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Introduction to Real Time Systems - Vanderbilt University


Real Time Systems

       Akos Ledeczi
    EECE 354, Fall 2012
   Vanderbilt University
• 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
• RTS: Correctness of the system depends not only on the
  logical results, but also on the time in which the results are

                       Real Time       Embedded
                        Systems         Systems

• 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.
                                            past     future
  instant              instant

                                                              time line
event                     event

                            digital clock

   tick                      granule
•   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

•   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               Instrumentation
                      Interface                   Interface

        Operator                  Computer

•   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.

reference                          rk
                            A/D                              uk
input r(t)                               control-law
                                    yk   computation                D/A

                         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;
             set timer to interrupt periodically with period T;
            at each timer interrupt do
             at each timer interrupt do
                  do analog-to-digital conversion to get y;
                   do analog-to-digital conversion to get y;
                  compute control output u;
                   compute control output u;
                  output uuand do digital-to-analog conversion;
                   output and do digital-to-analog conversion;
            end do
             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

          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
• 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
• 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
• Matches characteristics of many real
• 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).
• Single rate:
  – One period in the system
  – Simple but inflexible
  – Used in implementing a lot of wireless sensor
• Multi rate:
  – Multiple periods
  – Should be harmonics to simplify system design

         Taxonomy: Aperiodic
• Are also called sporadic, asynchronous, or
• Creates a dynamic situation
• Bounded arrival time interval are easier to
• 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
• Low-speed mode for
  rush hour traffic

                        United States Patent 7096109
Data Acquisition and Signal-Processing
• Examples:
   –   Video capture.
   –   Digital filtering.
   –   Video and voice compression/decompression.
   –   Radar signal processing.

• Response times range from a few milliseconds to a few
• 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
• 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

  – Sufficient but scarce: It is possible to realize the timing requirements of
    the application, but careful resource allocation is required.
           Example: Interactive/Multimedia
         Requirements Applications
    (performance, scale)
                                                             The interesting
                                                              The interesting
                                      sufficient             real-time
Interactive     insufficient          but scarce             applications
Video           resources             resources              are here
                                                              are here
File Access                                 abundant

                  1980         1990                2000
                                       Hardware resources in year X
                      OS or not?

    User Programs                    User Program
      Operating                    Including Operating

      Hardware                         Hardware

                                   System Components

Typical OS Configuration     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
• Code change affects
• 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
  àRequires a Real Time OS underneath (e.g., no Windows support)
Classification of Scheduling Algorithms

                          All scheduling algorithms

      static scheduling                                dynamic scheduling
 (or offline, or clock driven)                    (or online, or priority driven)

                                      static-priority                dynamic-priority
                                        scheduling                     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.

    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
• 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.

                               Time-Sharing              Real-Time
                                 Systems                  Systems
          Capacity             High throughput           Schedulability

      Responsiveness        Fast average response     Ensured worst-case
          Overload                 Fairness                 Stability

    – 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

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