International Journal of Computer Applications (0975 8887) Volume 81 - No. 12, November 2013 Real-time Operating System for Wireless Sensors Powered by Renewable Energy Source Hussein El Ghor El-Hadi M Aggoune Lebanese University - IUT Saida Electrical Engineering Department SNCS Research Center, UT, Saudi Arabia Sensor Networks and Cellular Systems (SNCS) Research Center B.P. 813 Saida, Lebanon University of Tabuk, 71491 Tabuk, Saudi Arabia ABSTRACT cating devices (sensor networks). Such devices, mainly character- ized by restrictions in terms of energy capacity, memory size and Energy management is a central problem in real-time systems de- computing power, impose new constraints to real-time operating sign, in particular for embedded wireless devices such as sensor systems (RTOS). Today, the challenge, raised by the emergence of devices. In our work, we aim at the improvement of real-time op- embedded systems powered by renewable energy, is to ensure sus- erating systems that are powered by renewable energy source (so- tainable autonomy if not perpetual that involves new guidelines for lar energy, for example). The objective of this work is to develop RTOS in terms of scheduling and power management. software components for the design of real-time operating systems. In this paper, we address the problem of scheduling hard real-time We provide an on-line scheduling scheme, named Earliest Deadline tasks under energy constraints. The scope of the paper is to develop with energy guarantee (EDeg), in order to address the limitations in software components for the design of real-time operating systems energy harvesting systems. We also integrate EDeg scheduling al- for wireless sensors that are powered by renewable energy sources gorithm into CLEOPATRE open-source component library, a patch such as solar panel and relies on a battery with limited capacity. to Linux/RTAI and evaluate the scheduling overheads of EDeg ob- The remainder of the paper is organized in the following manner. served under Linux/RTAI. In the next section, we summarize the state of art relative to en- ergy management in both real-time systems and real-time operat- Keywords: ing systems. Section 3 describes the CLEOPATRE project includ- ing scheduling and fault tolerance mechanisms. In section 4, we Energy Management, Real-time, Operating Systems, Energy Har- present the energy guarantee scheduler. Section 5 introduces the vesting, RTAI. integration of EDeg into Linux based systems. Performance eval- uation is studied in section 6. Section 7 concludes the paper and 1. INTRODUCTION gives some new directions of work. Real-time computing systems play a critical role in our society. This is due to the fact that many complex systems rely, in part or 2. STATE OF ART completely, on computer control. Examples of applications that re- 2.1 Energy Management in Real-Time Systems quire real-time computing include: sensor and cellular networks, plant and process control, detection and tracking, etc. Energy-aware real-time scheduling has been the subject of inten- A robust guarantee of the performance of a real-time system un- sive research. Most of the works focus on either minimizing the der all possible operating conditions can be achieved only by using energy consumption or maximizing the system performance such more sophisticated design methodologies. Such methodologies are as the lifetime achieved under energy constraints [1]. However, combined with a static analysis of the source code and specific op- the rechargeability of the energy storage unit is disregarded. Other erating systems mechanisms, purposely designed to support com- works use the techniques of Dynamic Voltage and Frequency Scal- putation under time constraints. Moreover, in critical applications, ing (DVFS) and Dynamic Power Management (DPM) [2]. But the control system must be capable of handling all anticipated sce- solely applying these techniques has limitations in energy harvest- narios, including peak load situations, and its design must be driven ing systems because they minimize CPU power, rather than dy- by pessimistic assumptions on the events generated by the environ- namically manage power according to the profiles of both available ment. energy and processor workload. Typically, a real-time system is implemented as a set of concurrent In the last decade, researchers started to address power and schedul- tasks that are managed by a software called Real-Time Operating ing issues but most of them do not consider both rechargeability of System (RTOS). Each task performs a computational activity ac- the batteries and real-time constraints. In the work by Allavena et cording to a set of constraints. Thus, the objective of the RTOS is to al. in [3], power scavenged by the energy source is constant and manage and control the assignment of system resources (including all tasks consume energy at a constant rate. Later in [4], Moser et the microprocessor) to the tasks in order to meet such constraints. al. propose LSA (Lazy scheduling Scheduling Algorithm) to opti- Nowadays, energy management becomes the central problem in mally schedule tasks with deadlines, periodic or not. In that work, real-time systems design, in particular for autonomous communi- the total energy consumption of every task is directly connected 1 International Journal of Computer Applications (0975 8887) Volume 81 - No. 12, November 2013 to its execution time through the constant power of the processing guidelines of RTOS for autonomous communicating devices (sen- device. But in a real application, instantaneous power consumed sor networks). by tasks may vary along time depending on circuitry and devices required by the tasks. 3. CLEOPATRE PROJECT 3.1 Generalities 2.2 Real-time Operating Systems A project named CLEOPATRE (Composants Logiciels sur In recent years, wireless sensor networks (WSN) have received Etageres` Ouverts Pour les Applications Temps Reel´ Embarquee)´ tremendous attention in the research community, with applications was labeled in April 2001 and notified in January 2002 by the ranging from home automation to industrial and environmental French Ministry of Education and Research. The objective of this monitoring. This attention imposes additional challenges on the de- project was to provide solutions to the development of embedded sign of special purpose operating systems deviated from the tradi- real-time applications by providing free and open source software tional ones. However, most of these OSs are non-real-time and can- components based on Linux [7]. CLEOPATRE aims to enhance the not support real-time applications. Moreover, various problems will real-time services of existing versions of Linux, such as RTLinux arise when attempting to convert a non-real-time OS to a real-time [6] or Linux/RTAI [5]. version. Some RTOS are available, which include ThreadX from In addition, many components, dedicated to real-time systems, Xpresslogic, VxWorks by Wind River, QNX Neutrino by QNX like dynamic scheduling, aperiodic task service, resource con- Software Systems, and Real-time Linux (by several vendors). trol access, fault-tolerance and QoS scheduling are offered by the Real-time operating systems are classified into two categories: sys- CLEOPATRE library (figure 1). tems that use light and standard versions of optimized operating RTAI permits Linux to fulfill some real-time constraints in a few system and those who seek the best determinism by liberating milliseconds deadline and with no event loss. It is based on a Hard- any operating system. The main difficulty with standard systems ware Abstraction Layer (HAL) [7]. The HAL defines a clear inter- is to find a balance between providing services of classical sys- face which exports some Linux data and functions related to the tems (such as dynamic allocation of memory pages that is non- hardware. deterministic) and the predictability of these services. This pre- RTAI has a microkernel, which is very close to the proposed archi- dictability is a key factor for application development with strict tecture for CLEOPATRE. In fact, CLEOPATRE components can be real-time constraints. A lot of commercial operating systems based directly treated as microkernel system servers since they have the on UNIX are derived from this approach such as QNX53, OS-954 same goal. We therefore choose RTAI to base our development and and LynxOS55. Moreover, research projects have been emerged to contribute its evolution by providing significant improvements. such as Mach Real-Time system that is an extension of the Mach Moreover, the choice of RTAI enables us to develop a complete project [11]. operating system where we can benefit from the rich Linux envi- Systems from the second category are derived from research ronment (drivers, compilers, interfaces, etc.). projects such as Spring [12], Maruti [14] and YARTOS [15]. Some- times real-time operating systems of this category are based on standard systems such as Linux. But, unlike previous systems, they are modified to give priority to real-time applications that use only CL1 CL2 CLn deterministic services. The standard operating system is relegated to the status of the task background of the real-time system that con- trols scheduling. These systems can be found as patches to Linux Fault-tolerant Aperiodic Synchronization QoS Schedulers which render real-time: RTLinux [6] and RTAI [5]. Schedulers Servers Regrettably, all these systems consider time as the only limiting CLEOPATRE factor, leaving energy efficiency as a hopeful consequence of em- Linux Processes piric decisions. Linux is now the host of most RTOS, especially for wireless sensor TCL T T T networks that operate with an ambient energy source and cannot 1 2 n Task rely on a power outlet on the wall. Thus, a power management de- sign is needed to make the best use of the available power and to adapt the performance of the environmental energy harvesting to RTAI Scheduler LINUX the available energy profile. Scheduler The most well-known versions of real-time Linux are: Real Time RTAI Application Interface (RTAI) [5] and RTLinux [6].