2.1.9

Types of Scheduling

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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

  • 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.

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)

  • 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.

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

  • 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.

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

  • 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.

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 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.

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.
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1

Components of a Computer

1.1

Structure & Function of the Processor

1.1.1Von Neumann Architecture1.1.2Registers1.1.3Buses1.1.4Fetch, Decode, Execute Cycle1.1.5Factors Affecting CPU Performance1.1.6Pipelining1.1.7CPU Architecture
1.2

Types of Processors

1.2.1CISC & RISC1.2.2GPUs & Their Uses1.2.3Multicore & Parallel Systems
1.3

Input, Output & Storage

1.3.1Elements of Computer Systems1.3.2Types of Computer Systems1.3.3How Magnetic Storage Works1.3.4Properties of Magnetic Storage1.3.5Examples of Magnetic Storage1.3.6How Optical Storage Works1.3.7Properties of Optical Storage1.3.8Examples of Optical Storage1.3.9Types of Optical Disc1.3.10Random Access Memory1.3.11Read Only Memory1.3.12Uses of Flash Memory1.3.13Properties of Flash Memory1.3.14What to do When We Run Out of Memory1.3.15How Virtual Memory Works
2

Software & Software Development

2.1

Systems Software

2.1.1Operating Systems2.1.2Managing Memory2.1.3Paging2.1.4Segmentation2.1.5Device Management & Drivers2.1.6Interrupts2.1.7Interrupt Service Routine (ISR)2.1.8Scheduling2.1.9Types of Scheduling2.1.10Types of Operating System2.1.11BIOS2.1.12Virtual Machines
2.2

Applications Generation

2.2.1Applications Software2.2.2Utilities2.2.3Encryption Software2.2.4Defragmentation Software2.2.5Data Compression Software2.2.6Backup Software2.2.7Open Source Software2.2.8Proprietary Software2.2.9Licensing Issues2.2.10Compilers2.2.11Interpreters2.2.12Assemblers2.2.13Compiling a Program2.2.14Lexical Analysis2.2.15Compilation Stages2.2.16Linkers, Loaders & Libraries
2.3

Software Development

2.3.1Algorithmic Thinking2.3.2Waterfall Lifecycle2.3.3Waterfall Lifecycle - Strengths & Weaknesses2.3.4Agile Methodology2.3.5Agile Methodology - Strengths & Weaknesses2.3.6Extreme Programming2.3.7Extreme Programming - Strengths & Weaknesses2.3.8Spiral Methodology2.3.9Spiral Methodology - Strengths & Weaknesses2.3.10Rapid Application Development2.3.11RAD - Strengths & Weaknesse
2.4

Types of Programming Language

2.4.1High Level Programming Languages2.4.2Low Level Programming Languages2.4.3Programming Paradigms2.4.4Procedural Programming Languages2.4.5Assembly2.4.6Machine Code2.4.7Modes of Addressing Memory2.4.8Object-Oriented Languages2.4.9Classes2.4.10Objects2.4.11Encapsulation2.4.12Polymorphism
3

Exchanging Data

3.1

Compression, Encryption & Hashing

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3.2

Databases

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3.3

Networks

3.3.1The Benefits of Networks3.3.2Network Performance3.3.3Types of Networks3.3.4Network Protocols3.3.5Transmission Protocols3.3.6What is the Internet?3.3.7Uniform Resource Locators3.3.8Domain Name Service3.3.9Web Hosting3.3.10Layering Concepts3.3.11TCP &. OSI Models3.3.12The Advantages of Layering3.3.13What's in a Packet?3.3.14How do Packets get Routed?3.3.15Did my Data Arrive Safely?3.3.16Network Hardware3.3.17Transmission Media3.3.18Firewalls3.3.19Proxies3.3.20Client-Server Model3.3.21Advantages of the Client Server Model3.3.22Disadvantages of the Client Server Model3.3.23Peer-to-Peer Model3.3.24Advantages of the Peer-to-Peer Model3.3.25Disadvantages of the Peer-to-Peer Model
3.4

Web Technologies

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4

Data Types, Data Structures & Algorithms

4.1

Data Types

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4.2

Data Structures

4.2.13D Arrays4.2.2Linked List4.2.3Queues4.2.4Graphs4.2.5Hash Tables4.2.6Stacks4.2.7Trees4.2.8Binary Search Trees
4.3

Boolean Algebra

4.3.1Binary Systems4.3.2Logic Circuits4.3.3Boolean Operators4.3.4Evaluating Expressions4.3.5Boolean Logic4.3.6Karnaugh Maps4.3.7Boolean Identities4.3.8Boolean Rules4.3.9Flip Flops4.3.10Adder Circuits
5

Legal, Moral, Cultural & Ethical Issues

5.1

Computing 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 Licensing5.1.5Regulation of Investigatory Powers Act (2000)
5.2

Moral & Ethical Issues

5.2.1Online Activity Tracking5.2.2Censorship5.2.3Positive Cultural Impacts5.2.4Negative Cultural Impacts5.2.5E-Waste5.2.6Energy Consumption5.2.7Positive Environmental Impact5.2.8Layout, Colour Paradigms & Character Sets5.2.9Computers in the Workplace5.2.10Automated Decision-Making5.2.11Artificial Intelligence5.2.12Monitoring Behaviour5.2.13Analysing Personal Information5.2.14Piracy & Offensive Communication
6

Elements of Computational Thinking

6.1

Thinking Abstractly

6.1.1Abstraction
6.2

Thinking Ahead

6.2.1Inputs & Outputs
6.3

Thinking Procedurally

6.3.1Decomposition
6.4

Thinking Logically

6.4.1Pseudocode6.4.2Flow Diagrams
6.5

Thinking Concurrently

6.5.1Concurrent Processing
7

Problem Solving & Programming

7.1

Programming Techniques

7.1.1Variables & Constants7.1.2Comments7.1.3Inputs & Outputs7.1.4Subroutines7.1.5Scope
7.2

Programming Construction

7.2.1Integrated Development Environments7.2.2The Purpose of Testing7.2.3Types of Testing7.2.4Test Plans
8

Algorithms

8.1

Algorithms

8.1.1Interpreting Algorithms8.1.2Correcting Algorithms8.1.3Completing Algorithms8.1.4Sort Algorithms8.1.5Bubble Sort8.1.6Insertion Sort8.1.7Merge Sort8.1.8Search Algorithms8.1.9Linear Search8.1.10Binary Search8.1.11Quick Sort Algorithm8.1.12A* Algorithm

Practice questions on Types of Scheduling

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    What is one drawback of using multilevel feedback queues for scheduling?Fill in the list
  2. 2
    Which is the most complex form of scheduling?Multiple choice
  3. 3
    Which scheduler is designed to minimize waiting time?Multiple choice
  4. 4
    Which type of scheduler evaluates the length of tasks as they come in, and fast tracks the shortest time to the front of the queue?Multiple choice
  5. 5
    Why do shortest remaining time schedulers require less CPU usage?Multiple choice
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Scheduling

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