PiCAS: New Design of Priority-Driven Chain-Aware Scheduling for ROS2

In ROS (Robot Operating System), most applications in time- and safety-critical domain are constructed in the form of callback chains with data dependencies. Due to the shortcomings in its real-time support, ROS does not provide a strong timing guarantee and may lead to disastrous results. Although ROS2 claims to enhance the real-time capability, ensuring predictable end-to-end chain latency still remains a challenging problem. In this paper, we propose a new priority-driven chain-aware scheduler for the ROS2 framework and present end-to-end latency analysis for the proposed scheduler. With our scheduler, callbacks are prioritized based on the given timing requirements of the corresponding chains so that the end-to-end latency of critical chains can be improved with a predictable bound. The proposed scheduling design includes priority assignment and resource allocation considering all ROS2 scheduling-related abstractions, e.g., callbacks, nodes, and executors. To the best of our knowledge, this is the first work to address the inherent limitations of ROS2 in end-to-end latency by proposing a new scheduler design. We have implemented our scheduler in ROS2 running on NVIDIA Xavier NX. We have conducted case studies and schedulability experiments. The results show that the proposed scheduler yields a substantial improvement in end-to-end latency over the default ROS2 scheduler and the latest work in real-world scenarios.