Timer at high rate causes a high CPU usage on armhf

I installed ROS2 Eloquent on Raspberry Pi Zero W and I have issues publishing messages at a high rate (~31Hz). The problem is high CPU usage and it is caused by calling a callback via create_timer(). Therefore, even though the callback function does nothing (pass) the CPU usage is still very high. Also, it seems the CPU usage rises linearly with callback calling rate, but I didn’t save the results support this (CPU usage is low for the timer running at 0.5s interval).

Required Info:

• Operating System: Raspbian Buster

• Installation type: from source

• Version or commit hash: 42cabefef332c394cfa96f08f1e35fe1cd82e6b8

• DDS implementation: Fast-RTPS

• Client library (if applicable): rclpy

Steps to reproduce issue

For tests I used a simple publisher example given at: https://raw.githubusercontent.com/ros2/examples/master/rclpy/topics/minimal_publisher/examples_rclpy_minimal_publisher/publisher_member_function.py with the following modifications:

• timer interval is set to 32/1000 (32ms) and
• logger method is deleted (self.get_logger().info('Publishing: "%s"' % msg.data))

Expected behaviour

This is CPU and memory usage for equivalent example in C++ running on the same device: https://i.imgur.com/N093gQV.png

Actual behavior

The same test for the Python example: https://i.imgur.com/wO1cmsF.png

[lukicdarkoo]

I cannot test it on Raspberry Pi now, but it seems it works ok on a PC:

import rclpy
from rclpy.node import Node


PUBLISH_PERIOD = 1e-3


class Test(Node):
    def __init__(self):
        super().__init__('test')
        self.timer = self.create_timer(PUBLISH_PERIOD, self.timer_callback)

    def timer_callback(self):
        pass


def main(args=None):
    rclpy.init(args=args)
    test = Test()
    rclpy.spin(test)
    test.destroy_node()
    rclpy.shutdown()


if __name__ == '__main__':
    main()

CPU reaches 100% utilization (on PC) only when PUBLISH_PERIOD = 1e-4 (10KHz), that seems to be reasonable I guess.

[hidmic]

I do not have your exact same hardware to reproduce, nor a 32 bit ARM platform at hand, but I can see the significant gap between rclcpp and rclpy, both on amd64 and arm64. I’ll try do some profiling next week, see if there’s a clear bottleneck.

You’re welcome to contribute as well. Thanks for reporting.