[PlanetCCRMA] What about the real time kernel

[Be]

It looks like the 'preempt' boot option is new. It was introduced in
Linux 5.12:

https://www.phoronix.com/scan.php?page=news_item&px=Linux-5.12-Dynamic-Preempt

It can also be changed at runtime. The default value is 'full' according
to that Phoronix article, though I don't know if Fedora changes that or
the default has changed since it was written.

On 11/11/21 21:43, Be wrote:
> Thank you for sharing this. It would be great to document somewhere. I
> though the PREEMPT_RT code that has been merged upstream was only
> available with a build time option. It is great that at least some of it
> can be enabled with boot time options on generic kernel builds.
>
> Many computers' motherboard firmware doesn't expose a switch to disable
> SMT, but this can be done with the 'nosmt' boot option for Linux. I
> found that this helps more than boot option or configuration to make
> audio less likely to glitch at low latencies, but I haven't tried yet
> the boot options you just posted about.
>
> On 11/11/21 14:20, Fernando Lopez-Lezcano wrote:
>> 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:
>>
>> ----
>> preempt=full
>> ----
>>
>> 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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>>> PlanetCCRMA at ccrma.stanford.edu
>>> https://cm-mail.stanford.edu/mailman/listinfo/planetccrma
>>>
>>
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