IN4343 Real-Time Systems

Transcription

1 IN4343 Real-Time Systems Koen Langendoen, TA (TBD) Delft University of Technology Challenge the future

2 Course outline Real-time systems Lectures theory instruction Exam Reinder Bril TU/e Practicum hands-on MSP430 lab Erik Luit TU/e 2 37

3 Course organization Real-time systems Lectures / instruction Tuesday 10:45 12:30 Thursday 15:45 17:30 register at brightspace Exception: Feb 15 Feb 23, 10:45 12:30 Practicum Friday 08:45 12:30 (start week 3.3) Teams of 2 register with this form 3 37

4 Grading Real-time systems Practicum = pass (1) fail (0) Written exam = mark [1-10] book reading list Grade = prac * exam 4 37

5 Other relevant courses Taught by ES faculty TI2726-B Embedded Software (Q2) ET4394 CS4140 Wireless Networking (Q3) Embedded Systems Laboratory (Q4) 5 37

6 Disclaimer The course slides are a compilation of material found on the web and obtained from friendly colleagues... it s amazing what is out there! Thanks to Akos Ledeczi Gerd Gross Giorgio Buttazzo Reinder Bril Zonghua Gu and many others 6 37

7 Real-Time Systems INTRODUCTION 7 37

8 Embedded vs. Real-Time Systems Embedded system A computer system that performs a limited set of specific functions. It often interacts with its environment. Real-Time Systems Embedded Systems Real-Time System Examples? Correctness of the system depends not only on the logical results, but also on the time in which the results are produced. 8 37

9 Examples Real-Time Embedded: Nuclear reactor control Flight control GPS, TomTom Mobile phone Real Time, but not Embedded: Stock trading system Skype Embedded, but not Real Time: Home temperature control Washing machine, refrigerator, etc. Blood pressure meter 9 37

10 Characteristics of RTS Event-driven, reactive High cost of failure timeliness Collision Too early Too late time 10 37

11 Classes of RTS Hard real-time systems where it is absolutely imperative that responses occur within the required deadline. Soft real-time systems where deadlines are important but which will still function correctly if deadlines are occasionally missed. Firm real-time systems which are soft real-time but in which there is no benefit from late delivery of service

12 Criticality / timeliness Utility functions soft firm hard criticality 12 37

13 Reliability and Safety Reliability probability that the system will provide the specified service for a given time period (cf. MTTF) Safety reliability regarding critical failure modes watchdog Fail-safe system has a guaranteed safe state that can be reached in case of a critical failure is a property of the controlled object and not the computer system 13 37

14 Control systems Man-Machine Interface Instrumentation Interface Operator Real-Time Computer System Controlled Object Man-machine interface: I/O devices, e.g. keyboard and display Instrumentation interface: sensors and actuators that transform between physical signals and digital data Most control systems are hard real-time Deadlines are determined by the controlled object, i.e. the temporal behavior of the physical phenomenon 14 37

15 Control systems Example A simple one-sensor, one-actuator control system reference input r(t) A/D A/D r k y k control-law computation u k D/A y(t) u(t) sensor plant actuator Outside effects The system being controlled 15 37

16 Control systems Example r(t) A/D A/D r k y k control-law computation u k D/A Pseudo-code for this system: set timer to interrupt periodically with period T; at each timer interrupt do do analog-to-digital conversion to get y; compute control output u; output u and do digital-to-analog conversion; end do y(t) u(t) sensor plant actuator T is called the sampling period. T is a key design choice. Typical range for T: seconds to milliseconds

17 Control systems Classes Open-loop control simple, convert to syst. setting Closed-loop control feedback to account for env. Multi-level feedback control at different time scales controller controller system system planning controller system 17 37

18 RTS for control Different tasks mapped to microcontroller sensing actuation control planning communication user interface RTOS must schedule tasks such that all meet their deadlines while sharing common resources 18 37

19 Taxonomy of Real-Time Systems 19 37

20 Taxonomy of Real-Time Systems 20 37

21 Taxonomy of Real-Time Systems 21 37

22 Taxonomy Static Task arrival times can be predicted Static (compile-time) analysis possible task length is known WCET Allows good resource usage (high CPU utilization) 22 37

23 Taxonomy Dynamic Arrival times unpredictable Static (compile-time) analysis possible only for simple cases Processor utilization decreases dramatically In many real systems, this is very difficult to handle Must avoid over-simplifying assumptions e.g., assuming that all tasks are independent, when this is unlikely 23 37

24 Taxonomy Soft Real-Time Allows more slack in the implementation Timings may be suboptimal without being incorrect Problem formulation can be much more complicated than hard real-time Common ways of handling non-trivial soft realtime system requirements Set somewhat loose hard timing constraints Informal design and extensive testing 24 37

25 Taxonomy Hard Real-Time Creates difficult problems Some timing constraints are inflexible Simplifies problem formulation 25 37

26 Taxonomy Periodic Each task (or group of tasks) executes repeatedly with a particular period Allows some static analysis techniques to be used Matches characteristics of many real problems It is possible to have tasks with deadlines smaller, equal to, or greater than their period The later are difficult to handle (i.e., multiple concurrent task instances occur) 26 37

27 Taxonomy Types of periodic systems Single rate: One period in the system Simple but inflexible Used in implementing a lot of sensor networks Multi rate: Multiple periods Should be harmonics to simplify system design 27 37

28 Taxonomy Aperiodic Also known as sporadic, asynchronous, or reactive Creates a dynamic situation Bounded arrival time intervals are easier to handle Unbounded arrival time intervals are impossible to handle with resource-constrained systems 28 37

29 Example: Adaptive Cruise Control Demo video United States Patent Control system Hard Real Time Multi-rate periodic Camera GPS Low-speed mode for rush-hour traffic 29 37

30 Example: Autonomous Robot Multi-level system motor control obstacle avoidance path planning Mixture of hard and soft static and dynamic (a)periodic multi rate 30 37

31 Are all systems Real-Time Systems? Question: Is a payroll processing system a RT system? It has a time constraint: Print the pay checks every two weeks yes: in a definitional sense no: it s too easy We are interested in systems for which it is not a priori obvious how to meet timing constraints 31 37

32 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 Sufficient, but scarce It is possible to realize the timing requirements of the application, but careful resource allocation is required 32 37

33 The Window of Scarcity Example: Interactive/Multimedia Applications Requirements (performance, scale) Interactive Video High-quality Audio Network File Access Remote Login insufficient resources sufficient but scarce resources abundant resources The interesting real-time applications are here Hardware resources in year X 33 37

34 OS or not? User Programs Operating Hardware System User Program Including Operating Hardware System Components Typical OS Configuration Typical Embedded Configuration 34 37

35 RTOS Programming by hand is difficult strict timing requirements different stimuli / time scales / priorities shared resources a simple sequential loop is not adequate Real-Time Operating Systems provide light-weight concurrency schedule task set within given requirements 35 37

36 Murphy s law Anything that can go wrong, will go wrong Accidents due to SW Task overrun during LEM lunar landing First flight of the Space Shuttle (synch) Ariane 5 (overflow) Airbus 320 (cart task) Airbus 320 (holding task) Pathfinder (reset for timeout) 36 37

37 Scheduling Course focus Design predictable RTSs fast!= real-time Proofing vs. testing Hard vs. soft real-time Worst case vs. best effort Study various scheduling policies theory lectures practice MSP430 lab 37 37