Work in Progress: Middleware-Transparent Callback Enforcement in Commoditized Component-Oriented Real-time Systems

TL;DR

This paper introduces a middleware-transparent callback enforcement model for real-time systems, enabling direct application of OS scheduling parameters to callbacks, reducing overhead and simplifying scheduling in ROS 2 systems.

Contribution

It proposes a novel ROS 2 Executor, CallbackIsolatedExecutor, that bypasses nested middleware scheduling, improving efficiency and predictability in real-time ROS 2 applications.

Findings

CallbackIsolatedExecutor has lower costs than MultiThreadedExecutor.

Cost ratio of CallbackIsolatedExecutor to SingleThreadedExecutor remains fixed.

Enables future real-time scheduling research to ignore middleware layer complexity.

Abstract

Real-time scheduling in commoditized component-oriented real-time systems, such as ROS 2 systems on Linux, has been studied under nested scheduling: OS thread scheduling and middleware layer scheduling (e.g., ROS 2 Executor). However, by establishing a persistent one-to-one correspondence between callbacks and OS threads, we can ignore the middleware layer and directly apply OS scheduling parameters (e.g., scheduling policy, priority, and affinity) to individual callbacks. We propose a middleware model that enables this idea and implements CallbackIsolatedExecutor as a novel ROS 2 Executor. We demonstrate that the costs (user-kernel switches, context switches, and memory usage) of CallbackIsolatedExecutor remain lower than those of the MultiThreadedExecutor, regardless of the number of callbacks. Additionally, the cost of CallbackIsolatedExecutor relative to SingleThreadedExecutor stays within a fixed ratio (1.4x for inter-process and 5x for intra-process communication). Future ROS 2 real-time scheduling research can avoid nested scheduling, ignoring the existence of the middleware layer.