set the priority of callback to high introduces some delay compared to simple MultiThreadedExecutor

[xibeisiber]

hi, I try to set one callback higher priority following the example cbg_executor . But It seems it introduces some delay compared to using MultiThreadedExecutor.

I publish on the topic "high_pong" at rate 100Hz. In the pong_node.cpp, I set the parameter "high_busyloop" to 1.0, and print the timestamp when receiving the msg.
Use the code in pong.cpp (setting high_prio_executor to ThreadPriority::HIGH), the output is (msg receiving interval is 1050ms):

But if I simply use MultiThreadedExecutor and add the pong node:

  auto pong_node = std::make_shared<PongNode>();
  rclcpp::executors::MultiThreadedExecutor multi_executor1;
  multi_executor1.add_node(pong_node);
  multi_executor1.spin();
  rclcpp::shutdown();
  return 0;

the output is (msg receiving interval is 1000ms):

[fujitatomoya]

@xibeisiber can you provide the self-contained reproducible example to see the difference?

[xibeisiber]

See attached file:
cbg_executor.zip

compile:
colcon build --packages-up-to examples_rclcpp_cbg_executor

In one terminal, publish on the topic at 100Hz
ros2 topic pub /high_ping std_msgs/msg/Int32 -r 100

case1: run (setting high_prio_executor to ThreadPriority::HIGH)
ros2 run examples_rclcpp_cbg_executor pong

case2: run (simply use MultiThreadedExecutor and add the pong node:)
ros2 run examples_rclcpp_cbg_executor pong_multi_executor