King Saud University
College of Computer & Information Sciences
Information Technology Department
IT425: Operating Systems
Assignment 6
Second Semester 1433/1434H – 2012/2013
Homework policy:
1) Copying any part of other people's solutions or from any other sources is strictly prohibited.
2) If your source is the web, write your references in details.
3) Solutions quality (understandable and organized document, not handwriting).
4) Always start doing your assignment EARLY. Waiting until the last few hours is definitely not a wise
move.
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Question 1.
Consider a system with a single processor. The system has a single disk that is available SJF to
only one process at any given time non-preemptively(i.e., if one process is accessing the disk, the
other process has to wait for that process to finish using the disk before it can access it).
Suppose that the following three processes arrive at the times indicated in the table below. Each
process makes a CPU burst, then an I/O burst to access the disk, another CPU burst, another I/O
burst to access the disk and finally terminates with a CPU burst. The lengths of these bursts are
given in milliseconds in the table below:
Process
1
2
3
CPU burst 1
2
2
1
Burst Time
I/O burst 1 CPU burst 2
4
2
2
3
2
1
We make the following two assumptions:
I/O burst 2
2
3
1
CPU burst 3
2
1
1
Arrival
Time
0
1
1
1. If a new process arrives at the same time that a process finishes using the CPU, then the
new process will be added to the ready queue first.
2. If an I/O completion and a CPU timeout of two processes occur at the same time, the
process completing I/O is added to the ready queue first.
Part A.Fill in the three Gantt charts that illustrate the execution of these processes using the
following scheduling algorithms:
1. Shortest Job First (SJF) algorithm
2. Round robin (RR) algorithm with quantum = 1
On each Gantt chart, use the following symbols to indicate the state of the process:
Symbol
x
r
w
i
t
State
Process is running (executing)
Process is ready
Process is waiting
Process is doing I/O
Process has terminated
Also show the processes on the ready queue and disk queue.
Disk queue
Ready queue
Process
1. Shortest Job First (SJF) algorithm
Time
P1
P2
P3
Pid
(head
)
Pid
Pid
(tail)
Pid
(head
)
Pid
Pid
(tail)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
13
14
15
16
17
18
19
20
Disk queue
Ready queue
Process
2. Round robin (RR) algorithm with time quantum = 1
0 1 2 3 4 5 6 7 8 9 10 11 12
Time
P1
P2
P3
Pid
(head)
Pid
Pid
(tail)
Pid
(head)
Pid
Pid
(tail)
Part B: What is the waiting time of each process for each of the above scheduling algorithms?
What is the average waiting time?
Process
Wait time
SJF
RR (q=1)
P1
P2
P3
Average
Part C: What is the turnaround time of each process for each of the above scheduling
algorithms? What is the average turnaround time?
Process
P1
P2
P3
Average
Turnaround time
SJF
RR (q=1)
21
Question 2
Consider two user processes P1 and P2 in a uniprocessor system. P1 has two threads. P2 has
three threads. The CPU burst times for the threads are shown in the table below:
Process
P1
P2
Thread
T11
T12
T21
T22
T23
CPU burst time
1
3
3
2
1
In the following questions, assume the context switch time to be zero.
a)Suppose the one-to-one model is being used to map the user threads to the kernel threads. The
scheduling algorithm being used by the kernel is Round Robin with time quantum = 1 ms. The
ready queue at time 0 is as follows:
T22 T11 T23 T12 T21
where T21 is at the head of the queue.
Draw a Gantt chart that illustrates the execution of these threads.
Time
0
1
2
3
4
5
6
7
8
9
b)Suppose the threads are implemented entirely at the user level. A many-to-one model is used
for mapping the user threads to the underlying kernel thread for the process. The scheduling
algorithm used for the kernel threads is Round Robin with a time quantum of 2 ms.The
scheduling algorithm for the user threads is Round Robin with a time quantum of 1 ms. Suppose
the processes are moved to the ready state in the following order:
P1andP2
InP1:T11andT12
InP2:T21andT22andT23
Draw a Gantt chart that illustrates the execution of these threads.
Time
0
1
2
3
4
5
6
7
8
9