Execution Time Analysis for Embedded Real-Time Systems

1 Introduction This chapter deals with the problem of how to estimate and analyze the execution time of embedded real-time software, in particular the worst-case execution time. A real-time system must react within precise time constraints, related to events in its environment and the system it controls. This means that the correct behavior of a real-time system depends not only on the result of the computation but also on the time at which the result is produced. Thus, knowing the execution-time characteristics of a program is fundamental to the successful design and execution of a real-time system [22, 57]. The size and the complexity of the software in real-time system are constantly increasing. This makes it hard, or even impossible, to perform exhaustive testing of the execution time. Furthermore, the hardware used in real-time systems is steadily becoming more complex, including advanced computer architecture features such as caches, pipelines, branch prediction, and out-of-order execution. These features increase the speed of execution on average, but also makes the timing behavior much harder to predict, since the variation in execution time between fortuitious and worst cases increase. Execution time analysis is any structured method or tool applied to the problem of obtaining information about the execution time of a program or parts of a program. The fundamental problem that a timing analysis has to deal with is the following: the execution time of a typical program (or other relevant piece of code) is not a fixed constant, but rather varies with different probability of occurrence across a range of times. 1 INTRODUCTION 2 No real-time Soft real-time Hard real-time System simulation User interface Internet video Vehicle control Tele communication Aircraft control Figure 1.1: Continuum of timing criticality Variations in the execution time occur due to variations in input data, as well as the characteristics of the software, the processor and the computer system in which the program is executed. The worst-case execution time (WCET) of a program is defined as the longest execution time that will ever be observed when the program is run on its target hardware. It is the most critical measure for most real-time work. The WCET is needed for many different types of system analysis for real-time systems. For example, it is a critical component of schedulability analysis techniques, it is used to ensure that interrupts are handled with sufficiently short reaction times, that periodic processing is performed …