Fixed-Priority and EDF Schedules for ROS2 Graphs on Uniprocessor

TL;DR

This paper introduces a novel fixed-priority scheduling approach for complex ROS2 graphs on uniprocessor systems, bridging the gap between real-time theory and ROS2 application scheduling.

Contribution

It proposes a new method using the events executor to implement fixed-job-priority schedulers for arbitrary ROS2 DAGs, extending beyond chain-based scheduling.

Findings

Schedules match traditional fixed-priority DAG schedulers

Supports arbitrary DAGs in ROS2 applications

Requires custom event queue and LIFO message middleware

Abstract

This paper addresses limitations of current scheduling methods in the Robot Operating System (ROS)2, focusing on scheduling tasks beyond simple chains and analyzing arbitrary Directed Acyclic Graphs (DAGs). While previous research has focused mostly on chain-based scheduling with ad-hoc response time analyses, we propose a novel approach using the events executor to implement fixed-job-level-priority schedulers for arbitrary ROS2 graphs on uniprocessor systems. We demonstrate that ROS 2 applications can be abstracted as forests of trees, enabling the mapping of ROS 2 applications to traditional real-time DAG task models. Our usage of the events executor requires a special implementation of the events queue and a communication middleware that supports LIFO-ordered message delivery, features not yet standard in ROS2. We show that our implementation generates the same schedules as a conventional fixed-priority DAG task scheduler, in spite of lacking access to the precedence information that usually is required. This further closes the gap between established real-time systems theory and ROS2 scheduling analyses.