Real-Time Robotics — Measured Latency & Jitter

I have analyzed your 500Hz (2ms period) control loop requirements. A 500us jitter represents 25% of your total period, which is high for a Raspberry Pi 4 running standard Linux.

Latency Audit and Tuning Plan

Kernel Optimization Commands

# 1. Isolate CPUs 2 and 3 for the control loop in /boot/cmdline.txt
# Add: isolcpus=2,3 nohz_full=2,3 rcu_nocbs=2,3

# 2. Set CPU governor to performance to prevent frequency scaling latency
echo performance | sudo tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

# 3. Apply RT priorities to the ROS 2 process (SCHED_FIFO)
chrt -f -p 98 <ros2_node_pid>

# 4. Move networking and disk IRQs away from isolated cores
echo 1 | sudo tee /proc/irq/eth0/smp_affinity  # Example for eth0 on Core 0

Measured Latency Analysis

Run the following to capture the baseline jitter before applying changes:
cyclictest -l100000 -m -n -p99 -i2000 -h400 > jitter_baseline.txt

Then process with
python3 scripts/latency_report.py jitter_baseline.txt --period-us 2000 --deadline-us 500 --cyclictest

Next steps

1. Apply mlockall() in your C++ node to prevent page faults during execution.
2. Set your ROS 2 QoS to Reliable with a small history depth to reduce middleware jitter.
3. Disable C-states in the firmware to prevent wake-up latency from deep sleep.