CPU Scheduling Algorithms

CPU Schedulin

gAlgorithm

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1Group No. 2

Overview

Basic Concepts

Scheduling Criteria

Scheduling Algorithms

Implementation in C++

Demonstration

Involvement of Operating System

2CPU Scheduling Algorithms

CPU Scheduling Algorithms 3

Basic Concepts Main objective of multiprogramming is to

keep on running processes all the time for maximum CPU utilization.

Scheduling is fundamental function of OS.

The task of selecting the processes in memory that are ready to execute, and allocating them to the CPU is performed by the CPU Scheduler.

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CPU Scheduler CPU scheduling decisions may take

place when a process:o 1. Switches from running to waiting

stateo 2. Switches from running to ready

stateo 3. Switches from waiting to readyo 4. Terminates

Scheduling under 1 and 4 is non preemptive.

All other scheduling is preemptive.

CPU Scheduling Algorithms

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Nonpreemptive Once a process is allocated the CPU, it does not leave unless: o it has to wait, e.g., for I/O request o it terminates

Preemptive o OS can force (preempt) a process

from CPU at anytime o E.g., to allocate CPU to another

higher-priority process

CONT…


CPU Scheduler

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Scheduling Criteria

CPU utilization: keep the CPU as busy as possible ◦ Maximize

Throughput: No of processes that complete their execution per time unit ◦ Maximize

Turnaround time: amount of time to execute a particular process (time from submission to termination)◦ Minimize


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CONT…


Waiting time: amount of time a process has been waiting in the ready queue (sum of time waiting in ready queue)o Minimize

Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment) o Minimize

Scheduling Criteria

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First Come, First Served

Shortest Job First

Priority

Round Robin




Class: cpuschedule Attributes:

o n – number of processeso Bu[ ] – Array to store Burst

Timeo A[ ] – Array to store Arrival

Timeo Wt[ ] – Array to store Waiting

Timeo Twt – Total Waiting Timeo Awt – Average Waiting Time



o Getdata() – To get number of processes and Burst Times from the user

o Fcfs() – First Come, First Served Algorithm

o Sjf() – Shortest Job First (normal) Algorithm

o SjfP() – Shortest Job First (Preemption) Algorithm

o SjfNp() – Shortest Job First (non preemption) Algorithm

o Priority() – Priority Algorithmo RoundRobin() – Round Robin

Algorithm

CONT…

Operations:

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Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order:P1 , P2 , P3

The Gantt Chart for the schedule is:

Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17


First Come, First Served

P1 P2 P3

24 27 300

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Suppose that the processes arrive in the order : P2 , P3 , P1 (P1:24,P2:3,P3:3)

The Gantt chart for the schedule is:

Waiting time for P1 = 6; P2 = 0; P3 = 3 Average waiting time: (6 + 0 + 3)/3 = 3 Much better than previous case Convoy effect - short process behind long

process

CONT…


First Come First Served

P1P3P2

63 300


Shortest Job FirstNormal SJF

Process Burst Time P1 7

P2 3

P3 4

The Gantt Chart for SJF (Normal) is:

Average waiting time = (0 + 3 + 7)/3 = 3.33

P2 P3 P1

730 14

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Process Arrival Time Burst

TimeP1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4 The Gantt Chart for SJF (non-preemptive)

is:

Average waiting time = (0 + 6 + 3 + 7)/4 = 4

Non-Preemptive SJF


Shortest Job FirstCONT…

P1 P3 P2

72 160

P4

8 124 5

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CONT…

Process Arrival Time Burst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4

The Gantt Chart for SJF (preemptive) is:

Average waiting time = (9 + 1 + 0 +2)/4 = 3

P1 P3P2

42 110

P4

5 7

P2 P1

16


Preemptive SJF

Shortest Job First

Associate with each process the length of its next CPU burst.

Use these lengths to schedule the process with the shortest time.

Two schemes: o Non-Preemptive: once CPU given to the

process it cannot be preempted until completes its CPU burst.

o Preemptive: if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF).

SJF is optimal: gives minimum average waiting time for a given set of processes.

Shortest Job FirstCONT…



PriorityProcess Burst Time Priority P1 10 3 P2 1 1

P3 2 4

P4 1 5 P5 5 2

Gantt Chart

Average waiting time = (6 + 0 + 16 + 18 + 1)/5 = 8.2

P2 P1P5

61 160

P3

18

P4

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A priority number (integer) is associated with each process.

Lager the CPU burst lower the priority.The CPU is allocated to the process with

the highest priority (smallest integer highest priority)

Starvation (Infinity blocking): low priority processes may never execute.

Aging: as time progresses increase the priority of the process.

PriorityCONT…



Round RobinProcess Burst Time

P1 24P2 3P3 3

Quantum time = 4 millisecondsThe Gantt chart is:

Average waiting time = {[0+(10-4)]+4+7}/3 = 5.6

P1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30

Typically, higher average turnaround than SJF, but better response

Each process gets a small unit of CPU time (time quantum), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue.

Performanceoq large FCFSoq small q must be large with

respect to context switch, otherwise overhead is too high


Round RobinCONT…


Involvement of OSSource Code (.c)

Conversion

Executable (.exe)

Compiler

MicrosoftWindows

Micro-kernel

(.i , .o)

Memory

CPU

(load executable directly to memory)

Execute


CPU Schedulin

gAlgorithm

sGroup No. 2 22

Group No. 2

CPU Scheduling Algorithms

Technology

Transcript of CPU Scheduling Algorithms