Real Time Systems
A real- time system is defined as a system in which the correctness of computations depends not only on the logical correctness of the computation but also on the time at which the result is produced. So, we can say that it has strict time constraints. The inputs, data or outputs should be available within this time period otherwise disasters can happen. Sensors bring data to the computer. The computer must analyze the data and adjust controls to modify the sensor’s inputs. These systems are characterized by having time as a key parameter.
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Introduction:-
Recently, computer hardware
and software are increasingly embedded in a majority of these real-time systems
to monitor and control their operations. These computer systems are called
embedded systems, real-time computer systems, or simply real-time systems.
Unlike conventional, non-real-time computer systems, real-time computer systems
are closely coupled with the environment being monitored and controlled.
Examples of real-time systems include computerized versions of the braking
controller and the vital-sign monitor, the new generation of airplane and
spacecraft avionics, the planned Space Station control software,
high performance network and telephone switching systems, multimedia tools,
virtual reality systems, robotic controllers, battery-powered instruments,
wireless communication devices (such as cellular phones and PDAs), astronomical
telescopes with adaptive-optics systems, and many safety-critical industrial
applications. These embedded systems must satisfy stringent timing and
reliability constraints in addition to functional correctness requirements.
Figure below shows a model of a real-time system. A real-time
system has a decision component that interacts with the external environment
(in which the decision
sensor readings and computing control decisions based on sensor readings and stored state information. We can characterize this real time system model with seven components:
1.
A sensor vector ¯ x ∈ X,
2.
A decision vector ¯y ∈ Y,
3.
A system state vector ¯s ∈ S,
4.
A set of environmental constraints A,
5.
A decision map D, D : S × X → S × Y ,
6.
A set of timing constraints T , and
7.
A set of integrity constraints I.
In this model, X is the space of sensor input
values, Y is the space of decision values, and S is the space of system state
values. Let ¯ x(t) denote the value of the
sensor input ¯ x at time t , and so on.
The environmental constraints A are relations
over X, Y , S and are assertions about the effect of a control decision on the
external world which in turn affect future sensor input values. Environmental
constraints are usually imposed by the physical environment in which the
real-time decision system functions. The decision map D relates ¯y(t 1), ¯s(t
1) to ¯ x(t ), ¯s(t), that is, given the current system state and sensor input,
D determines the next decisions and system state
values. Decision maps can be implemented by
computer hardware and software components. A decision system need not be
centralized, and may consist of a network of coordinating, distributed
monitoring/decision-making components.
The decisions specified by D must conform to a
set of integrity (safety) constraints I . Integrity constraints are relations
over X, S, Y and are assertions that the decision map D must satisfy to ensure
safe operation of the physical system under control. The implementation of the
decision map D is subject to a set of timing constraints T , which are
assertions about how fast the map D has to be performed. In addition, timing
constraints exist on the environment (external to the decision system) that
must be satisfied for the correct functioning of this environment.
There are two ways to ensure system safety and
reliability. One way is to employ engineering (both software and hardware)
techniques, such as structured programming principles, to minimize
implementation errors and then utilize testing techniques to uncover errors in
the implementation. The other way is to use formal analysis and verification
techniques to ensure that the implemented system satisfy the required safety
constraints under all conditions given a set of assumptions. In a realtime
system, we need to not only satisfy stringent timing requirements but also
guard against an imperfect execution environment, which may violate pre-runtime
design assumptions. The first approach can only increase the confidence level
we have on the correctness of the system because testing cannot guarantee that
the system is error-free [Dahl, Dijkstra, and Hoare, 1972]. The second approach
can guarantee that a verified system always satisfies the checked safety
properties, and is the focus of this text.
However, state-of-the-art techniques, which have
been demonstrated in pedagogic systems, are often difficult to understand and
to apply to realistic systems. Furthermore, it is often difficult to determine
how practical a proposed technique is from the large number of mathematical
notations used. The objective of this book is to provide a more readable
introduction to formal techniques that are practical for actual use. These
theoretical foundations are followed by practical exercises in employing these
advanced techniques to build, analyze, and verify different modules of
real-time systems. Available specification analysis and verification tools are
also described to help design and analyze real-time systems.
Real time systems span several domain of computer science. The term “real-time system” refers to
any information processing system with hardware and
software
components that perform real-time application functions and can respond to
events
within
predictable and specific time constraints. Common examples of real-time systems
include air
traffic control
systems, process control systems, and autonomous driving systems.
Real-Time systems are those which must
produce correct responses within a
definite time limit. Should computer responses exceed these time bounds then
performance degradation and/or malfunction results.
Examples of the real-time operating systems: Airline traffic control systems, Command Control Systems, Airlines reservation system, Heart Peacemaker, Network Multimedia Systems, and Robot etc.
Types of Real Time Operating Systems
The real-time operating systems
can be of 3 types –
1.
Hard Real-Time operating system:
Actions are not
necessarily disjoint in time, and the sequence of some of program
an action is not determined by the designer but the environment
(by events occurring in the outside world which occur in real-time and without
reference to the internal operations of the computer). Another important
characteristic in real-time systems is their ability to perform concurrent
execution of real-time and non-real-time workloads in order to avoid critical
system failure.
Finally, it’s important to understand how real-time systems are
typically categorized. They are designated as either a soft real-time system or
a hard real-time system based on timing constraints.
These operating systems guarantee that critical tasks be completed within a
range of time.
For example, a robot is hired to weld a car body. If
the robot welds too early or too late, the car cannot be sold, so it is a hard
real-time system that requires complete car welding by robot hardly on the
time.
A Hard real-time is when a system will cease to function if a deadline is missed,
which can result in catastrophic consequences. As in hard real time systems,
the scheduler plays an important roe in soft real time systems also. A carefull
design of scheduler is required. It must handle priority scheduling and the
dispatch latency must be small. In hard real time systems, late data is bad
data. For example, in industrial process- control systems, real time computers
have to collect data about production process and use it to control machines in
the factory. Often there are hard deadlines that must be met. For example, if a
car is moving down an assembly line, certain actions must take place at certain
instants of time. If for example, a wielding robot weilds too early or too
late, the car will be reined. If the action occur at a certain moment (or
within a certain range). Many of these are found in industrial process control,
avionics, military, and similar application areas. These systems must provide
absolute guarantees that a certain action will occur by a certain time.
However, there are certain industries, such as robotics, automotive, utilities, and healthcare, where use cases have higher requirements for synchronization, time lines, and worst-case execution time guarantee. Those examples fall within the hard real-time classification. Since meeting deadlines is crucial in (hard) real time systems, sometimes the operating system is simply a library linked in with the application programs, with everything tightly coupled and no protection between parts of the system. As example of this type of real-time system is eCos.
Soft real-time operating system:
This
operating system provides some relaxation in the time limit.
For example – Multimedia
systems, digital audio systems etc. Explicit, programmer-defined and controlled
processes are encountered in real-time systems. A separate process is changed with handling
a single external event. The process is activated upon occurrence of the
related event signaled by an interrupt.
A Soft real-time is when a
system continues to function even if it’s unable to execute within an allotted
time. If the system has missed its deadline, it will not result in critical
consequences. The system can continue to function, though with undesirable
lower quality of output. In this critical task are given priority over
noncritical task. Delays need to be bounded (i.e., Predictable) so the critical
tasks do not wait forever but these bounds are not as serve as in hard systems.
In soft real time systems, late data is good data. It is one where missing an
occasional deadline, while not desirable, is acceptable and does not cause
any permanent damage. Digital audio, multimedia systems and smart phones are
also soft real time systems.
Multitasking operation is accomplished by scheduling processes for execution independently of each other. Each process is assigned a certain level of priority that corresponds to the relative importance of the event that it services. The processor is allocated to the highest priority processes. This type of schedule, called, priority-based preemptive scheduling is used by real-time systems.
Firm Real-time Operating System:
RTOS of this type have to follow deadlines as well.
In spite of its small impact, missing a deadline can have unintended
consequences, including a reduction in the quality of the product. Example:
Multimedia applications. A Firm real time system is a combination of both hard and soft
timeliness requirements. The computation has a shorter soft requirement and a
longer hard requirement.
For Example: a patient ventilator must mechanically ventilate the patient a certain amount in a given time periods. A few seconds delay in the initiation of the breath in allowed but not more than that.
Soft Real-Time Systems vs. Hard Real-Time Systems:-
|
Hard Real Time Systems |
Soft Real Time Systems |
|
1. Autonomous error detection for hard real time
systems. 2. These systems have limited utility for rollback/
recovery mechanism due to short-term integer of data. 3. The data file sizes are small/medium. 4. Their safety is often critical. 5. Predictable (well defined) peak load
performance for these systems. Examples: (a) Air Traffic Control. (b) An Embedded System controlling the operation
of a refinery. |
1. User assisted error detection for soft real
time systems. 2. These systems have long-term integrity of data. 3. The data file sizes are large. 4. Their safety is non –critical. 5. Degraded peak load performance. Examples: (a) Telecom (Voice) (b) Delayed information about fight path will lose
its utility. It is a soft system as it will not be catastrophic but the
output loses its utility. |
Advantages:
The advantages of real-time operating systems are as
follows-
1.
Maximum consumption –
Maximum
utilization of devices and systems. Thus more output from all the
resources.
2. Task
Shifting –
Time assigned for shifting tasks in these systems is very less. For example, in
older systems, it takes about 10 microseconds. Shifting one task to another and
in the latest systems, it takes 3 microseconds.
3. Focus
On Application –
Focus on running applications and less importance to applications that are in
the queue.
4. Real-Time Operating System in
Embedded System –
Since the size of programs is small, RTOS can also be embedded systems like in
transport and others.
5. Error
Free –
These types of systems are error-free.
6. Memory
Allocation –
Memory allocation is best managed in these types of systems.
Disadvantages:
The disadvantages of real-time
operating systems are as follows-
1. Limited
Tasks –
Very few tasks run simultaneously, and their concentration is very less on few
applications to avoid errors.
2. Use
Heavy System Resources –
Sometimes the system resources are not so good and they are expensive as
well.
3. Complex
Algorithms –
The algorithms are very complex and difficult for the designer to write
on.
4. Device
Driver And Interrupt signals –
It needs specific device drivers and interrupts signals to respond earliest to
interrupts.
5. Thread
Priority –
It is not good to set thread priority as these systems are very less prone to
switching tasks.
6. Minimum
Switching –
RTOS performs minimal task switching.
Real-Time Program: “A program for which the
correctness of operation depends both on the logical
results of the computation and at which the results are produced”
Note: All real time systems are distributed
systems but all distributed systems are not real systems.
Real-time systems are defined as those systems in
which the correctness of the system depends not only on the logical result of
computation but also on the time at which the results are produced. The journal
Real-Time Systems publishes papers that concentrate on real-time computing
principles and applications. Real-Time Systems publishes research papers
invited papers project reports and case studies standards and corresponding
proposals for general discussion and a partitioned tutorial on real-time
systems as a continuing series. Much of the work in building sophisticated
modern real-time systems is interdisciplinary in nature and up until now has
been found scattered throughout the primary literature. Real-Time Systems
provides a single-source coverage of the state of the art in this exciting and
expanding field. The editorial board the writers and the readers of the journal
are drawn from all parts of the world from industry and from academe. Papers
published in Real-Time Systems cover the following topics among others:
requirements engineering specification and verification techniques design
methods and tools programming languages operating systems scheduling algorithms
architecture and hardware (especially interfacing) fault tolerance distributed
and other novel architectures communications distributed databases AI expert
systems and application case studies. Applications are found in command and
control systems process control systems automated manufacturing flight control
avionics space avionics and defense systems shipborne systems vision and
robotics and robot teams in hazardous environments.
The Need for Real-Time Systems
Growing global connectivity, changing consumer demands for
always-available data, and always-on, sensor-enabled enterprise environments
are driving the creation, collection, and analysis of exponential amounts of
data. By 2025, IDC estimates that there will be 79.4 zettabytes of data
created and nearly 30 percent of it will require real-time processing
enabled by real-time systems.
The need for real-time processing is especially crucial for businesses in
robotics, manufacturing, healthcare, and high-precision industries, such as oil
and gas and power, that rely on real-time data for continuous improvement in
safety, efficiency, and reliability.
One key factor in ensuring data is processed in real-time for businesses in
these types of industries is a system’s ability to prioritize, manage, and
execute real-time workloads over non-real-time workloads.
For example, modern automotive manufacturers are highly reliant on robots to work together on a production line to assemble a car. The robots will pass each other parts, drill or weld, or perform safety inspections—all of which require a high level of precision and meticulous timing. In this use case, a real-time system must have the ability to not only process data in a defined, predictable time frame but also ensure that critical tasks, such as safety-related workloads, are completed prior to less critical task.
Benefits
of Real-Time Systems for Applications
Real-time systems offer several benefits:
Real-Time
System Components
For a real-time system to be capable of real-time
computing, it must satisfy two requirements:
- Timeliness: The ability to produce the expected result by a specific deadline.
- Time synchronization: The capability of agents to coordinate independent clocks and operate together in unison.
- Latency: Measurement of time between two events
- Compute jitter: Latency variation between iterations
· Clock- based tasks (cyclic, periodic)
· Event –based tasks(a periodic)
· Interactive systems
Synchronization between the external processes and internal actions (tasks) carried out
by the computer may be defined in terms of the passage of time, or the actual time of
day, in which case the system is said to be “Clock-based system” or it may be defined in
terms of events, and the system is said to be “Event-based system”.
If the relationship between the actions in the computer and the system is much more
loosely defined, then the system is said to be “interactive system”.
· Clock-Based Tasks: (Cyclic and Periodic):
– The completion of the operations within the specified time is dependent on the
number of operations to be performed and the speed of the computer.
– Synchronization is usually obtained by adding a clock to the computer system,
and using a signal from this clock to interrupt the operation of the computer at
predetermined fixed time interval.
· Event-Based Tasks: (A periodic):
– Action are to be performed not at particular times or time intervals but in
response to some event. The system must respond within a given max. Time to a
particular event.
– Events occur at non-deterministic intervals and event-based tasks are referred to
as “a -periodic” task.
· Interactive Systems:
- They represent the largest class of RTSs such as automatic bank tellers,
reservation systems for hotels, airlines and car rental……etc.
The real-time requirement is usually expressed in terms such as “the average
response time must not exceed some predetermined time”
Example: an automatic bank teller system might require an average response time
not exceeding 20 sec.
- An interactive system responds at a time determined by internal state of the
computer without reference to the external environment.
Classification Based on Timing Constraints:
Based on timing constraints real-time tasks are
classified into two types
·
Hard real-time (these are systems that must satisfy the deadlines on each and
every occasion)
·
Soft real-time (these are systems for which an occasional failure to meet a
deadline does not comprise the correctness of the system).
Classification Of Programs:
Programs can be broadly classified into the following 3 categories:
· Sequential
· Multi-tasking
· Real-time
Sequential:
- Actions are ordered as a time sequence, the program
behaviour depends only on the
effects of the individual actions and their order but not on the time taken to
perform
those actions
Multi-tasking:
performed in parallel. Hence in multitasking programs various tasks are executed concurrently and communicate through shared variables and synchronization signals.
Applications of Real-Time Systems
 |
Working of real time Systems Process Control Systems Process control systems are used
in industrial applications where production is continuous and interruptions cannot happen. These systems help businesses maintain quality and
improve performance by testing processes, collecting relevant data, and returning that
data for monitoring and possible troubleshooting. Companies in the oil and gas
sector are key users of process control systems and often realize numerous
benefits, from increased efficiency to safer operation of facilities to less
downtime and fewer losses.
Machine Vision Machine vision is used to help machines rapidly interpret data so they can see
their surroundings and make decisions quickly based on that visual input. These
machines are often key to ensuring production keeps flowing or critical processes
continue. Real-time systems help ensure machines such as these are able to
process that data in near real-time.
Robotics Robotics technologies are used for a variety of complex applications, many of
which require precise timing constraints to ensure a safe workload execution as
well as the ability to continuously function. Real-time systems are a valuable
part of robot operating systems because of the need for real-time computing and
processing.
Manufacturing Future-focused manufacturers rely on insights gained from real-time
applications to avoid product quality issues, improve efficiency and
performance, and, ultimately, gain a competitive edge.
Embedded realtime systems can help
manufacturers maximize productivity, improve product quality and consistency, and enhance safety on the factory floor. Healthcare and Patient Monitoring How quickly data is processed in healthcare can often mean the difference
between life and death. Real-time systems are key to ensuring data from patient
monitoring systems, such as heart rate monitors, is available to clinicians when
and where they need it to keep patients safe and healthy. |
Summary:
The key ideas described in
this chapter are:
·
A brief history of computer control.
·
Digital computers are sequential
devices.
·
Real-time systems have to carry out
both Periodic and a-periodic activities.
·
Real-time systems have to satisfy time
constraints that can be either a hard or a
soft constraint.
·
Real-time software is more difficult
to specify, design and construct than non real-time software.
The categories of handhelds, embedded systems and real -time
systems overlap considerably. Nearly all of them have some soft real-time
aspects. The embedded and real-time systems run only software put in by the
system designers, users cannot add their own software, which makes protection
easier. The handhelds and embedded systems are intended for consumers, whereas
real –time systems are more for industrial usage. Nevertheless , they have a
certain amount in common.