‫اململكة العربية السعودية‬
Kingdom of Saudi Arabia
Ministry of Higher Education
Prince Sattam Bin Abdul-Aziz University
College of Computer
Engineering and sciences
‫وزارة التعليم العايل‬
‫جامـعـة األمري سطام بن عبدالعزيز‬
‫كل ـي ــة هـنــدس ــة وعلـ ــوم احلـاســب‬
Course Title: CS3701-Operating System
CSC3701
Instructor Name: Mr. Abdul Haseeb Khan
Tutorial Lecture # 05
Date: - - -2015
Week# 05
2440 & 2544 :‫الشعبة‬
CH: CPU Scheduling
Objective: How CPU Schedule different tasks among different process in multiprogramming
Q. 1. What is CPU scheduling?
In multiprogramming CPU is switched among different processes in the memory. This is
called CPU scheduling.
Q. 2. Briefly describe CPU-I/O Burst cycle.
Processes have the property that process execution consists of a cycle of CPU execution (CPU
Burst) and I/O wait (I/O Burst). This property is important for success of CPU scheduling.
Q. 3. What is the function of a CPU scheduler?
When CPU becomes idle OS must select one of the processes in the ready queue to be
executed on the CPU. This selection is carried out by CPU scheduler (also called short-term
scheduler).
Q. 4. What is the function of a dispatcher?
Dispatcher gives control of the CPU to the process selected by CPU scheduler. It performs the
context switch, switches to user mode and starts the program.
Q. 5.What is non-preemptive and preemptive scheduling?
Non-preemptive scheduling: When a process running on the CPU terminates or has to
go into a wait state, CPU must be allocated to a new process. This is called nonpreemptive s cheduling or cooperative scheduling.
Preemptive scheduling: When a process switches from running state to ready state or
from a wait state to ready state, the scheduling done under this situation is called
preemptive scheduling.
Q. 6.Name the criteria used to judge scheduling algorithms.
(a) CPU Utilization – percent of time CPU is busy (usually 40% to 90%)
(b) Throughput – number of processes completing per unit time.
(c) Turnaround time – time between process submission and completion.
(d) Waiting time – time spent by a process waiting in the ready queue.
(e) Response time – time between submissions of a request to the first response.
Q. 7. Name and briefly describe various scheduling algorithms.
(a) First Come First Served (FCFS) scheduling – the process that arrived first in the ready
queue is given the CPU first.
(b) Shortest Job First (SJF) scheduling – with each process is associated the length of its
next CPU burst and the process with smallest next CPU burst is given the CPU.
(c) Priority scheduling – a priority is associated with a process and CPU is given to the
process of highest priority.
(d) Round Robin scheduling – each process gets a small unit of CPU time (time quantum
or time slice) usually 10 -100 milliseconds. When the time quantum finishes the
process is added to the end of the ready queue.
(e) Multilevel Queue scheduling – ready queue is partitioned into several separate queues.
A process is permanently assigned to one queue based on some property of the process
e.g. memory size, process priority. Each queue has its own scheduling algorithm.
(f) Multilevel Feedback-queue scheduling – allows a process to move between queues
according to CPU time requirements. If a process uses too much CPU time it is moved
to a lower-priority queue. A process waiting too long in a lower priority queue is
moved to a higher priority queue.
Q. 8. What is the problem of starvation? How is it dealt with?
A low priority process waiting indefinitely in the ready queue because of other high
priority processes is said to be facing starvation.
Solution to the problem of starvation is aging – a technique of slowly increasing the
priority of processes that wait too long.
Problem Solving Part
Consider the following set of processes, with the
length of CPU burst time given in milliseconds:
Question #
Process
Arrival Time Burst Time
P1
0
3
P2
1
1
P3
3
7
P4
4
0.5
P5
5
2
Draw the Gant charts illustrating the execution of these processes.
i.
FCFS:
P1
P2
0
3
P3
P4
4
11
P5
11.5
13.5
SJF (NON-PRE-EMPTIVE):
0
ii.
P1 P2
P4 P3
3
4 4.5
P5
11.5
13.5
What is the turnaround time of each process?
FCFS: TAT:
P1: 3
P2:
SJF (Non-Preemptive) TAT: P1: 3
iii.
3
P2: 3
P3: 8
P4:
7.5
P5: 8.5
P3: 8.5
P4: 0.5
P5: 8.5
P3: 1
P4: 0.5
P5: 6.5
What is the waiting time of each process?
FCFS: W.T:
P1: 0
P2: 2
AWT: 3.3
SJF (Non-Preemptive) W.T: P1: 0
AWT: 2
P2: 2
P3: 1.5
P4: 0
P5: 6.5
Q
Consider the following set of processes, with the length of CPU burst time given
in milliseconds:
Process
P1
P2
P3
P4
P5
iv.
Burst Time
3
1
8
5
2
Draw the Gant chart for SJF PRE-EMPTIVE:
P1
0
v.
P2
1
P1
2
P4
4
P5
5
P4
7
P3
11
19
What is the turnaround time of each process of the scheduling algorithm?
SJF TAT:
vi.
Arrival Time
0
1
3
4
5
P1: 4
P2: 1
P3: 16
P4: 7
P5: 2
What is the waiting time of each process for each of the scheduling algorithm?
SJF W.T:
P1: 1
AWT: 1+0+8+2+0=11/5=
P2: 0
P3: 8
P4: 2
P5: 0