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why does rclpy timer have no update method and stored in ns?

asked 2021-12-16 16:20:07 -0500

masynthetic gravatar image

using rclpy timer there does not appear to be any method for updating the timer period/interval there is simply a timer_period_ns variable which can be changed but it feels a bit off, especially the whole multiply by a million part. Is there a reason it is this way?

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answered 2021-12-18 14:48:12 -0500

osilva gravatar image

Hi @masynthetic

To better answer your question, let's step back a little to review the following concepts. Source: ROS2 Documentation

Executors:

An Executor uses one or more threads of the underlying operating system to invoke the callbacks of subscriptions, timers, service servers, action servers, etc. on incoming messages and events. The explicit Executor class (in executor.hpp in rclcpp, in executors.py in rclpy, or in executor.h in rclc) provides more control over execution management than the spin mechanism in ROS 1, although the basic API is very similar.

Scheduling

If the processing time of the callbacks is shorter than the period with which messages and events occur, the Executor basically processes them in FIFO order. However, if the processing time of some callbacks is longer, messages and events will be queued on the lower layers of the stack. The wait set mechanism reports only very little information about these queues to the Executor. In detail, it only reports whether there are any messages for a certain topic or not. The Executor uses this information to process the messages (including services and actions) in a round-robin fashion - but not in FIFO order. In addition, it prioritizes all timer events over the messages. The following flow diagram visualizes this scheduling semantics.

image description

There is a great presentation by Ralph Lange that explains Executor Design as well. And one key principle, Timers overrule everything as there may be conflicting priorities in real time with the other executors.

Given these 2 concepts of Executors and Scheduling, we also need to appreciate that

"ROS multiplexes independent message handlers onto shared threads using custom scheduling policies. Consequently, applications running on top of ROS are subject to the scheduling decisions of the underlying operating system and the middleware layer, with complex and interdependent effects on timing."

Source: Response-Time Analysis of ROS 2 Processing Chains Under Reservation-Based Schedulingby D. Casini, T. Blaß, I. Lütkebohle, and B. B. Brandenburg

Timers can be set in nano-seconds due to the important role they play in Scheduling, thus the need to multiply by a million. Even though your application may not need but the capability is there.

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Asked: 2021-12-16 16:20:07 -0500

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Last updated: Dec 18 '21