Hi everyone,

I've got a very weird problem here. I have a topic published on at around 50hz. On the other end there's a subscriber. The callback of the subscriber is a relatively long running process, slower than those 50hz.

That subscriber is initialized with queue_size=1 and buff_size=2**24. That way I want to make sure, messages come in as fast as possible and if there's currently one message processed, further messages get dropped. That's the desired behavior. I don't need all messages, but I need them fast.

When I leave queue_size=1 and buff_size=2**24 out, I see an increasing delay when I take a look at the age of the messages processed (now - header.stamp).

queue_size=1 and buff_size=2**24 work when I use rospy directly. However, it does not with rosbridge. I have already changed rosbridge in a way that the actual rospy.Subscriber is created in the exact same way as descrbed above, including queue_size=1 and buff_size=2**24. it does not help. I still see the increasing delay.

Where do I have to look to make sure I don't have that delay when using rosbridge?

Thanks

Update 1

Here's a code example with which I can reproduce the problem. I'm just not sure if it's caused by roslibpy or by rosbridge. Maybe someone with a little spare time could help me investigate this behavior. Or tell me that they cannot reproduce it and that I'm the only one seeing this behavior. Or that I'm doing something inherently wrong.

import roslibpy
import time
import json
import logging
logger = logging.getLogger("roslibpy")
handler = logging.StreamHandler()
logger.addHandler(handler)

def to_epoch(stamp):
    stamp_secs = stamp["secs"]
    stamp_nsecs = stamp["nsecs"]
    return stamp_secs + stamp_nsecs*1e-9

def from_epoch(stamp):
    stamp_secs = int(stamp)
    stamp_nsecs = (stamp - stamp_secs) * 1e9
    return {"secs": stamp_secs, "nsecs": stamp_nsecs}


def sub(msg):
    # The `age` of the message should remain constant, but it increases seemingly indef.
    age = time.time() - to_epoch(msg["stamp"])
    print("Age of message: %6.3f seconds" % age)
    time.sleep(1./5.)


ros = roslibpy.Ros("ws://localhost:9090")
ros.run()

tt = roslibpy.Topic(ros, "/test", "std_msgs/Header", queue_size=1)

ts = roslibpy.Topic(ros, "/test", "std_msgs/Header", queue_length=1)
ts.subscribe(sub)

try:
    seq = 0
    while True:
        seq += 1
        header = {"frame_id": "test", "stamp": from_epoch(time.time()), "seq": seq}
        tt.publish(roslibpy.Message(header))
except KeyboardInterrupt:
    ros.terminate()

Update 2

Three things that I've already ruled out:

1. in rosbridge_library subscribers.py setting rospy.Subscriber(..., queue_size = 1, ...) has no effect
2. in rosbridge_library subscribers.py setting rospy.Subscriber(..., buff_size = 2**24, ...) has no effect
3. in rosbridge_server rosbridge_websocket.py setting listenWS(..., backlog = 1, ...) or backlog = 0 has no effect

Update 3

I've now tested rosbridge_server/rosbridge_udp. It shows the same behavior! So it is not related to websockets.

Update 4

As suggested below I've added a sleep to the send loop. It has no effect on my side.

import roslibpy
import time
import json
import ...