Introduction to Real Time Systems Akos Ledeczi EECE 354, Fall 2015 Vanderbilt University

Disclaimer Some of the material/slides are adapted from various presentations found on the internet: Johnnie W. Baker Ian Sommerville Alan Burns and Andy Wellings Others And Prof. Kopetz’s Real Time Systems book

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

Examples Real Time Embedded: Real Time, but not Embedded: Nuclear reactor control Flight control Basically any safety critical system GPS MP3 player Mobile phone Real Time, but not Embedded: Stock trading system Skype Pandora Embedded, but not Real Time: Home temperature control Sprinkler system Washing machine, refrigerator, etc. Blood pressure meter

Characteristics of RTS Event-driven, reactive. High cost of failure. Concurrency/multiprogramming. Stand-alone/continuous operation. Reliability/fault-tolerance requirements. Predictable behavior.

Time now instant past future duration event time line digital clock tick granule

Definitions Hard real-time — systems where it is absolutely imperative that responses occur within the required deadline. E.g. Flight control systems. Soft real-time — systems where deadlines are important but which will still function correctly if deadlines are occasionally missed. E.g. Data acquisition system. Real real-time — systems which are hard real-time and which the response times are very short. E.g. Missile guidance system. Firm real-time — systems which are soft real-time but in which there is no benefit from late delivery of service. A single system may have all hard, soft and real real-time subsystems. In reality many systems will have a cost function associated with missing each deadline

Real-Time Computer System Control systems Operator Controlled Object Real-Time Computer System Man-Machine Interface Instrumentation Man-machine interface: input devices, e.g. keyboard and output devices, e.g. 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

Control system example Example: A simple one-sensor, one-actuator control system. rk reference input r(t) A/D control-law computation uk D/A yk A/D y(t) u(t) sensor plant actuator Outside effects The system being controlled

Control systems cont’d. 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 T is called the sampling period. T is a key design choice. Typical range for T: seconds to milliseconds.

Reliability and Safety Reliability: probability that the system will provide the specified service for a given time period. (Also see Failure Rate or Mean Time To Failure: MTTF) Safety: reliability regarding critical failure modes Fail-safe system: if the system has a guaranteed safe state that can be reached in case of a critical failure. It is a property of the controlled object and not the computer system. Watchdog: external device that gets periodic life sign from the computer system. If it does not get it, it forces the controlled object into a safe state. Fail-operational system: no such safe state exists, so the computer system must provided (limited) functionality in case of failures to avoid a catastrophic failure. Alarm monitoring. Primary event Secondary alarms. Temporal order is very important. Alarm shower Rare events

Taxonomy of Real-Time Systems

Taxonomy of Real-Time Systems

Taxonomy of Real-Time Systems

Taxonomy: Static Task arrival times can be predicted Static (compile-time) analysis possible Allows good resource usage (low idle time for processors).

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.

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 Two common and an uncommon way of handling non-trivial soft real-time system requirements Set somewhat loose hard timing constraints Informal design and testing Formulate as an optimization problem

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

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).

Periodic Single rate: Multi rate: One period in the system Simple but inflexible Used in implementing a lot of wireless sensor networks. Multi rate: Multiple periods Should be harmonics to simplify system design

Taxonomy: Aperiodic Are also called sporadic, asynchronous, or reactive. Creates a dynamic situation Bounded arrival time interval are easier to handle Unbounded arrival time intervals are impossible to handle with resource-constrained systems.

Example: Adaptive Cruise Control Demo video Control system Hard Real Time Multi-rate periodic Camera GPS Low-speed mode for rush hour traffic United States Patent 7096109

Data Acquisition and Signal-Processing Systems Examples: Video capture. Digital filtering. Video and voice compression/decompression. Radar signal processing. Response times range from a few milliseconds to a few seconds. Typically simpler than control systems

Other Real-Time Applications Real-time databases. Examples: stock market, airline reservations, etc. Transactions must complete by deadlines. Main dilemma: Transaction scheduling algorithms and real-time scheduling algorithms often have conflicting goals. Data is subject temporal consistency requirements. 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).

Are All Systems Real-Time Systems? Question: Is a payroll processing system a real-time 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.

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.

Example: Interactive/Multimedia Applications Requirements (performance, scale) The interesting real-time applications are here sufficient but scarce resources Interactive Video insufficient resources High-quality Audio Network File Access abundant resources Remote Login 1980 1990 2000 Hardware resources in year X

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

Foreground/Background Systems Task-level, interrupt level Critical operations must be performed at the interrupt level (not good) Response time/timing depends on the entire loop Code change affects timing Simple, low-cost systems

RTS Programming Concurrent programming Because of the need to respond to timing demands made by different stimuli/responses, the system architecture must allow for fast switching between stimulus handlers. Because of different priorities, unknown ordering and different timing requirements of different stimuli, a simple sequential loop is not usually adequate. Real-time systems are therefore usually designed as cooperating processes with a real-time kernel controlling these processes. Concurrent programming

Real Time Java? Java supports lightweight concurrency (threads and synchronized methods) and can be used for some soft real-time systems. Java is not suitable for hard RT programming but real-time versions of Java are now available that address problems such as Not possible to specify thread execution time; Uncontrollable garbage collection; Not possible to access system hardware; Etc. Real-Time Specification for Java Sun Java Real-Time System Requires a Real Time OS underneath (e.g., no Windows support)

Classification of Scheduling Algorithms All scheduling algorithms static scheduling (or offline, or clock driven) dynamic scheduling (or online, or priority driven) static-priority scheduling dynamic-priority scheduling

Scheduling strategies Non pre-emptive scheduling Once a process has been scheduled for execution, it runs to completion or until it is blocked for some reason (e.g. waiting for I/O). Pre-emptive scheduling The execution of an executing processes may be stopped if a higher priority process requires service. Scheduling algorithms Round-robin; Rate monotonic; Shortest deadline first; Etc.

Real-time operating systems Real-time operating systems are specialised operating systems which manage the processes in the RTS. Responsible for process management and resource (processor and memory) allocation. Do not normally include facilities such as file management. 14

Operating system components Real-time clock Provides information for process scheduling. Interrupt handler Manages aperiodic requests for service. Scheduler Chooses the next process to be run. Resource manager Allocates memory and processor resources. Dispatcher Starts process execution.

Interrupt servicing Control is transferred automatically to a pre-determined memory location. This location contains an instruction to jump to an interrupt service routine. Further interrupts are disabled, the interrupt serviced and control returned to the interrupted process. Interrupt service routines MUST be short, simple and fast.

What’s Important in Real-Time Metrics for real-time systems differ from that for time-sharing systems. schedulability is the ability of tasks to meet all hard deadlines latency is the worst-case system response time to events stability in overload means the system meets critical deadlines even if all deadlines cannot be met Time-Sharing Systems Real-Time Systems Capacity High throughput Schedulability Responsiveness Fast average response Ensured worst-case response Overload Fairness Stability