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

2. 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

3. 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?

4. 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

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

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

7. 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

8. 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

9. 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

10. 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.

11. 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

12. Taxonomy of Real-Time Systems

13. Taxonomy of Real-Time Systems

14. Taxonomy of Real-Time Systems

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

16. 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.

17. 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

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

19. 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).

20. 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

21. 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.

22. 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

23. 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

24. 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).

25. 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.

26. 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.

27. 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

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

29. 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

30. 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

31. 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)

32. 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

33. 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.

34. 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

35. 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.

36. 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.

37. 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