[python]prevent callbacks from slowing down the main loop

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Hi,

I have a rather basic python2 node (it's the interface to the motor controllers)

• a main loop running (theoretically) at 10Hz (using rate.sleep() )
• 2 subscribers for velocities commands (only one is used at a given time, depending of the navigation mode) : the frequency should be about 10Hz on the active one (and 0 on the other one). Both have a queu size of 1
• 1 subscriber for proximity sensors (about 10Hz). Queue_size=1
• 2 publishers (publishing done from the main loop

Most of the time everything runs OK. However, sometimes, the main loop seems to be "stuck" for sometimes more than 1 second in a random place, while the message callbacks seem to be handled fine.

My guess is that sometimes, the main thread isn't executed for about 1 second, while the callback thread is still running.

From my understanding (please correct me if I'm wrong, I'm quite unsure about it), in Python there is only one thread running at a given time (even if multiple CPU cores are available). From what I understood in python (contrary to C++), the message callbacks are handled on a separate (python) thread).

So it seems that the callback thread gets executed but not the main thread.

NB : this even is rather execeptionnal (about once per minute under very heavy CPU load from other nodes, far less under lighter load).

For this specific use case, The main loop is more important than the message callbacks.

Is there a way to make sure the main loop gets executed (dropping messages in the input queues if needed) rather than processing all incomming messages and not executting fat enough the main loop?

A few potential ideas (but I'm not sure if they are and if so how) :

• increase the main thread priority (or decrease the message handling thread) : from my understanding it is not possible because python is handling itself it's threads)

• disable the message processing thread and une something like the C++ spinOnce function

• have a way to explicitly aks for the latest data (ie not having any callbacks at all). If no new data arrived, getting "None" or getting the last recieved data are both fine to me

Thanks a lot in advance

Best

Felix

EDIT : as asked by mgruhler, I add my code (it's now the full version). NB : The code was started by a collegue who left the company and that I replaced (maybe based on an example he found on the web), but I did several changes afterwards (but I never modified the mutex part, so I'm not even sure if it is usefull).

#!/usr/bin/env python

import rospy
from math import pi as PI
import threading
import time

from roboclaw import Roboclaw
#import roboclaw_driver as roboclaw
from std_msgs.msg import String
from std_msgs.msg import Float64
from std_msgs.msg import Float32
from std_msgs.msg import Bool
from geometry_msgs.msg import Twist
from borobo.msg import capteur_proximite_msg

r = 0.155
e ...