PREEMPT_RT kernel 5.10 latency on Raspberry Pi

[kdoren]

I posted a release of PREEMPT_RT kernel 5.10.52-rt47-v7l+
This is the first 5.10 PREEMPT_RT kernel for RPi that I think is solid enough for production use.
In earlier releases of 5.10 PREEMPT_RT kernel for RPi, while most applications were stable, it was possible to lock up a thread (jamulus server with multitheading enabled would do this). That seems to have been fixed in 5.10.52-rt47.

So how does it perform in terms of latency? I ran some tests using cyclictest: https://www.osadl.org/Create-a-latency-plot-from-cyclictest-hi.bash-script-for-latency-plot.0.html

Initial tests using cyclictest with no other load show definite latency improvement over low-latency kernel on Pi4.

For reference, I have included a plot for stock raspian kernel 5.10.17 pm Pi4. As expected, max latency is much worse (557us vs 32 us for PREEMPT_RT and 144us for PREEMPT). Avg latency (not the concern in realtime) of the stock kernel is actually better than PREEMPT.

Raspberry Pi 4 with PREEMPT_RT kernel 5.10.52-rt47:

Raspberry Pi 4 with low-latency PREEMPT kernel 5.10.52:

Raspberry Pi 4 with stock raspian kernel 5.10.17:

Raspberry Pi 3 with PREEMPT_RT kernel 5.10.52-rt47:

Raspberry Pi 3 with low-latency PREEMPT kernel 5.10.52:

[rtlinux]

Works great on RPi3 with my custom NativeDSD kernel for Savitech SA9227 (inc. Accuphase/McIntosh DACs).

case USB_ID(0x21ed, 0xd37a): /* Accuphase DC-37 */
case USB_ID(0x262a, 0x9227): /* Savitech SA9227 */

My initial latency test shows an impressive improvement of 63 vs 294 us

Allo USBridge Sig CM3 with PREEMPT kernel 5.10.52-1:

Raspberry Pi 3 with PREEMPT_RT kernel 5.10.52-rt47:

The DSD/DoP sample rate switching is improved as well. Most importantly it sounds great! :)

Thank you so much for sharing this!