Real-time Systems Engineering (Raspberry Pi)

Real-time does not mean fast. It means predictable: a result must arrive by its deadline, every time. A correct answer delivered too late can be a failure.

• Hard real-time: a missed deadline is a system failure, as in airbags and flight control.
• Firm real-time: a late result is worthless, but the system survives.
• Soft real-time: lateness reduces quality, as in video calls.
• Jitter: variation in timing. Often this matters more than the average speed.
• WCET: worst-case execution time, the number that really counts.

When several tasks share a processor, the scheduler picks who runs. With rate monotonic scheduling, the most frequent task gets the highest priority. Done carefully, you can prove that all deadlines will be met.

• Preemption: a high-priority task can interrupt a lower one.
• Rate monotonic: a shorter period gets a higher priority.
• Utilisation: U is the sum of C/T for each task. For two tasks, RM guarantees success up to about 83%.
• Priority inversion: a low task holding a lock blocks a high task, while a medium task runs.
• Priority inheritance: the lock holder briefly borrows the high priority.

Task A: C = 3 ms, T = 10 ms
Task B: C = 6 ms, T = 20 ms

U = 3/10 + 6/20 = 0.6
0.6 ≤ 0.83, so all deadlines are met

A Raspberry Pi runs Linux, which is built for fairness and throughput, not deadlines. Normal Linux can delay a task unpredictably. You can improve this a lot, but for hard deadlines a microcontroller is still the safer choice.

• SCHED_FIFO: a real-time policy. Your task runs until it blocks or something of higher priority needs the CPU.
• PREEMPT_RT: makes the kernel far more preemptible. It became part of mainline Linux in version 6.12.
• Measure it: cyclictest measures your real worst-case delay.
• Lock memory: mlockall stops page faults causing surprise delays.
• Hybrid: let a Pi Pico or Arduino handle the strict timing, and the Pi do the heavy thinking.

# run with a real-time priority
sudo chrt -f 80 ./control_loop

# measure worst-case latency
sudo cyclictest --mlockall --priority=90 --interval=1000