Kernel Setting for Pi

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When optimizing a Raspberry Pi for the lowest possible latency and jitter, there are various kernel parameters that can be added to /boot/cmdline.txt which may help. Some of these parameters disable certain features that can introduce latency, while others tweak the way the system handles interrupts or scheduling.

Here are some parameters you might consider adding to /boot/cmdline.txt:

  1. dwc_otg.fiq_enable=1: Enables FIQ (Fast Interrupt Request) support in the USB driver, which can improve USB throughput and reduce latency (only applicable to certain Pi models).
  2. dwc_otg.fiq_fsm_enable=1: Enables FIQ FSM (Finite State Machine), which further optimizes USB interrupt handling.
  3. dwc_otg.fiq_fsm_mask=0x7: This masks which USB endpoints use the FIQ FSM. The default 0x7 enables it for all endpoints, but you might need a different configuration depending on your setup.
  4. dwc_otg.nak_holdoff=0: This parameter can be used to disable NAK holdoff in the USB driver, which might improve response time for USB devices.
  5. sdhci_bcm2708.enable_llm=0: Disables low-latency mode for the SD host interface, which can sometimes improve performance.
  6. isolcpus: This parameter can be used to isolate certain CPU cores from the general kernel scheduling and interrupt handling. For example, isolcpus=3 would isolate CPU core #3. This can be useful in combination with setting certain real-time tasks to run exclusively on the isolated core to reduce jitter.
  7. nohz_full: This is used to run a CPU in full "tickless" mode, further reducing jitter by eliminating periodic timer ticks. For example, nohz_full=1 would set CPU #1 to be tickless.
  8. threadirqs: This parameter forces all interrupt handlers to run in thread context. This can be beneficial for real-time performance because it can reduce interrupt handling latency and allow finer-grained interrupt priority levels.
  9. idle=poll: This disables CPU idle states, which can reduce latency at the cost of increased power consumption and heat generation.
  10. audit=0: Disables the Linux auditing system, which can reduce overhead if you don't need auditing features.

Here's an example of how you might add some of these parameters to /boot/cmdline.txt:

dwc_otg.fiq_enable=1 dwc_otg.fiq_fsm_enable=1 dwc_otg.fiq_fsm_mask=0x7 dwc_otg.nak_holdoff=0 sdhci_bcm2708.enable_llm=0 isolcpus=3 nohz_full=1 threadirqs audit=0

Remember that /boot/cmdline.txt should contain all parameters in a single line. Before making changes, ensure you have a backup of your current configuration, and understand that some settings may affect the stability or functionality of your system. It's also essential to verify compatibility with your specific Raspberry Pi model and its peripherals, as not all settings are appropriate for all models.

After modifying /boot/cmdline.txt, reboot your Raspberry Pi for the changes to take effect. Monitor your system closely to make sure the changes have the desired effect and do not negatively impact system stability.