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Frank DrewsReal-Time Systems Introduction Frank Drews

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1 Frank DrewsReal-Time Systems Introduction Frank Drews

2 Frank DrewsReal-Time Systems Real-time Systems A real-time system is 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.

3 Frank DrewsReal-Time Systems Embedded Systems []: 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

4 Frank DrewsReal-Time Systems 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.

5 Frank DrewsReal-Time Systems 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.

6 Frank DrewsReal-Time Systems 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.

7 Frank DrewsReal-Time Systems Misconceptions (Continued) It is not meaningful to talk about guaranteeing real-time performance when things can fail. –Though things may fail, we certainly dont 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.

8 Frank DrewsReal-Time Systems 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 doesnt 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

9 Frank DrewsReal-Time Systems 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.

10 Frank DrewsReal-Time Systems Example: Interactive/Multimedia Applications

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

12 Frank DrewsReal-Time Systems 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.

13 Frank DrewsReal-Time Systems Time

14 Frank DrewsReal-Time Systems 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

15 Frank DrewsReal-Time Systems Hierarchical Control Systems

16 Frank DrewsReal-Time Systems Air Traffic Control [Reddaway et al. WMPP05]

17 Frank DrewsReal-Time Systems Signal-Processing System Signal-processing systems transform 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.

18 Frank DrewsReal-Time Systems Example: Radar System

19 Frank DrewsReal-Time Systems 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 Server Domain 1 Domain 2 Domain 3 static content dynamic content streaming video/audio

20 Frank DrewsReal-Time Systems 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).

21 Frank DrewsReal-Time Systems Hard vs. Soft Real Time Task: A sequential piece of code. Job: Instance of a task Jobs require resources to 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.

22 Frank DrewsReal-Time Systems Example Job is released at time 3. Its absolute deadline is at time 10. Its relative deadline is 7. Its response time is 6.

23 Frank DrewsReal-Time Systems Hard Real-Time Systems A hard deadline must be met. –If any hard deadline is ever missed, then the system is incorrect. –Requires a means for validating that 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.

24 Frank DrewsReal-Time Systems Soft Real-Time Systems A soft deadline may 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.

25 Frank DrewsReal-Time Systems Performance Two particular factors are important –How fast does a system respond –When it fails, what happens?

26 Frank DrewsReal-Time Systems 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

27 Frank DrewsReal-Time Systems Periodic vs. Aperiodic Tasks Periodic Tasks –Tasks run at regular, pre-defined intervals –Example: closed loop digital controller having fixed, pre-set sampling rates t0t0 t1t1 t2t2 time Execute control task Idle time Sampling interval Synchronous real-time clock signals

28 Frank DrewsReal-Time Systems 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 e0e0 time Execute event task Idle time Asynchronous events e1e1 e2e2 Execute event task

29 Frank DrewsReal-Time Systems 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

30 Frank DrewsReal-Time Systems 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

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