85 lines
3.1 KiB
Markdown
85 lines
3.1 KiB
Markdown
# FIFO priority for FRI
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## Why it is needed
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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:
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```text
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Could not enable FIFO RT scheduling: Operation not permitted
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Overrun might occur ... Read time: 11500 us
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```
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`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.
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Configure this on the **ROS 2 computer** that runs `cobot run`. It does not require changes to the Sunrise Cabinet or `cobot-setting.yaml`.
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## Ubuntu 24.04 setup
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On the ROS 2 computer, run these commands as the user who starts `cobot`:
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```bash
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sudo groupadd -f realtime
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sudo usermod -aG realtime "$USER"
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sudo tee /etc/security/limits.d/99-ros2-realtime.conf >/dev/null <<'EOF'
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@realtime soft rtprio 99
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@realtime soft priority 99
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@realtime soft memlock unlimited
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@realtime hard rtprio 99
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@realtime hard priority 99
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@realtime hard memlock unlimited
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EOF
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```
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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.
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## Verification
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In a new terminal, verify the group and limits:
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```bash
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id -nG | tr ' ' '\n' | grep -x realtime
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ulimit -r
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ulimit -l
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```
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The expected output is `realtime`, then `99`, then `unlimited`.
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Start FRI without commanding robot motion and check the log for:
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```text
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FRI worker uses FIFO priority 80
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```
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You can also inspect the policies of all threads in the process:
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```bash
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PID=$(pgrep -n ros2_control_node)
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ps -L -p "$PID" -o pid,tid,cls,rtprio,pri,comm
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```
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Realtime threads have `FF` in the `CLS` column. The FRI worker has `RTPRIO` 80 and the `controller_manager` main loop has 50.
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!!! warning "Before the first motion"
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First confirm that FRI connects and that the log contains `FRI worker uses FIFO priority 80`. Then make a short move at speed 0.1 with an operator supervising. FIFO reduces jitter on the computer; it does not replace network checks, robot limits, or a clear work area.
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## If setup does not work
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- If `realtime` is missing from `id`, log out and log in again.
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- If `ulimit -r` is less than 80, check `/etc/security/limits.d/99-ros2-realtime.conf` and that PAM loads `pam_limits.so`.
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- If the log still says `Operation not permitted`, make sure `cobot run` is started by the same user passed to `usermod`.
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- A Python virtualenv needs no extra setup: permissions are assigned to the user and inherited by Python.
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- Docker requires `--cap-add=sys_nice --ulimit rtprio=99 --ulimit memlock=-1`.
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## Rollback
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To remove the setup, run:
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```bash
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sudo rm /etc/security/limits.d/99-ros2-realtime.conf
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sudo gpasswd -d "$USER" realtime
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```
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After logging in again, processes use the regular Linux scheduler.
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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.
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