Sloth on Time: Efficient Hardware-Based Scheduling for Time-Triggered RTOS

Abstract

Abstract-Traditional time-triggered operating systems are implemented by multiplexing a single hardware timer-the system timer-in software, having the kernel maintain dispatcher tables at run time. Our SLOTH ON TIME approach proposes to make use of multiple timer cells as available on modern microcontroller platforms to encapsulate dispatcher tables in the timer configuration, yielding low scheduling and dispatching latencies at run time. SLOTH ON TIME instruments available timer cells in different roles to implement time-triggered task activation, deadline monitoring, and time synchronization, amongst others.By comparing the SLOTH ON TIME kernel implementation to two commercial kernels, we show that our concept significantly reduces the overhead of time-triggered operating systems. The speed-ups in task dispatching that it achieves range up to a factor of 171 x, and its dispatch latencies go as low as 14 clock cycles. Additionally, we demonstrate that SLOTH ON TIME minimizes jitter and increases schedulability for its real-time applications, and that it avoids situations of priority inversion where traditional kernels fail by design. I. INTRODUCTION AND MOTIVATIONIn operating system engineering, the overhead induced by the kernel is a crucial property since operating system kernels do not provide a business value of their own. This is especially true in embedded real-time systems, where superfluous bytes in RAM and ROM as well as unnecessary event latencies can decide whether a kernel is used for the implementation of an embedded device or not. In previous work on the SLOTH approach, we have shown that by using commodity microcontroller hardware in a more sophisticated manner in the kernel, we can achieve lower footprints in RAM and ROM as well as very low system call overheads [7]. To achieve this, the SLOTH kernel maps run-to-completion tasks to interrupt handlers and lets the interrupt hardware schedule them, eliminating the need for a software task scheduler completely. Additionally, we have been able to show that implementing a full thread abstraction with blocking functionality in the SLEEPY SLOTH kernel still yields a significant performance boost over traditional, software-based embedded kernels [8].However, both the SLOTH and the SLEEPY SLOTH kernels target event-triggered real-time systems with event-driven task dispatching. In this paper, we discuss how the SLOTH principle of making better use of hardware facilities in the implementation of embedded kernels can be applied to timetriggered operating systems. The resulting SLOTH ON TIME kernel uses the fundamental task dispatching mechanisms as