Evaluating Robustness of Resource Allocation in Uniprocessor Real Time System

Abstract

ABSTRACT Real time systems (RTS) operate in an unpredictable changing environment that causing certain system performance features to degrade. Such systems need robustness to guarantee limited degradation despite variations in environmental parameters. EDF (Earliest Deadline First) scheduling has been used to evaluate the robustness of uniprocessor system with makespan as performance parameter. Robustness of real time system is directly proportional to the makespan of the resource allocation. EDF produces the optimal schedule that maximizes the robustness in the system. Keywords Robustness, QoS, Resource Allocation, Real Time System. 1. INTRODUCTION Real Time systems are widely used in our daily life and production industry such as the robotic control, telecommunications, chemical plant control, satellite control, flight control systems, military systems, multimedia systems, and so on. A real-time system is the one whose logical correctness is based on correctness of outputs as well as timeliness. It consists of a controlling system (computer) and a controlled system (environment). Real time systems guarantee that all the timing requirements can be met by the real time scheduling and schedulability analysis. Real time systems may operate in an environment where certain system performance features degrades due to unpredictable circumstances, such as sudden machine failures, higher than expected system load, or inaccuracies in the estimation of system performance parameters like makespan, slack etc [15, 16]. Resource allocation is very important to achieve a given level of QoS (Quality of Service). The resource allocation is defined as the act of assigning (matching) each task to a machine and ordering (scheduling) the execution of the tasks on each machine. Resource allocation is generally performed based on estimated computation time of each task on each class of machines. A resource allocation is said to be robust with respect to specified system performance features against perturbations in given system parameters if degradation in these features is within acceptable limits when certain perturbations occur [2]. For example, if a resource allocation has been declared to be robust with respect to satisfying a makespan requirement against perturbations in the estimated execution time, then the system configured under that allocation should continue to operate without a makespan violation when the actual execution time is greater than estimated execution time. Makespan (the completion time for an entire set of tasks) is mostly used as the performance feature that requires to be optimized in such systems. Robustness has been defined in different ways by different researchers. According to [5] robustness is the degree to which a system can function correctly in the presence of inputs different from those assumed. In a more general sense a robust system continues to operate correctly across a wide range of operating conditions. Robustness guarantees the maintenance of certain desired system characteristics despite variations in the behavior of its component parts or its environment [4]. A robust system is one that continues to perform at desired level of service in spite of perturbations in some components that constitute the system. In this paper resource allocation and scheduling are synonymous for uniprocessor system. The rest of the paper is organized as follows. Section 2 describes system model and defines the resource allocation problem. Section 3 provides the work dealing with robustness and some robustness metrics. Section 4 presents some experiments and their results that highlight the usefulness of the robustness metric. Related work is given in section 5. Section 6 concludes the paper.