Ubuntu Linux for Robotics · Lesson 84 of 96

Real-time Linux

PREEMPT_RT, priorities, CPU isolation, memory locking and measuring latency.

Estimated time · 60–90 min

Learning goals

What you will understand

Explain real-time linux in your own words
Use it in a small workstation example
Verify the result instead of assuming it worked

Understand the idea first

The mental model

Real-time means meeting a deadline every cycle, not just being fast on average. Ubuntu Pro offers a real-time kernel (PREEMPT_RT), which makes most of the kernel preemptible so a high-priority thread runs when it needs to. Check the running kernel with uname -a (look for PREEMPT_RT).

Run the control thread with a real-time policy (chrt -f 80 or SCHED_FIFO), lock memory (mlockall), pin it to an isolated CPU core (isolcpus, taskset) and keep other load off that core. Measure with cyclictest and your own jitter histogram; never claim hard real-time without measurements, and keep allocation, logging and blocking calls out of the loop.

Guided lesson

Work through these steps

1.
Open a terminal and record the current state before changing anything.
2.
Read the real-time linux commands and predict what each one does.
3.
Run the commands one at a time and compare each output with your prediction.
4.
Write the commands that worked in a setup note you can repeat later.

Worked example

Read it before you run it

uname -a | grep -i preempt
# Check whether the running kernel is a real-time kernel.
sudo apt install rt-tests
# Install cyclictest and related tools.
sudo cyclictest -m -p80 -i1000 -l10000
# Measure wake-up latency at priority 80 with a 1 ms interval.
sudo chrt -f 80 taskset -c 3 ./control_loop
# Run the control program with a FIFO priority pinned to CPU 3.
cat /sys/devices/system/cpu/isolated
# Show which CPU cores are isolated from the scheduler.

What the example is doing

1.Lines without sudo only inspect or change files you own; sudo authorizes a system-wide change.
2.Read command output before moving to the next line. Stop if names, versions, disks, or paths differ from the lesson.
3.Comments beginning with # explain safety checks and are not executed by the shell.

Watch it run · The robot's computer

See what this code does

The terminal types this lesson's practice solution and runs it. The scene shows the kind of robot behaviour that code drives. It plays by itself and loops.

Practice code

Now type it yourself

Open a terminal, run the starter line by line (or save it as practice.sh and run bash practice.sh), and replace every TODO with your own command.

# TODO: run cyclictest with memory locking (-m), priority 80 (-p80), a 1000 us interval (-i1000) and 10000 loops (-l10000)

Expected output

The latency statistics of cyclictest in microseconds

Stuck? Show the full solution (try it yourself first)

sudo cyclictest -m -p80 -i1000 -l10000

Expected result

How you know it worked

Each command completes without an unexplained error.
Your notes contain the command, the important output, and what it proves.

Practical exercise

Apply the lesson

Measure latency with cyclictest on the normal kernel and, if available, on PREEMPT_RT, while running a CPU stress test. Record the maximum and 99th percentile in each case.

Before you continue

You recorded the starting state
Every command and output is understood
The change can be repeated from your notes

If something goes wrong

A command says permission denied.

Check ownership and permissions first. Do not add sudo unless the command changes a documented system location.

A command or package is not found.

Check spelling, Ubuntu version, enabled repositories, and whether the previous installation command succeeded.