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

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 Frank Drews Real-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. Frank Drews Real-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. Frank Drews Real-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. Frank Drews Real-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 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. Frank Drews Real-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 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 Frank Drews Real-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. Frank Drews Real-Time Systems

Example: Interactive/Multimedia Applications Frank Drews Real-Time Systems

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

Time Frank Drews Real-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 Frank Drews Real-Time Systems

Hierarchical Control Systems Frank Drews Real-Time Systems

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

Example: Radar System Frank Drews Real-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 static content dynamic content streaming video/audio Server Domain 1 Domain 3 Domain 2 Frank Drews Real-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). Frank Drews Real-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”. Frank Drews Real-Time Systems

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. Frank Drews Real-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. Frank Drews Real-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. Frank Drews Real-Time Systems

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

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

Periodic vs. 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 Execute event task Execute event task Execute event task Idle time time e0 e1 e2 Asynchronous events Frank Drews Real-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 Frank Drews Real-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 Frank Drews Real-Time Systems