CS4730 Real-Time Systems and Modeling Fall 2010 José M. Garrido Department of Computer Science & Information Systems Kennesaw State University

CS4730Real-Time Systems and Modeling

Fall 2010

José M. Garrido

Department of Computer Science & Information Systems

Kennesaw State University

Fall 2010 (C) J M Garrido 2

Real-Time Systems

RTS are computer systems that

• monitor,

• respond to, or

• control an external environment

Environment

The environment is connected to the computer system through:

• Sensors

• Actuators

• Other I/O devices



General Architecture of A Real-Time System


Real-Time System


Constraints

• A real-time system must meet various timing and other constraints imposed by the real-time behavior of the environment

• A real-time system is also called a reactive system because it must respond to or react to signals from the environment


Real-Time Systems

• A reactive system that maintains an on-going interaction with its environment, and with given time windows for its output

• Includes a combination of hardware and software

• There is a wide variety of RTS – small embedded systems– very large systems on WANs.


Examples of RTS

• Vehicle systems for automobile, subways, aircraft, railways, ships, …

• Traffic control

• Process control for power plants, chemical plants, …

• Medical systems

• Military applications

• Manufacturing systems with robots


Examples of RTS (Cont.)

• Communication systems

• Computer games

• Multimedia systems that provide text, graphic, audio, and video interfaces

• Consumer appliances


Embedded Systems

• A real-time system that is part of another larger system

• Does not usually interact with humans users or operators

• Interfaces directly with sensors and actuators

• Very specialized to suit a specific purpose

• The software is usually implemented in firmware


Embedded Real Time System


Categories of Real_time Systems

Different systems will have different requirements for meeting deadlines:

• Hard real-time systems - missing even a single deadline is considered unacceptable, e.g., The new U.S. Air Traffic Control System.

• Soft real-time systems - missing a deadline occasionally is considered acceptable, e.g., telephony systems.


Real-Time Systems

General characteristics:• Reactive, the system must respond timely to

internal and external input events

• Concurrent, the system needs to manage several tasks (or processes)

• Each task has timing constraints, i.e., it has to respond in a timely fashion.


Other Major Characteristics

• Reliability - a measure of how often a system will fail. Also stated as the probability that a system will perform correctly for a given period

• Fault tolerance - recognition and handling of failures. Avoidance of failures is important, but responding appropriately to failures is challenging


Criticality

• A measure of the failure cost. The higher the cost of failure, the more critical the system.

• A critical system might control an aircraft or a nuclear power plant

• A RTS might be a hard system, even though it is not a critical one. For example, a computer game.


Architecture of Real-Time Systems

• Hardware components– used to interact with the environment– consists of sensors and actuators

• Software components– controls the actions of the hardware– computations


Properties of Real-Time Systems

• Timeliness - the system must perform operations in timely manner

• Reactiveness - the system continuously responds to (random) events

• Concurrency - multiple simultaneous activities are carried out

• Distribution - tasks cooperate in multiple computing sites


Timeliness Issues

The goal is to reduce two specific time intervals:

• service time - the interval taken to compute a response to a given input

• latency - the interval between the time of occurrence of an input and the time at which it starts being serviced

Timeliness Issues

• The sum of these two intervals represents the response time.

• This must be shorter than the deadline for this type of input.


Deadline

• The most common type of timing constraint

• It is the absolute latest time that a computation must complete


General Properties of RTS

• Safety property

• Progress or liveness property



Safety Properties

• The system should never be in certain (undesirable) states

• “The system should not do bad things”

• In the Train-gate System, the gate should never be open when a train is in the intersection area


Progress or Liveness Properties

• These describe the required “live” and desirable behavior of a system

• For example, in the Train-Gate System, the gate should be open whenever there are no trains in the area


Techniques for Studying RT Systems

The techniques for dealing with timeliness problems belong to two main groups:

• Analytic techniques - use formal mathematical models of the systems and mathematical techniques

• Simulation - use computer models to compute behavior characteristics.


Major Subsystems

• In a high-level view, a RTS is considered a closed world containing two components: an environment and a computer system

• These components interact via messages, signals, or events at their interface

• At any time, the RT system may be in one of several states


Basic Elements in Behavior

• Events, are instantaneous occurrences that trigger transitions and operations on some objects. Events can be external or internal.

• States, represent a unique behavior with respect to the other states in an object. An event or message can cause a change of state.

• Transitions are state changes in the objects.


States

• Each state denotes a particular set of values of variables or set of relations among variables

• The system changes state due to events that change the values of, or relations among these state variables

• These state changes are called transitions


Model of RTS

• The software that implement RT systems deal with states, and transitions and a set of interacting processes

• Processes interact by – sending messages and signals to each other– sharing hardware and software– competing for resources

• A process is an active component (object) of a RTS


Reactiveness Issues

• Transformational systems - some initial data is transformed by a series of computations into the desired output data, then the system terminates

• Reactive systems - are involved in a continuous interaction with the environment, these systems do not terminate.


Nondeterminism

• In general, real-time systems are non-deterministic systems since they have no control over the relative order or time of occurrence of input events.

• This characteristic is a reflection of the unpredictable nature of the real-world in which such systems operate.


Concurrency Issues

• A task is a sequential thread of control, a system with concurrency contains two or more simultaneous threads of control that dynamically interact

• Threads may progress at different speeds, their interactions can be defined by:– synchronization– communication


Communication

The communication of tasks in real-time systems can be:

• Synchronous - a direct communication between two tasks. Both the sender task and the receiver task have to be ready

• Asynchronous - an indirect communication that uses mailboxes.

UML Sequence diagrams can used to describe this.


Distribution Issues

• Consists of a set of computing sites loosely coupled through a communication network

• Message loss is just one of a collection of truly difficult problems facing designers of distribution systems


Active and Passive Agents

• Active agents are the major components of a system with their own thread of control

• Passive agents only “executes” when one of its operations is invoked - it is only active during interactions with other objects

• Design and programming with agents involves constructing “machines” and interconnecting them.


Types of Processes in RTS

Real-time systems contain two general types of processes:

• Periodic processes - are activated in a regular basis between fixed time intervals. Used for systematically monitoring, polling, or sampling data from sensors

• Sporadic processes - are event-driven and activated by an external signal, or change in some relationship


Periodic Process

• A periodic process is characterized by a triple (c,p,d).

• The computation time interval is denoted by c

• The period or cycle time is denoted by p

• The deadline interval is denoted by d

• The first important relationship is:

c d p.


Sporadic Process

• A sporadic process is also characterized by a triple (c,p,d), with c d p.

• On the occurrence of the event, the computation must be completed within the specified deadline. The completion time t must satisfy:

t te + d

te is the event occurrence time

• The minimum time interval between events is p


Further Timing Effects

• The interactions among processes further add more delays in the computation times of the processes

• Another effect is jitter. This is the variation from cycle to cycle, or from event to event, of the time to complete a task.


Performance Measures for RTS

• Latency

• Number of deadlines missed

• Worst case reaction time


General RTS Approach

Documents

CS4730 Real-Time Systems and Modeling Fall 2010 José M. Garrido Department of Computer Science & Information Systems Kennesaw State University