I have question at AI:
ethercat on debian 10 with kernel 4.19 and linuxcnc has less latency than debian 13 with kernel 6.12. Why?
reply , but only for curiosity :
This is a very insightful observation, and while it may seem counterintuitive (new = better?), in the world of Real-Time (RT) and numerical control (NC) like LinuxCNC, "old and stable" often trumps "new and complex."
The move from a 4.19 kernel (Debian 10) to a 6.12 kernel (Debian 13) introduces several layers of complexity that can degrade deterministic latency. Here are the main reasons:
1. Security Mitigations (The "Price" of CPUs)
Since the 4.19 kernel era, numerous hardware vulnerabilities have been discovered in CPUs (Spectre, Meltdown, L1TF, MDS, etc.).
Kernel 4.19: Many of these mitigations were not yet implemented or were less invasive.
Kernel 6.12: Includes a comprehensive suite of security patches that add overhead to every syscall and every switch between user-space and kernel-space. This increases jitter and maximum latency.
2. Kernel RT Integration (Preempt-RT)
Kernel 4.19: The PREEMPT_RT patch was a very mature set of external changes focused exclusively on pure real-time performance.
Kernel 6.12: Much of the real-time code has now been integrated into the Mainline Kernel. While this is a huge technical achievement, the overall integration must make compromises with other modern kernel features, making it sometimes less "extreme" than dedicated patches of the past.
3. Power Management (C-States and P-States)
Newer kernels are optimized for power efficiency.
Kernel 6.12 aggressively attempts to put CPU cores into power-saving states or vary the frequency.
Waking a CPU from a deep "C-state" requires precious microseconds, which, in EtherCAT communication (which often runs at 1kHz or more), translates into latency spikes.
4. EtherCAT Driver (IgH or SOEM)
The EtherCAT master (usually the IgH EtherCAT Master) must be patched to work with each kernel version.
On older kernels like 4.19, "native" drivers (those that replace the standard network card drivers, e.g., ec_8139too) are extremely stable and fast.
On very recent kernels, you're often forced to use the "generic" driver, which runs through the standard Linux network stack, inevitably adding latency compared to an optimized native driver.
5. Operating System Complexity
Debian 13 loads many more background services, more complex systemd versions, and more intensive D-Bus management than Debian 10. Even though LinuxCNC runs at high priority, background operating system noise can affect determinism.
How can I improve the situation on Debian 13?
If you absolutely must use the latest version, you can try "neutering" modern kernel features to regain speed:
Disable mitigations: Add mitigations=off to the GRUB boot parameters. (Warning: This reduces PC security.)
Core isolation: Use isolcpus=X (where X is the core dedicated to LinuxCNC) to prevent the operating system from using it for generic processes.
Idle=poll: Add idle=poll or intel_idle.max_cstate=0 to prevent the CPU from "sleeping."
Check Drivers: Make sure you don't use the generic EtherCAT driver if your network card supports an updated native driver for the 6.x kernel.
In summary: Debian 10 with kernel 4.19 is a "minimalist" and hardware-friendly combination. Debian 13 is a modern, hardened system optimized for power savings—the exact opposite of what's needed to drive a brushless motor in real time.
