Rospy subscriber to Image topic high CPU in noetic vs. melodic

[franzpeter124]

During migration from melodic to noetic, I found that a subscription to a Image topic is very costly on noetic.
I found this on my laptop as well as on Nvidia Jetson.
Even though the callback does nothing and just pass, the subscriber node will take up much CPU.

I have compared melodic vs. noetic on the same laptop, both with plain ROS installation on a separate OS.

	Image size	Frequency	CPU publisher	CPU subscriber
melodic	2000	15	25%	12%
noetic	2000	15	15%	25%
melodic	3000	30	95%	45%
noetic	3000	30	60%	90%

In my application I have multiple nodes subscribing to one Image topic, so this issue sums up over multiple nodes and results in very high overall CPU usage.

Questions:

• What is the reason for this? Looks like melodic and noetic have different underlying data transport
• How can I get back to the low subscriber CPU usage from melodic?
• Can a simple parameter change like queue_sizeor similar help?

Reproduce

Minimal example with publisher and subscriber of Image topic:
Publisher:

#!/usr/bin/env python

import rospy
from sensor_msgs.msg import Image
import numpy as np
from cv_bridge import CvBridge

def publisher():
    # Initialize the ROS node
    rospy.init_node('image_publisher_node', anonymous=True)

    # Create a publisher to the 'white_image' topic
    pub = rospy.Publisher('white_image', Image, queue_size=1)

    # Set the loop frequency
    rate = rospy.Rate(30)  # 15 Hz

    # Create an all-white image
    size = 3000
    white_image = np.ones((size, size, 3), np.uint8) * 255

    # Initialize the CvBridge class
    bridge = CvBridge()

    while not rospy.is_shutdown():
        # Convert the image to a ROS Image message
        ros_image = bridge.cv2_to_imgmsg(white_image, encoding="bgr8")

        # Publish the image
        pub.publish(ros_image)

        # Sleep to maintain the desired loop rate
        rate.sleep()


if __name__ == '__main__':
    try:
        publisher()
    except rospy.ROSInterruptException:
        pass

Subscriber:

#!/usr/bin/env python

import rospy
from sensor_msgs.msg import Image


def callback(data):
    pass

def subscriber():
    # Initialize the ROS node
    rospy.init_node('image_subscriber_node', anonymous=True)

    # Subscribe to the 'white_image' topic
    rospy.Subscriber('white_image', Image, callback, queue_size=1)

    # Spin to keep the script running
    rospy.spin()


if __name__ == '__main__':
    subscriber()