Introductionto

Real Time Systems

Akos LedecziEECE 354, Fall 2012

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.

EmbeddedSystems

Real TimeSystems

Examples?

Examples• Real Time 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

digital clock

tick granule

now

instantinstantpast future

durationevent event

time line

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

Control systems

• 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

Operator Controlled Object

Real-Time Computer

System

Man-Machine Interface

Instrumentation Interface

Control system example

Example: A simple one-sensor, one-actuator control system.

control-lawcomputation

A/D

A/DD/A

sensor plant actuator

rk

yk

y(t) u(t)

uk

referenceinput r(t)

The systembeing controlled

Outside effects

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

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. Typicalrange 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

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Taxonomy of Real-Time Systems

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Taxonomy of Real-Time Systems

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Taxonomy of Real-Time Systems

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Taxonomy: Static

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

processors).

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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.

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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

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Taxonomy: Hard Real-Time

• Creates difficult problems.– Some timing constraints are inflexible

• Simplifies problem formulation.

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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).

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Periodic

• Single 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

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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

sufficientbut scarceresources

abundantresources

insufficientresources

Requirements(performance, scale)

1980 1990 2000Hardware resources in year X

RemoteLogin

NetworkFile Access

High-qualityAudio

InteractiveVideo

The interestingreal-timeapplicationsare here

The interestingreal-timeapplicationsare here

OS or not?

Hardware

Operating

System

User Programs

Typical OS Configuration

Hardware

Including Operating

System Components

User Program

Typical Embedded 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• 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-priorityscheduling

dynamic-priorityscheduling

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.

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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.

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

What’s Important in Real-Time

Time-Sharing Systems

Real-Time Systems

Capacity High throughput Schedulability

Responsiveness Fast average response Ensured worst-case response

Overload Fairness Stability