docs: add FRI realtime scheduling guide

2026-09-16 15:08:55 +10:00
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+# FIFO priority for FRI
+
+## Why it is needed
+
+FRI exchanges a position command on every cycle. With a 10 ms period, a new packet should be handled about every 10 ms. The regular Linux scheduler can delay the control thread behind other processes. The log then contains messages such as:
+
+```text
+Could not enable FIFO RT scheduling: Operation not permitted
+Overrun might occur ... Read time: 11500 us
+```
+
+`SCHED_FIFO` is Linux scheduling for threads with fixed priority. This project requests priority 50 for `controller_manager` and priority 80 for the FRI worker. The FRI worker receives the higher priority so it can receive a KUKA packet and send the reply on time.
+
+Configure this on the **ROS 2 computer** that runs `cobot run`. It does not require changes to the Sunrise Cabinet or `cobot-setting.yaml`.
+
+## Ubuntu 24.04 setup
+
+On the ROS 2 computer, run these commands as the user who starts `cobot`:
+
+```bash
+sudo groupadd -f realtime
+sudo usermod -aG realtime "$USER"
+sudo tee /etc/security/limits.d/99-ros2-realtime.conf >/dev/null <<'EOF'
+@realtime soft rtprio 99
+@realtime soft priority 99
+@realtime soft memlock unlimited
+@realtime hard rtprio 99
+@realtime hard priority 99
+@realtime hard memlock unlimited
+EOF
+```
+
+Log out of the desktop session completely and log in again, or reboot. An already open terminal does not receive the new group and limits.
+
+## Verification
+
+In a new terminal, verify the group and limits:
+
+```bash
+id -nG | tr ' ' '\n' | grep -x realtime
+ulimit -r
+ulimit -l
+```
+
+The expected output is `realtime`, then `99`, then `unlimited`.
+
+Start FRI without commanding robot motion and check the log for:
+
+```text
+FRI worker uses FIFO priority 80
+```
+
+You can also inspect the policies of all threads in the process:
+
+```bash
+PID=$(pgrep -n ros2_control_node)
+ps -L -p "$PID" -o pid,tid,cls,rtprio,pri,comm
+```
+
+Realtime threads have `FF` in the `CLS` column. The FRI worker has `RTPRIO` 80 and the `controller_manager` main loop has 50.
+
+!!! warning "Before the first motion"
+    First confirm that FRI connects and that the log contains `FRI worker uses FIFO priority 80`. Then make a short move at speed 0.1 with an operator supervising. FIFO reduces jitter on the computer; it does not replace network checks, robot limits, or a clear work area.
+
+## If setup does not work
+
+- If `realtime` is missing from `id`, log out and log in again.
+- If `ulimit -r` is less than 80, check `/etc/security/limits.d/99-ros2-realtime.conf` and that PAM loads `pam_limits.so`.
+- If the log still says `Operation not permitted`, make sure `cobot run` is started by the same user passed to `usermod`.
+- A Python virtualenv needs no extra setup: permissions are assigned to the user and inherited by Python.
+- Docker requires `--cap-add=sys_nice --ulimit rtprio=99 --ulimit memlock=-1`.
+
+## Rollback
+
+To remove the setup, run:
+
+```bash
+sudo rm /etc/security/limits.d/99-ros2-realtime.conf
+sudo gpasswd -d "$USER" realtime
+```
+
+After logging in again, processes use the regular Linux scheduler.
+
+See the official [ros2_control documentation](https://control.ros.org/jazzy/doc/ros2_control/controller_manager/doc/userdoc.html#determinism) for controller-manager priorities and limits.