Real Time System Resume

7/29/2019 Real Time System Resume

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REAL TIME SYSTEM RESUME

Real Time System (RTS) is a system must generate an appropriate response within

the time limit specified. If the computer response out of time limit, it can make

performance degradation or system failure.

Classify of Real Time System based on time response:

1. Hard Real Time System

In this system needed to solve the critical task within the time limit. If the

time limit not fulfilled, the application will failure. The system can’t tolerate

more than 100ms.

Example: flight control

2. Soft Real Time System

In this system, the critical process received the higher priority than others.

The usefulness of the result degrades after its deadline, thereby degrading

the system quality of service (QoS).

Example: data acquisition system, audio-video system.

3. Real Real Time System

This method is a like Hard Real Time System, but the response times are very

short.

Example: Missile guidance system

4. Firm Real Time System

Infrequent deadline misses are tolerable, but may degrade the system quality

of service. The usefulness of a result is zero after its deadline.

The goal of Hard Real Time System is to ensure that all deadlines are met, but for

Soft Real Time System the goal becomes meeting a certain subset of deadlines in

order to optimize some application specific criteria.



Time Triggered System versus Event Triggered System

Time triggered systems derive system of follows the principle global progression of

time, such allowing precise temporal specification of interfaces and interoperability

and replica determinism.

An event triggered system follows the principle of reaction on demand, where

temporal control is enforced from environment onto the system in unpredictable

manner (interrupts), with all undesirable problems of jitter, missing precise

temporal specification of interfaces and membership, scheduling etc, but good for

sporadic action/data, low-power sleep modes, best-effort soft real time system with

high utilization of resources.

NETWORK CONTROL SYSTEM

Network control system is a network wherein the control loops are closed through areal time network. The defining feature of a Network Control System (NCS) is that

control and feedback signals are exchanged among the system’s components in the

form of information packages through the network.

The functionality of a typical NCS is established by the use of four basic elements:

1. Sensors, to acquire information

2. Controllers, to provide decision or commands,

3. Actuators, to perform the control command,

4. Communication network, to enable exchange of information

The most important feature of a NCS is that it connects cyberspace to physical

space enabling the execution of several task from long distance. In addition, the

NCS eliminate the unnecessary wiring reducing the complexity and the overall cost

in designing and implementation of control system. They can also be easily

modified or upgraded by adding sensors, actuators, and controllers to them with

relative low cost and no major changes in their structure. Moreover, featuring

efficient sharing of data between their controllers, NCS are able to easily fuse global

information to make intelligence decisions over large physical spaces.

Their potential applications are numerous and cover a wide range of industries such

as: space and terrestrial exploration, factory automation, experimental facilities,

domestic robots, aircraft, automobiles, manufacturing plant monitor.



REAL TIME SOFTWARE SYSTEM AND OPERATING SYSTEM

FOR EMBEDDED SYSTEMS

The important features in Real Time System

Time-SharingSystem

Real-Time System

Capacity High Throughput Schedule abilityResponsive

nessFast average response

Ensure worst-caseresponse

Overload fairness Stability

• schedule ability is the ability of task to meet all hard deadlines

• -Latency is the worst-case system response time in event

• -Stability in the overload means the system meets the critical deadlines

even if all deadlines cannot be met.

1. REAL TIME OPERATING SYSTEM

RTOS is an operating system (OS) intended to serve real time application

request. It must be able to process data as it comes in, typically without

buffering delays. Processing time requirements (including any OS delays) are

measured in tenth of seconds or shorter.

An RTOS has an advanced algorithm for scheduling. Scheduler flexibility

enables a wider, computer system orchestration of process priority, but a real

time OS is more frequently dedicated to a narrow set of applications. Key

factors in a RTOS are minimal interrupt latency and minimal thread switching

latency. A real time OS is valued more for how quickly or how predictably it

can respond than for amount of work it can perform in a given period of time.

DESIGN PHILOSOPY

The most common designs are:



• Event-driven which switches tasks only when an event on higher

priority needs servicing, called preemptive priority, or priority

scheduling.

• Time-sharing design switch task on regular clocked interrupt, and on

events, called round robin.

Time sharing designs switch tasks more often than strictly needed, but give

smoother multitasking, giving the illusion that a process or user has sole use

of a machine.

SCHEDULLING

In typical designs, a task has three states:

1. Running (executing on the CPU)

2. Ready (ready to be executed)

3. Blocked (waiting for an event, I/O for example)

Most tasks are blocked or ready most of the time because generally only one

task can run at a time per CPU.

ALGORITHM

Some commonly used RTOS scheduling algorithm are:

1. Cooperative scheduling

2. Preemptive scheduling

a. Rate-monotonic scheduling

b. Round-robin scheduling

c. Fixed priority pre-emptive scheduling, an implementation of preemptive time slicing

d. Fixed-priority scheduling with deferred preemption

e. Fixed-priority non-preemptive scheduling

f. Critical section preemptive scheduling



g. Static time scheduling

3. Earliest Deadline First approach

4. Stochastic Digraphs with multi-threaded graph traversal

Real Time System Resume

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