Types of Scheduling

Operating systems use schedulers to manage the jobs that are being completed by the CPU to ensure maximum use of processing time.

Round robin

Round robin is the most basic of scheduler.
In this system each time a job comes in to be completed it is added to the end of a queue.
Each job is given an amount of CPU time to be completed in.

Round robin cont.

If the job completes within this time, then the next job is loaded.
If the job is not completed, then it is pushed to the bottom of the queue and waits for its next slot of CPU time.
- This is fine if all the jobs are similar in size and similar in priority.
This system ignores any priority of a job and ignores that each job will take different amounts of time so can be very inefficient.

First come first served (FCFS)

First come first served (FCFS) is another scheduler that is easy to set up and manage.
As jobs come in to be completed, they are added to a queue.
Jobs are completed in the order they came in regardless of the time taken to complete.

First come first served (FCFS) cont.

This system is very easy to set up and great for getting things done in the set order of arrival.
This system can cause a long delay in getting jobs done, especially if higher up jobs in the queue are big tasks that take a long time.
First come first served scheduling generates bad performance within a computer system.

Multilevel feedback queues

Multilevel feedback queues are the most complex form of scheduling but also give the best results.
The scheduler maintains many queues of jobs, usually grouped by priority and similarity of job.
The CPU will switch between queues to get jobs to complete.

Multilevel feedback queues cont.

If a job is waiting too long in one queue it will be moved to a higher priority location in another queue to get the job completed quicker.
Each separate queue has its own scheduler to maintain that queue.
This system offers the best results, but is very CPU intensive.

Types of Scheduling

Operating systems use schedulers to manage the jobs that are being completed by the CPU to ensure maximum use of processing time.

Shortest job first

Shortest job first is a scheduler that selects the shortest job in the queue to complete first.
Shortest job first can have the shortest waiting time of all the schedulers.

Shortest job first cont.

As it is always looking for the shortest job first, larger processes can face “starvation” and not get completed.
Shortest job first is not very feasible for implementation as the operating system rarely knows the amount of time to complete each new job.
The operating system has to use estimation based on a record of all previous jobs.

Shortest remaining time

Shortest time remaining is a preemptive version of shortest job first.
As a job arrives, it is compared to the currently running job (which is currently the shortest job) if it is longer than the current job then it is added to a queue.
If the new job is shorter than the current job, then the current job is pushed to the queue and the new job is worked on.

Shortest remaining time cont.

As a job completes, the scheduler assesses the queue and selects the next shortest job to complete.
This scheduler requires very little CPU usage as it only makes a comparison as a job arrives or as it completes a job.
Short jobs are always completed quickly, but bigger jobs will face starvation.

1Components of a Computer

2Software & Software Development

2.1Systems Software

2.1.1Operating Systems

2.1.2Managing Memory

2.1.3Paging

2.1.4Segmentation

2.1.5Device Management & Drivers

2.1.6Interrupts

2.1.7Interrupt Service Routine (ISR)

2.1.8Scheduling

2.1.9Types of Scheduling

2.1.10Types of Operating System

2.1.11BIOS

2.1.12Virtual Machines

2.2Applications Generation

2.2.1Applications Software

2.2.2Utilities

2.2.3Encryption Software

2.2.4Defragmentation Software

2.2.5Data Compression Software

2.2.6Backup Software

2.2.7Open Source Software

2.2.8Proprietary Software

2.2.9Licensing Issues

2.2.10Compilers

2.2.11Interpreters

2.2.12Assemblers

2.2.13Compiling a Program

2.2.14Lexical Analysis

2.2.15Compilation Stages

2.2.16Linkers, Loaders & Libraries

2.3Software Development

2.3.1Algorithmic Thinking

2.3.2Waterfall Lifecycle

2.3.3Waterfall Lifecycle - Strengths & Weaknesses

2.3.4Agile Methodology

2.3.5Agile Methodology - Strengths & Weaknesses

2.3.6Extreme Programming

2.3.7Extreme Programming - Strengths & Weaknesses

2.3.8Spiral Methodology

2.3.9Spiral Methodology - Strengths & Weaknesses

2.3.10Rapid Application Development

2.3.11RAD - Strengths & Weaknesse

2.4Types of Programming Language

2.4.1High Level Programming Languages

2.4.2Low Level Programming Languages

2.4.3Programming Paradigms

2.4.4Procedural Programming Languages

2.4.5Assembly

2.4.6Machine Code

2.4.7Modes of Addressing Memory

2.4.8Object-Oriented Languages

2.4.9Classes

2.4.10Objects

2.4.11Encapsulation

2.4.12Polymorphism

3Exchanging Data

3.1Compression, Encryption & Hashing

3.1.1Compression

3.1.2Run Length Encoding

3.1.3Dictionary Coding

3.1.4Encryption

3.1.5Symmetric Encyrption

3.1.6Hashing

3.2Databases

3.2.1Records

3.2.2Databases

3.2.3Relational Databases

3.2.4Entity Relationship Modelling

3.2.5Managing Databases

3.2.6Normalisation

3.2.7SQL

3.2.8Principles of Data Storage

3.2.9Atomicity, Consistency, Isolation & Durability

3.3Networks

3.4Web Technologies

3.4.1HTML

3.4.2CSS

3.4.3JavaScript

3.4.4Search Engine Indexing

3.4.5PageRank Algorithm

3.4.6Server & Client-Side Processing

4Data Types, Data Structures & Algorithms

4.1Data Types

4.2Data Structures

4.2.13D Arrays

4.2.2Linked List

4.2.3Queues

4.2.4Graphs

4.2.5Hash Tables

4.2.6Stacks

4.2.7Trees

4.2.8Binary Search Trees

4.3Boolean Algebra

4.3.1Binary Systems

4.3.2Logic Circuits

4.3.3Boolean Operators

4.3.4Evaluating Expressions

4.3.5Boolean Logic

4.3.6Karnaugh Maps

4.3.7Boolean Identities

4.3.8Boolean Rules

4.3.9Flip Flops

4.3.10Adder Circuits

5Legal, Moral, Cultural & Ethical Issues

5.1Computing Related Legislation

5.1.1Data Protection Act (1998)

5.1.2Computer Misuse Act (1990)

5.1.3Copyright Designs & Patents Act (1988)

5.1.4Creative Commons Licensing

5.1.5Regulation of Investigatory Powers Act (2000)

5.2Moral & Ethical Issues

5.2.1Online Activity Tracking

5.2.2Censorship

5.2.3Positive Cultural Impacts

5.2.4Negative Cultural Impacts

5.2.5E-Waste

5.2.6Energy Consumption

5.2.7Positive Environmental Impact

5.2.8Layout, Colour Paradigms & Character Sets

5.2.9Computers in the Workplace

5.2.10Automated Decision-Making

5.2.11Artificial Intelligence

5.2.12Monitoring Behaviour

5.2.13Analysing Personal Information

5.2.14Piracy & Offensive Communication

6Elements of Computational Thinking

6.1Thinking Abstractly

6.1.1Abstraction

6.2Thinking Ahead

6.2.1Inputs & Outputs

6.3Thinking Procedurally

6.3.1Decomposition

6.4Thinking Logically

6.4.1Pseudocode

6.4.2Flow Diagrams

6.5Thinking Concurrently

6.5.1Concurrent Processing

7Problem Solving & Programming

7.1Programming Techniques

7.1.1Variables & Constants

7.1.2Comments

7.1.3Inputs & Outputs

7.1.4Subroutines

7.1.5Scope

7.2Programming Construction

7.2.1Integrated Development Environments

7.2.2The Purpose of Testing

7.2.3Types of Testing

7.2.4Test Plans

8Algorithms

8.1Algorithms

8.1.1Interpreting Algorithms

8.1.2Correcting Algorithms

8.1.3Completing Algorithms

8.1.4Sort Algorithms

8.1.5Bubble Sort

8.1.6Insertion Sort

8.1.7Merge Sort

8.1.8Search Algorithms

8.1.9Linear Search

8.1.10Binary Search

8.1.11Quick Sort Algorithm

8.1.12A* Algorithm

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