newer kernels have the 
PREEMPT_DYNAMIC option which allows you to turn on full preemption on 
boot. Fedora defaults to it being "off" which means you get 
PREEMPT_VOLUNTARY (I think), which is not good for low latency audio. 
But if you add this to the boot command line (in /etc/defaults/grub), 
and recreate the configuration in /boot, the next time you boot you will 
have full preemption:


You can verify it is on by doing:

dmesg|grep preempt

As many parts of the RT patch have been migrating over the years to the 
standard kernel the results are not bad at all.

My latest testing (for critical use) also uses these boot line switches:

processor.max_cstate=1 idle=poll

These disallow the CPU from going to lower clock speeds and that seems 
to fix xruns when using jackd at low latencies (in my latest test I also 
disabled hyperthreading in the BIOS, I still have to test again with it 
ON to see if it really makes a difference). A replacement computer to 
the control system that runs our Listening Room speaker array is now 
running at 64 frames per period - the best I could do before was 128.

If you have an Intel CPU upping the minimum clock speed in the 
intel_pstate driver (and disabling turbo) is also good (those tweaks are 
in /sys/devices/system/cpu/intel_pstate). I have not double checked if 
this is still needed if you do processor.max_cstat=1....

Hope this helps...
-- Fernando

PS: I have been building and using tweaked Fedora kernels that have a 
hacked Motu USB driver (as outlined in a LinuxMusicians thread) that is 
quite effective at using newer firmware hardware with 64 I/O channels on 
USB2 (not super easy to use and not 100% perfect but does the job). But 
I have not released them... Caveat: this driver does NOT really work 
with a fully RT patched kernel, something to do with the number of 
interrupt and thus context switches it generates.

> $ dnf search kernel-rt
> Dernière vérification de l’expiration des métadonnées effectuée il y a 2 
> days, 15:16:38 le lun. 08 nov. 2021 21:49:52.
> ==================================================================== Nom 
> correspond à : kernel-rt 
> =====================================================================
> kernel-rt-mao.x86_64 : The Linux Real Time Kernel
> kernel-rt-mao.src : The Linux Real Time Kernel
> kernel-rt-mao-devel.x86_64 : Development package for building real time 
> kernel modules to match the 5.14.2.rt21 kernel
> kernel-rt-mao-headers.x86_64 : Header files for the Linux real time 
> kernel for use by glibc
> The last one I uploaded is 5.14.2-rt21.
> The 5.15 is still based on a .0 kernel, so I am waiting for a new 
> release before compiling this version.
> Best regards,
> Yann
> Le 11/11/2021 à 13:02, Yoann LE BARS a écrit :
>> Hello, everybody out there!
>>     Well, I have not used Fedora for several months, but I will 
>> probably install it back. So I have been looking to Planet CCRMA 
>> repository, as well to ycollet/linuxmao COPR: there is no more 
>> real-time kernel. As the RT patch is well on its way to be mainlined, 
>> does it mean that the standard kernel is now considered fulfilling the 
>> requirement for music production?
>>     Best regards.
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