ROS2 Latency Experiments

Package to compare pointcloud (PC) publishing latency using rclypp or rclpy.

Issue (has been solved)

See rclpy issue: ros2/rclpy#763
See question on answers.ros.org: https://answers.ros.org/question/375827/ros2-performance-rclpy-is-30x-100x-slower-than-rclcpp/

Getting Started

You need either PCL to create dummy pointclouds in the source node or any other pointcloud source (Gazebo, Realsense, ...).

1. Build the package
2. Run ros2 launch ros2_latency experiment.launch.py
   (You can add megabytes:=<integer> to have a specific payload size)

Nodes

• source (C++)
  Publishes dummy pointcloud (PC) data of a specific size
• repeater (C++)
  Copies the incoming PC and re-publishes on another topic
• repeater.py (Python)
  Same as the C++ repeater, but using rclpy. A significant latency on the pub.publish(...) call is observed.
• measure (C++) Measures the age of an incoming PC by comparing the stamp with the current ROS time. This is to double-check if the latency is actually observed on the receiving end.

Launch Files

• experiment.launch.py
  Launches all nodes. Arguments:
  • megabytes (int) The size of the created dummy pointcloud, to compare different sizes.

Observations

On a fast desktop system with:

• Ryzen 3800X CPU
• 16GB RAM
• SSD

Comparison with 1 MB Pointclouds

(like from a LIDAR sensor)

>> ros2 launch ros2_latency experiment.launch.py megabytes:=1
[source-1]      [INFO] [1617894568.132222845] [ros2_latency]: creating   took 3898 [us]
[source-1]      [INFO] [1617894568.132272495] [ros2_latency]: publishing took 118 [us]
[repeater-2]    [INFO] [1617894568.133056859] [ros2_latency]: copying    took 121 [us]
[repeater-2]    [INFO] [1617894568.133102729] [ros2_latency]: publishing took 116 [us]
[measure-4]     [INFO] [1617894568.133555481] [ros2_latency]: age of pointcloud via cpp is 611.183 [us]
[repeater.py-3] [INFO] [1617894568.145218816] [ros2_latency]: Age of source PC is  1346 [us]
[repeater.py-3] [INFO] [1617894568.145447607] [ros2_latency]: Copying took    92 [us]
[repeater.py-3] [INFO] [1617894568.145639368] [ros2_latency]: Publishing took 11310 [us]
[measure-4]     [INFO] [1617894568.145463717] [ros2_latency]: age of pointcloud via py  is 13350 [us]

-> Publishing the PC via Python takes 11ms, via C++ only 0.12ms

Comparison with 10 MB Pointclouds

(like from a 640x480 RGBD camera)

>> ros2 launch ros2_latency experiment.launch.py megabytes:=10
[source-1]      [INFO] [1617894523.025022662] [ros2_latency]: creating   took 28423 [us]
[source-1]      [INFO] [1617894523.025084873] [ros2_latency]: publishing took 1406 [us]
[repeater-2]    [INFO] [1617894523.035501434] [ros2_latency]: copying    took 2006 [us]
[repeater-2]    [INFO] [1617894523.035576994] [ros2_latency]: publishing took 2321 [us]
[measure-4]     [INFO] [1617894523.041298005] [ros2_latency]: age of pointcloud via cpp is 8112.71 [us]
[repeater.py-3] [INFO] [1617894523.133180090] [ros2_latency]: Age of source PC is 11477 [us]
[repeater.py-3] [INFO] [1617894523.133437451] [ros2_latency]: Copying took  1961 [us]
[repeater.py-3] [INFO] [1617894523.133638792] [ros2_latency]: Publishing took 95739 [us]
[measure-4]     [INFO] [1617894523.137496766] [ros2_latency]: age of pointcloud via py  is 113877 [us]

-> Publishing the PC via Python takes 96ms, via C++ only 2.3ms