Real-Time Systems

1. Real-Time Systems Introduction Frank Drews drews@ohio.edu Real-Time Systems

2. Real-time Systems • A real-time systemis a system whose specification includes both logical and temporal correctness requirements. • Logical Correctness: Produces correct outputs. • Can by checked, for example, by Hoare logic. • Temporal Correctness: Produces outputs at the right time. • In this course, we spend much time on techniques and technologies for achieving and checking temporal correctness. Real-Time Systems

3. Embedded Systems • [www.webopedia.com]: An embedded system is “a specialized computer system that is part of a larger system or machine. Typically, an embedded system is housed on a single microprocessor board with the programs stored in ROM. Virtually all appliances that have digital interfaces (e.g., watches, microwaves, VCRs, cars) utilize embedded systems […]” • Many embedded systems are real-time systems Real-Time Systems

4. Typical Characteristics of Real-Time Systems • Event-driven, reactive. • High cost of failure. • Concurrency/multiprogramming. • Stand-alone/continuous operation. • Reliability/fault-tolerance requirements. • Predictable behavior. Real-Time Systems

5. Misconceptions about Real-Time Systems(Stankovic ‘88) • There is no science in real-time-system design. • We shall see… • Advances in supercomputing hardware will take care of real-time requirements. • The old “buy a faster processor” argument… • Real-time computing is equivalent to fast computing. • Only to ad agencies. To us, it means PREDICTABLE computing. Real-Time Systems

6. Misconceptions (Continued) • Real-time programming is assembly coding, • We would like to automate (as much as possible) real-time system design, instead of relying on clever hand-crafted code. • “Real time” is performance engineering. • In real-time computing, timeliness is almost always more important than raw performance … • “Real-time problems” have all been solved in other areas of CS or operations research. • OR people typically use stochastic queuing models or one-shot scheduling models to reason about systems. • In other CS areas, people are usually interested in optimizing average-case performance. Real-Time Systems

7. Misconceptions (Continued) • It is not meaningful to talk about guaranteeing real-time performance when things can fail. • Though things may fail, we certainly don’t want the operating system to be the weakest link! • Real-time systems function only in a static environment. • Note true. We consider systems in which the environment may change dynamically. Real-Time Systems

8. Are All Systems Real-Time Systems? • Question: Is a payroll processing system a realtime system? • It has a time constraint: Print the pay checks every two weeks. • Perhaps it is a real-time system in a definitional sense, but it doesn’t pay us to view it as such. • We are interested in systems for which it is not a priori obvious how to meet timing constraints Real-Time Systems

9. The “Window of Scarcity” Resources may be categorized as: • Abundant: Virtually any system design methodology can be used to realize the timing requirements of the application. • Insufficient: The application is ahead of the technology curve; no design methodology can be used to realize the timing requirements of the application. • Sufficient but scarce: It is possible to realize the timing requirements of the application, but careful resource allocation is required. Real-Time Systems

10. Example: Interactive/Multimedia Applications Real-Time Systems

11. Example: Real-Time Applications Many real-time systems are control systems Example 1: A simple one-sensor, one-actuator control system Real-Time Systems

12. Simple Control System (Continued) Pseudo-code for this system: T is called sampling period. T is a key design choice. Typical range for T: seconds to milliseconds. Real-Time Systems

13. Time Real-Time Systems

14. Multi-rate Control Systems More complicated control systems have multiple sensors and actuators and must support control loops of different rates. Example 2: Helicopter flight controller. Note: Having only harmonic rates simplifies the system Real-Time Systems

15. Hierarchical Control Systems Real-Time Systems

16. Air Traffic Control [Reddaway et al. WMPP’05] Real-Time Systems

17. Signal-Processing System • Signal-processing systemstransform data from one form to another. • Examples: • Digital filtering. • Video and voice compression/decompression. • Radar signal processing. • Response times range from a few milliseconds to a few seconds. Real-Time Systems

18. Example: Radar System Real-Time Systems

19. Internet/Multimedia Applications • Web farms hosting multiple web domains • Each web domain receives a certain share of the overall resources (CPU, network, file system) • Each web domain consists of an application pool (static content, dynamic content, streaming video/audio, etc.) • Challenges • Sharing the resource among domains (i.e. application pools) may be hard in general purpose Operating Systems • Guarantee of a uniform, steady, jitter-free execution of time critical multimedia applications while not starving other applications • Support of multiprocessor server systems static content dynamic content streaming video/audio Server Domain 1 Domain 3 Domain 2 Real-Time Systems

20. Other Real-Time Applications • Real-time databases. • Transactions must complete by deadlines. • Main dilemma: Transaction scheduling algorithms and real-time scheduling algorithms often have conflicting goals. • Data may be subject to absolute and relative temporal consistency requirements. • Overall goal: reliable responses • Multimedia. • Want to process audio and video frames at steady rates. – TV video rate is 30 frames/sec. HDTV is 60 frames/sec. – Telephone audio is 16 Kbits/sec. CD audio is 128 Kbits/sec. • Other requirements: Lip synchronization, low jitter, low end-to-end response times (if interactive). Real-Time Systems

21. Hard vs. Soft Real Time • Task: A sequential piece of code. • Job: Instance of a task • Jobs require resourcesto execute. – Example resources: CPU, network, disk, critical section. – We will simply call all hardware resources “processors”. • Release time of a job:The time instant the job becomes ready to execute. • Deadline of a job: The time instant by which the job must complete execution. • Relative deadline of a job: “Deadline - Release time”. • Response time of a job: “Completion time - Release time”. Real-Time Systems

22. Example Job is released at time 3. It’s absolute deadline is at time 10. It’s relative deadline is 7. It’s response time is 6. Real-Time Systems

23. Hard Real-Time Systems • A hard deadlinemust be met. • If any hard deadline is ever missed, then the system is incorrect. • Requires a means for validatingthat deadlines are met. • Hard real-time system: A real-time system in • which all deadlines are hard. • We consider hard and soft real-time systems in this course. • Examples: Nuclear power plant control, flight control. Real-Time Systems

24. Soft Real-Time Systems • A soft deadlinemay occasionally be missed. • Question: How to define “occasionally”? • Soft real-time system: A real-time system in which some deadlines are soft. • Examples: multimedia applications. Real-Time Systems

25. Performance • Two particular factors are important • How fast does a system respond • When it fails, what happens? Real-Time Systems

26. The Speed of Response • All required responses are time-critical • The designer should predict the delivered performance of the systems with the required performance • Unfortunately, it may not be possible to give 100% guarantees Real-Time Systems

27. Periodic vs. Aperiodic Tasks • Periodic Tasks • Tasks run at regular, pre-defined intervals • Example: closed loop digital controller having fixed, pre-set sampling rates Executecontrol task Executecontrol task Idle time Idle time time Sampling interval t0 t1 t2 Synchronous real-time clock signals Real-Time Systems

28. Periodic vs. Aperiodic Tasks • Aperiodic Tasks • Occur when the computer must respond to (generally) external events which occur at random (asynchronous or aperiodic); have either soft or no deadlines • Sporadic Tasks • Similar to aperiodic tasks; however, the event must be serviced within a specific maximum time period; hard deadline Idle time Executeevent task Executeevent task Executeevent task Idle time time e0 e1 e2 Asynchronous events Real-Time Systems

29. Mixing Periodic and Aperiodic Tasks • We get into trouble in situations which involve a mix of periodic and aperiodic(sporadic) events, which are usual in real-time designs • Much thought and skill are needed to deal with the response requirements of periodic and aperiodic tasks Real-Time Systems

30. Real-Time Operating Systems (RTOSs) • RTOS:specialized operating system for RTS • Main responsibilities: • Process management • Resource allocation (processor, memory, network) • They may not include regular OS facilities such as file management, virtual memory, user/kernel level separation, etc. • Manage at least two priority levels: • Interrupt level, for processes that need fast response • Clock level, for periodic processes • Typical components: real-time clock, interrupt handler, scheduler, resource manager, dispatcher Real-Time Systems