rtcqs

Linux audio performance improvements

jeremy, , , , , , 2 Comments

This is a recap in blog form of the following Mastodon toot: https://mastodon.autostatic.net/@jeremy/115632831793380239

The biggest performance improvements when it comes to Linux audio you can do are in my experience:

  • Setting the CPU scaling governor to performance.
  • Disabling SMT (Simultaneous MultiThreading).
  • Allow your DAW to set CPU DMA latency. For instance Ardour and Reaper can do this.

The Ardour manual provides some great background information on these matters. CPU scaling governor and SMT are explained here: https://manual.ardour.org/setting-up-your-system/the-right-computer-system-for-digital-audio/. CPU DMA latency is explained here: https://manual.ardour.org/setting-up-your-system/the-right-computer-system-for-digital-audio/

All other recommendations that for instance rtcqs or Millisecond give are for those that really need stable, ultra low latency. So buffer sizes below 64 samples that result in round-trip latencies below 10 milliseconds. This is the area where threaded IRQs or disabling Spectre/Meltdown mitigations might contribute to getting rid of that stray xrun.

Regarding threaded IRQs, enabling those by itself doesn’t change anything. You will need to configure those threaded IRQs after you’ve enabled them. Tools that can do this are rtcirqus or rtirq. You could also do this manually by using the chrt command on the threaded IRQ process.

Modern systems use MSI(-X) interrupts though (Message Signaled Interrupts) so shared IRQs should be something of the past. On those systems there’s very little gain in prioritising threaded IRQs.

The main difference between rtcirqus and rtirq is that rtcirqus allows you to set the real-time priority of a thread based on ALSA card names. rtirq works differently, it sets the real-time priority based on kernel module names. So with rtcirqus you can be sure the desired audio interface gets the desired real-time prio, with rtirq you’re prioritising all the devices that make use of a specific kernel module (xhci_hcd, snd_hda_intel).

rtirq does allow for some finer grained control regarding USB2 ports and onboard audio devices that use the snd_hda_intel driver. The USB2 ehci_hcd driver and the snd_hda_intel driver add the bus name and card index number respectively to the IRQ thread process name so you can use that designation in the rtirq configuration file. In case of USB2 you’re still prioritising the IRQ of the whole USB bus though but then rtcirqus does the same.

Linux audio performance improvements

rtcqs v0.6.1 released

jeremy, , , , , , 2 Comments

A new version of rtcqs, a Linux audio performance analyzer, is now available. Most notable changes include:

  • Fixed inconsistent use of single and double quotes
  • Replaced audio group check with a group agnostic check (fixes #4)
  • Governor check can now deal with systems that have SMT disabled
  • Tickless check now deals with all CONFIG_NO_HZ* variants and with nohz being set on the kernel command line (fixes #8)
  • File systems check has been expanded
  • IRQ check now loops through /sys/kernel/irq instead of parsing /proc/interrupts
  • rtprio check now checks if a SCHED_FIFO priority can be set instead of a SCHED_RR priority
  • Improved preempt RT check, check if “preempt=full” is part of the kernel command line (fixes #7)
  • Refactoring, created separate classes for main app, resources and GUI
  • Moved all packaging directives into pyprojects.toml

While working on this release I found out PySimpleGUI is not open source anymore so rtcqs’ GUI has become a bit of a moving target. I’m looking at alternatives like pygubu or even popsicle but that will be something for in the long run. In the short run there are more improvements in the pipeline. The swappiness check needs some attention and same goes for the IRQ check. I’ve been working on a different project to automate prioritizing IRQs and I’m planning to to reuse some parts of that project for the IRQ check in rtcqs. The idea is to have rtcqs not only list the status of all audio related IRQs but also any audio devices attached to those IRQs.

rtcqs is available on Codeberg, PyPI and is also included in the AUR.

rtcqs v0.6.1 released

rtcqs v0.4.2 released

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This release comes with a new Power Management check which checks if the audio group has read/write permissions on /dev/cpu_dma_latency. If your user is a member of the audio group and permissions are set for this group then DAW’s like Ardour and Reaper can open this file as your user, keep it open and control power management this way. This allows a user to prevent CPU sleep states for example so your CPUs are always on and instantly available which could lower the chance running into xruns.

Ardour CPU DMA latency setting Under Edit – Preferences – Performance

This release also introduces a new basic and simple tkinter-based GUI. The Qt GUI does look fancy but to use it it also needs a fancy amount of dependencies. When building binaries with PyInstaller the result of the Qt GUI is a whopping 130MB package while the tkinter version stays below 12MB.

rtcqs tkinter GUI

Future plans are to get rid of some checks:

  • Max user watches as it’s not related to the overall performance of your system
  • System timer as it’s not relevant anymore, rtcqs already checks for the more relevant stuff (high res timers and tickless kernel)
  • Background processes as it’s merely a placeholder which checks for two processes that don’t exist anymore on modern systems

I’m having my doubts about swappiness too as it’s not really applicable anymore for modern machines. But I’m curious if it still applies for smaller systems like RPi’s for example. I’d like to add a filesystem mount option check, for Ext it would check if the filesystem is mounted at least with the relatime option or even noatime for example. And maybe a disk scheduler check but I’m not conviced yet that it really makes a difference.

The new release and binary packages of rtcqs and rtcqs_simple_gui can be found on the Codeberg repo: https://codeberg.org/rtcqs/rtcqs/releases/tag/v0.4.2

rtcqs v0.4.2 released

rtcqs released

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rtcqs v0.3.1 is now available on Codeberg and Github. rtcqs is the continuation of the realtimeconfigquickscan project but then rewritten in Python. It comes with a Qt GUI and a few extra checks.

Dear all,

I’d like to announce rtcqs, the continuation of the realtimeconfigquickscan project. It’s a port to Python with some added extra’s, like a Spectre/Meltdown mitigations check and a Qt GUI. It has the approval of the original author of realtimeconfigquickscan to whom I owe a debt of gratitude, not only for the original code but also for his helpfulness with the continuation, or maybe even evolution of the project.

So check it out, indulge me with bugs, issues, improvements or any other useful feedback on the Codeberg repo which you can find at at https://codeberg.org/rtcqs/rtcqs

Happy system tuning and happy holidays!

Jeremy

While setting up a solution to fully automate the deployment of SSL certificates at work I piggybacked on the flow and focus to rewrite the realtimeconfigquickscan Perl code in Python. As part of the certificate deployment project I wrote an application to decrypt, re-encrypt and base64 encode PFX files so they can be uploaded to a vault solution. This way I ran into PySimpleGUI which enabled me to quickly put together a nice looking Qt GUI.


rtcqs main window

The code could be more terse and probably contains some typical non-programmer idiosyncracies. First improvement will be to make the code more dynamic so the GUI gets generated instead of using hardcoded values like it does now. And I’d like to add a power management check but then I first need to read up on that subject. There are also some checks that might need some more scrutiny like the swappiness and max_user_watches checks to verify if those checks are really needed for a real-time audio environment.

rtcqs released