operating systems , ch-05, (CPU Scheduling), 3rd level, College of Computers, Seiyun University. انظمة التشغيل لطلاب المستوى الثالث بكلية الحاسبات بجامعة سيئون المحاضرة 05

1. Chapter 5: CPU Scheduling

2. • CPU scheduling is the central in multi-programming
system.
2
• Maximum CPU utilization obtained with
multiprogramming (prevent CPU from being idle).
•Processes residing in the main memory is selected by
the Scheduler that is:
 Concerned with deciding a policy about which process is
to be selected.
Process selection based on a scheduling algorithm.
Basic Concepts

3. Basic Concepts
3

4. Basic Concepts
4

5. • Process execution consists of a cycle of
CPU execution and I/O wait.
• Processes alternate between these two
states. Process execution begins with a
CPU burst. That is followed by an I/O
burst, which is followed by another
CPU burst, then another I/O burst, and
so on.
• CPU bursts vary greatly from process to
process and from computer to computer.
Basic Concepts
5

6. Schedulers
•Long-term scheduler chooses some of them to go to
memory (ready queue).
•Then, short-term scheduler (or CPU scheduler) chooses
from ready queue a job to run on CPU.
•Medium-term scheduler may move (swap) some
partially-executed jobs from memory to disk (to enhance
performance).
6
Basic Concepts

7. CPU Scheduler
•Whenever the CPU becomes idle, the operating system
must select one of the processes in the ready queue to be
executed. The selection process is carried out by the short-
term scheduler, or CPU scheduler.
7
Basic Concepts

8. CPU scheduling decisions may take place when a process:
1. Switches from running to waiting state
2. Switches from running to ready state
3. Switches from waiting to ready
4. Terminates
8
Basic Concepts

9. Scheduling can be
•Non-preemptive
 Once a process is allocated the
CPU, it does not leave until terminate.
•Preemptive
 OS can force (preempt) a process
from CPU at anytime.
 Say, to allocate CPU to another higher-priority process.
9
Basic Concepts

10. Non-preemptive and Preemptive
•Preemptive is harder: Need to maintain consistency of
data shared between processes, and more importantly,
kernel data structures (e.g., I/O queues).
10
Basic Concepts

11. Dispatcher
•Dispatcher module gives control of the CPU to the
process selected by the short-term scheduler; this involves:
 Switching context.
 Switching to user mode.
 Jumping to the proper location in the user program to restart
that program.
•Dispatch latency – time it takes for the dispatcher to stop
one process and start another running.
11
Basic Concepts

12. • CPU utilization – keep the CPU as busy as possible.
• Throughput – number of processes that complete their
execution per time unit.
• Turnaround time – amount of time to execute a particular
process. (time from submission to termination)
• Waiting time – amount of time a process has been waiting in
the ready queue.
• Response time – amount of time it takes from when a request
was submitted until the first response is produced, not output.
12
Scheduling Criteria

13. SchedulingAlgorithm Optimization Criteria
• Max CPU utilization.
• Max throughput.
• Min turnaround time.
• Min waiting time.
• Min response time.
13
Scheduling Criteria

14. • There are many different CPU-scheduling algorithms:
1. First Come, First Served (FCFS).
2. Shortest Job First (SJF).
 Preemptive SJF.
 Non-Preemptive SJF.
3. Priority.
4. Round Robin.
5. Multilevel queues.
14
Scheduling Algorithms

15. 1. First-Come, First-Served (FCFS) Scheduling
Scheduling Algorithms
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:
Note: A process may have many CPU bursts, but in the following
examples we show only one for simplicity.

16. First-Come, First-Served (FCFS) Scheduling
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:
P1 P2 P3
24 27 30
0
Waiting Time
𝑷
𝟏 𝑷
𝟐 𝑷
𝟑
0 24 27
Average waiting time:
(0 + 24 + 27)/3 = 17
Turnaround Time
𝑷𝟏 𝑷𝟐 𝑷𝟑
24 27 30
Average turnaround time:
(24 + 27 + 30)/3 = 27
Response Time
𝑷
𝟏 𝑷
𝟐 𝑷
𝟑
0 24 27
Average response time:
(0 + 24 + 27)/3 = 17

17. FCFS Scheduling (Cont.)
Suppose that the processes arrive in the order
P2 , P3 , P1
 The Gantt chart for the schedule is:
P1
P3
P2
6
3 30
0
Waiting Time
𝑷
𝟏 𝑷
𝟐 𝑷
𝟑
6 0 3
Average waiting time:
(6 + 0 + 3)/3 = 3
Turnaround Time
𝑷
𝟏 𝑷
𝟐 𝑷
𝟑
30 3 6
Average turnaround time:
(30 + 3 + 6)/3 = 13
Response Time
𝑷
𝟏 𝑷
𝟐 𝑷
𝟑
6 0 3
Average response time:
(6 + 0 + 3)/3 = 3
• Much better than previous case
• Convoy effect short process behind long process

18. Shortest-Job-First (SJR) Scheduling
 Associate with each process the length of its next CPU
burst. Use these lengths to schedule the process with the
shortest time
 Two schemes:
 nonpreemptive – once CPU given to the process it
cannot be preempted until completes its CPU burst
 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

19. Process Arrival Time Burst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
 SJF (non-preemptive)
 Average waiting time = (0 + 6 + 3 + 7)/4 = 4
Example of Non-Preemptive SJF
P1 P3 P2
7
3 16
0
P4
8 12

20. Example of Preemptive SJF
Process Arrival Time Burst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
 SJF (preemptive)
 Average waiting time = (9 + 1 + 0 +2)/4 = 3
P1 P3
P2
4
2 11
0
P4
5 7
P2 P1
16

21. Priority Scheduling
 A priority number (integer) is associated with each process
 The CPU is allocated to the process with the highest
priority (smallest integer  highest priority)
 Preemptive
 nonpreemptive
 SJF is a priority scheduling where priority is the predicted
next CPU burst time
 Problem  Starvation – low priority processes may never execute
 Solution  Aging – as time progresses increase the priority of the
process

22. 3. Priority Scheduling
(2/25)
Process Burst Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
Priority scheduling Gantt Chart
22

23. 3. Priority Scheduling
(2/25)
Process Burst Time Priority
P1 10 3
P2 1 1 Highest priority
P3 2 4
P4 1 5
P5 5 2
Priority scheduling Gantt Chart
23

24. 3
. Priority Scheduling
Scheduling Algorithms
Process Burst Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
Priority scheduling Gantt Chart
24

25. 3. Priority Scheduling
(3/25)
25
Process Burst Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
Priority scheduling Gantt Chart

26. 3. Priority Scheduling
(3/25)
26
Process Burst Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
Priority scheduling Gantt Chart

27. 3. Priority Scheduling
(3/25)
27
Process Burst Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
Priority scheduling Gantt Chart

28. 3. Priority Scheduling
Scheduling Algorithms
28
Process Burst Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
Priority scheduling Gantt Chart
Waiting Time
𝑷
𝟏 𝑷
𝟐 𝑷
𝟑 𝑷
𝟒 𝑷
𝟓
6 0 16 18 1
Average Waiting time =
[6+0+16+18+1]/5 = 8.2 msec
Turnaround Time
𝑷
𝟏 𝑷
𝟐 𝑷
𝟑 𝑷
𝟒 𝑷
𝟓
16 1 18 19 6
Average Turnaround time =
[16+1+18+19+6]/5 = 12 msec
Response Time
𝑷
𝟏 𝑷
𝟐 𝑷
𝟑 𝑷
𝟒 𝑷
𝟓
6 0 16 18 1
Average Response time =
[6+0+16+18+1]/5 = 8.2 msec

29. Round Robin (RR)
 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.
 If there are n processes in the ready queue and the time
quantum is q, then each process gets 1/n of the CPU time in
chunks of at most q time units at once. No process waits
more than (n-1)q time units.
 Performance
 q large  FIFO
 q small  q must be large with respect to context switch,
otherwise overhead is too high

30. Example of RR with Time Quantum = 20
Process Burst Time
P1 53
P2 17
P3 68
P4 24
 The Gantt chart is:
 Typically, higher average turnaround than SJF, but better response
P1 P2 P3 P4 P1 P3 P4 P1 P3 P3
0 20 37 57 77 97 117 121 134 154 162

31. Time Quantum and Context Switch Time

32. 4. Round Robin (RR) Scheduling
Scheduling Algorithms
Process Burst Time
P1 24
P2 3
P3 3
All the processes arrive at the same time 0
dnuoR .
:mutnauq fo gniludehcs )RR( niboR
4 ms
32

33. 4. Round Robin (RR) Scheduling
Scheduling Algorithms
33
Process Burst Time
P1 24
P2 3
P3 3
All the processes arrive at the same time 0
.
:mutnauq fo gniludehcs )RR( niboR dnuoR
4 ms
P1
0 4 7 10 14 18 22 26 30
P2 P3 P1 P1 P1 P1 P1

34. 4. Round Robin (RR) Scheduling
Scheduling Algorithms
34
Process Burst Time
P1 24
P2 3
P3 3
All the processes arrive at the same time 0
.
:mutnauq fo gniludehcs )RR( niboR dnuoR
4 ms
P1
0 4 7 10 14 18 22 26
30
P2
P3 P1 P1 P1 P1 P1

35. 4. Round Robin (RR) Scheduling
Scheduling Algorithms
35
Process Burst Time
P1 24
P2 3
P3 3
All the processes arrive at the same time 0
.
:mutnauq fo gniludehcs )RR( niboR dnuoR
4 ms
0 4 7 10 14 18 22 26 30
P1 P2 P3 P1 P1 P1 P1 P1

36. 4. Round Robin (RR) Scheduling
Scheduling Algorithms
36
Process Burst Time
P1 24
P2 3
P3 3
All the processes arrive at the same time 0
.
:mutnauq fo gniludehcs )RR( niboR dnuoR
4 ms
P1 P2 P3 P1
0 14 18 22 26 30
P1 P1 P1 P1
4 7 10

37. 4. Round Robin (RR) Scheduling
Scheduling Algorithms
37
Process Burst Time
P1 24
P2 3
P3 3
All the processes arrive at the same time 0
.
:mutnauq fo gniludehcs )RR( niboR dnuoR
4 ms
P1 P2 P3 P1 P1
0 18 22 26 30
P1 P1 P1
14
4 7 10

38. 4. Round Robin (RR) Scheduling
Scheduling Algorithms
38
Process Burst Time
P1 24
P2 3
P3 3
All the processes arrive at the same time 0
.
:mutnauq fo gniludehcs )RR( niboR dnuoR
4 ms
P1 P1
0 18 26 30
14
4 7 10 22
P1 P2 P3 P1 P1 P1

39. 4. Round Robin (RR) Scheduling
Scheduling Algorithms
39
Process Burst Time
P1 24
P2 3
P3 3
All the processes arrive at the same time 0
.
:mutnauq fo gniludehcs )RR( niboR dnuoR
4 ms
P1 P2 P3 P1 P1 P1 P1 P1
0 18 30
26
14
4 7 10 22

40. 4. Round Robin (RR) Scheduling
Scheduling Algorithms
40
Process Burst Time
P1 24
P2 3
P3 3
All the processes arrive at the same time 0
.
:mutnauq fo gniludehcs )RR( niboR dnuoR
4 ms
P1 P2 P3 P1 P1 P1 P1 P1
0 18 30
26
14
4 7 10 22

41. 4. Round Robin (RR) Scheduling
Scheduling Algorithms
41
Process Burst Time
P1 24
P2 3
P3 3
All the processes arrive at the same time 0
niboR dnuoR .
:mutnauq fo gniludehcs )RR(
4 ms
4 7 10
# of context switches = 7
P1 P2 P3 P1 P1 P1 P1 P1
0 18 30
26
14 22

42. 4. Round Robin (RR) Scheduling
Scheduling Algorithms
(11/25)
Process Burst Time
P1 24
P2 3
P3 3
All the processes arrive at the same time 0.
Round Robin (RR) scheduling of quantum: 4 ms
P1 P2 P3 P1 P1 P1 P1 P1
42
0 18 30
26
14
4 7 10 22
Waiting Time
𝑷
𝟏 𝑷
𝟐 𝑷
𝟑
0
( +
10
−
4
) 4 7
Average waiting time:
(6 + 4 + 7)/3 = 5.667 ms
Turnaround
Time
𝑷
𝟏 𝑷
𝟐 𝑷
𝟑
30 7 10
Average Turnaround time:
(30 + 7 + 10)/3 = 15.667 ms
Response Time
𝑷
𝟏 𝑷
𝟐 𝑷
𝟑
0 4 7
Average Response time:
(0 + 4 + 7)/3 = 3.667 ms

43. CHAPTER 5 - PART 2

44. Scheduling Algorithms
(16/25)
5. Multilevel Queue Scheduling
Scheduling must be done between the queues:
Fixed priority scheduling; (i.e., serve all from foreground
then from background). Possibility of starvation.
Time slice – each queue gets a certain amount of CPU time
which it can schedule amongst its processes; i.e., 80% to
foreground in RR.
20% to background in FCFS.

45. Scheduling Algorithms
(17/25)
5. Multilevel Queue Scheduling

46. Scheduling Algorithms
(18/25)
5. Multilevel Queue Scheduling
Three queues:
Q0 – RR with time quantum 8 milliseconds
Q1 – RR time quantum 16 milliseconds
Q2 – FCFS

47. Scheduling Algorithms
(19/32)
5. Multilevel Queue Scheduling
Scheduling
Anew job enters queue 𝑄
0 which is served FCFS
 When it gains CPU, job receives 8 milliseconds.
 If it does not finish in 8 milliseconds, job is moved to queue 𝑄
1
.
At 𝑄
1 job is again served FCFS and receives 16 additional milliseconds
 If it still does not complete, it is preempted and moved to queue 𝑄
2
.

48. Scheduling Algorithms
(20/32)
5. Multilevel Queue Scheduling
Now we add the concepts of varying arrival times and preemption to the
analysis
Process Arrival Time Burst Time
P1 0 7
P2 1 60
P3 2 20
P4 3 40
Multilevel Queue Fixed priority
non-preemptive
Using multi-processors
or multi-core processor

49. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(21/32)
Process
P1 P2 P
3
P4
Arrival Time Burst Time
0 7
1 60
2 20
3 40
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2

50. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(21/32)
3
𝑃1
Process Arrival Time Burst Time
P1 0 7
P2 1 60
P 2 20
P4 3 40
7ms
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
7ms

51. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(21/32)
3
Process Arrival Time Burst Time
P1 0 7
P2 1 60
P 2 20
P4 3 40
7ms
7ms 8ms
𝑃1 𝑃2
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2

52. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(21/32)
Process
P1 P2 P
Arrival Time Burst Time
0 7
1
2
3
60 52
20
40
𝑃1 𝑃2
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
3
P4
15ms
7ms 8ms

53. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(21/32)
Process
P1 P2 P
Arrival Time Burst Time
0 7
1
2
3
60 52
20
40
𝑃1 𝑃2
𝑃2
𝑃3
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
3
P4
15ms
7ms 8ms 8ms

54. Scheduling Algorithms
(21/32)
Process Arrival Time Burst Time
P1 0 7
P2 1 60 52 44
P3 2 20 12
P4 3 40
𝑃1 𝑃2
𝑃2
𝑃3
𝑄0
𝑄1
𝑄2
5. Multilevel Queue Scheduling
𝑄0
𝑄1
𝑄2
23ms
7ms 8ms 8ms

55. Scheduling Algorithms
(21/32)
𝑄0
𝑄1
𝑄2
5. Multilevel Queue Scheduling
𝑄0
𝑄1
𝑄2
Process Arrival Time Burst Time
P1 0 7
P2 1 60 52 44
P3 2 20 12
P4 3 40
23ms
7ms 8ms 8ms 8ms
𝑃1 𝑃2 𝑃3 𝑃4
𝑃2

56. Scheduling Algorithms
(21/32)
Process Arrival Time Burst Time
P1 0 7
P2 1 60 52 44 36
P3 2 20 12
P4 3 40 32
𝑃1 𝑃2
𝑃2
𝑃3
𝑄0
𝑄1
𝑄2
5. Multilevel Queue Scheduling
𝑄0
𝑄1
𝑄2
31ms
𝑃4
7ms 8ms 8ms 8ms
16ms

57. Scheduling Algorithms
(21/32)
Process Arrival Time Burst Time
P1 0 7
P2 1 60 52 44 36
P3 2 20 12
P4 3 40 32
𝑃1 𝑃2
𝑃2
𝑃3
𝑄0
𝑄1
𝑄2
5. Multilevel Queue Scheduling
𝑄0
𝑄1
𝑄2
31ms
𝑃4
7ms 8ms 8ms
𝑃2
𝑃3
8ms
16ms 12ms
36ms

58. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(21/32)
Process
P1 P2
P3 P4
Arrival Time Burst Time
0 7
1
2
3
60 52 44 36 24
20 12
40 32
𝑃1 𝑃2
𝑃2
𝑃3
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
43ms
𝑃4
7ms 8ms 8ms
𝑃2
𝑃3
8ms
16ms 12ms
36ms

59. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(21/32)
Process
P1 P2
P3 P4
Arrival Time Burst Time
0 7
1
2
3
60 52 44 36 24
20 12
40 32
𝑃1 𝑃2
𝑃2
𝑃3
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
43ms
𝑃4
7ms 8ms 8ms
𝑃2
𝑃3
8ms
16ms
𝑃4
12ms 16ms
36ms

60. Scheduling Algorithms
(21/32)
Process Arrival Time Burst Time
P1 0 7
P2 1 60 52 44 36 24 8
P3 2 20 12
P4 3 40 32 16
𝑃1 𝑃2
𝑃2
𝑃3
𝑄0
𝑄1
𝑄2
5. Multilevel Queue Scheduling
𝑄0
𝑄1
𝑄2
59ms
𝑃4
7ms 8ms 8ms
𝑃2
𝑃3
8ms
16ms
𝑃4
12ms 16ms
36ms

61. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(21/32)
Process
P1 P2
P3 P4
Arrival Time Burst Time
0 7
1
2
3
60 52 44 36 24 8
20 12
40 32 16
𝑃1 𝑃2
𝑃2
𝑃3
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
67ms
𝑃4
7ms 8ms 8ms
𝑃2
𝑃3
8ms
16ms
𝑃4
12ms 16ms
36ms

62. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(21/32)
Process
P1 P2
P3 P4
Arrival Time Burst Time
0 7
1
2
3
60 52 44 36 24 8
20 12
40 32 16
𝑃1 𝑃2
𝑃2
𝑃3
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
67ms
𝑃4
7ms 8ms 8ms
𝑃2
𝑃3
8ms
16ms
𝑃4
12ms 16ms
36ms
𝑃4
16ms

63. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(21/32)
Process
P1 P2
P3 P4
Arrival Time Burst Time
0 7
1
2
3
60 52 44 36 24 8
20 12
40 32 16
𝑃1 𝑃2
𝑃2
𝑃3
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
83ms
𝑃4
7ms 8ms 8ms
𝑃2
𝑃3
8ms
16ms
𝑃4
12ms 16ms
36ms
𝑃4
16ms

64. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(21/32)
Process
P1 P2
P3 P4
Arrival Time Burst Time
0 7
1
2
3
60 52 44 36 24 8
20 12
40 32 16
𝑃1 𝑃2
𝑃2
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
83ms
𝑃3 𝑃4
7ms 8ms
𝑃2
𝑃3
16ms
𝑃4
12ms 16ms
36ms
𝑃4
16ms
59ms67ms
43ms
7ms 15ms 23ms 31ms
8ms 8ms

65. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(22/32)
Process Arrival Time
P1
P2
P3
P4
0
1
2
3
𝑃1 𝑃2
𝑃2
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
83ms
𝑃3 𝑃4
7ms 8ms
𝑃2
𝑃3
16ms
𝑃4
12ms 16ms
36ms
𝑃4
16ms
59ms67ms
43ms
7ms 15ms 23ms 31ms
8ms 8ms
Waiting Time
𝑷𝟏 𝑷𝟐 𝑷𝟑 𝑷𝟒

66. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(22/32)
Process Arrival Time
P1
P2
P3
P4
0
1
2
3
𝑃1 𝑃2
𝑃2
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
83ms
𝑃3 𝑃4
7ms 8ms
𝑃2
𝑃3
16ms
𝑃4
12ms 16ms
36ms
𝑃4
16ms
59ms67ms
43ms
7ms 15ms 23ms 31ms
8ms 8ms
Waiting Time
𝑷𝟏 𝑷𝟐 𝑷𝟑 𝑷𝟒
0
= 𝟎

67. 5. Multilevel Queue Scheduling
(22/32)
Process Arrival Time
P1
P2
P3
P4
0
1
2
3
𝑃1 𝑃2
𝑃2
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
83ms
𝑃3 𝑃4
7ms 8ms
𝑃2
𝑃3
16ms
𝑃4
12ms 16ms
36ms
𝑃4
16ms
59ms67ms
43ms
7ms 15ms 23ms 31ms
8ms 8ms
Waiting Time
𝑷𝟏 𝑷𝟐 𝑷𝟑 𝑷𝟒
0 7 − 1
= 𝟎 = 𝟔

68. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(22/32)
Process Arrival Time
P1
P2
P3
P4
0
1
2
3
𝑃1 𝑃2
𝑃2
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
83ms
𝑃3 𝑃4
7ms 8ms
𝑃2
𝑃3
16ms
𝑃4
12ms 16ms
36ms
𝑃4
16ms
59ms67ms
43ms
7ms 15ms 23ms 31ms
8ms 8ms
Waiting Time
𝑷𝟏 𝑷𝟐 𝑷𝟑 𝑷𝟒
0 7 − 1 15 + 8 − 2
= 𝟎 = 𝟔 = 𝟐𝟏

69. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(22/32)
Process Arrival Time
P1
P2
P3
P4
0
1
2
3
𝑃1 𝑃2
𝑃2
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
83ms
𝑃3 𝑃4
7ms 8ms
𝑃2
𝑃3
16ms
𝑃4
12ms 16ms
36ms
𝑃4
16ms
59ms67ms
43ms
7ms 15ms 23ms 31ms
8ms 8ms
Waiting Time
𝑷𝟏 𝑷𝟐 𝑷𝟑 𝑷𝟒
0 7 − 1 15 + 8 − 2 23 + 12 + 8 − 3
= 𝟎 = 𝟔 = 𝟐𝟏 = 𝟒𝟎

70. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(23/32)
Process
P1 P
Arrival Time
2
P3
P4
0
1
2
3
𝑃1 𝑃2
𝑃2
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
83ms
𝑃3 𝑃4
7ms 8ms
𝑃2
𝑃3
16ms
𝑃4
12ms 16ms
36ms
𝑃4
16ms
59ms67ms
43ms
7ms 15ms 23ms 31ms
8ms 8ms
Turnaround Time
𝑷𝟏 𝑷𝟐 𝑷𝟑 𝑷𝟒

71. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(23/32)
Process
P1 P
Arrival Time
2
P3
P4
0
1
2
3
𝑃1 𝑃2
𝑃2
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
83ms
𝑃3 𝑃4
7ms 8ms
𝑃2
𝑃3
16ms
𝑃4
12ms 16ms
36ms
𝑃4
16ms
59ms67ms
43ms
7ms 15ms 23ms 31ms
8ms 8ms
Turnaround Time
𝑷𝟏 𝑷𝟐 𝑷𝟑 𝑷𝟒
7 − 0
= 𝟕

72. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(23/32)
Process
P1 P
Arrival Time
2
P3
P4
0
1
2
3
𝑃1 𝑃2
𝑃2
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
83ms
𝑃3 𝑃4
7ms 8ms
𝑃2
𝑃3
16ms
𝑃4
12ms 16ms
36ms
𝑃4
16ms
59ms67ms
43ms
7ms 15ms 23ms 31ms
8ms 8ms
Turnaround Time
𝑷𝟏 𝑷𝟐 𝑷𝟑 𝑷𝟒
7 − 0 67 − 1
= 𝟕 = 𝟔𝟔

73. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(23/32)
Process
P1 P
Arrival Time
2
P3
P4
0
1
2
3
𝑃1 𝑃2
𝑃2
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
83ms
𝑃3 𝑃4
7ms 8ms
𝑃2
𝑃3
16ms
𝑃4
12ms 16ms
36ms
𝑃4
16ms
59ms67ms
43ms
7ms 15ms 23ms 31ms
8ms 8ms
Turnaround Time
𝑷𝟏 𝑷𝟐 𝑷𝟑 𝑷𝟒
7 − 0 67 − 1 43 − 2
= 𝟕 = 𝟔𝟔 = 𝟒𝟏

74. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(23/32)
Process
P1 P
Arrival Time
2
P3
P4
0
1
2
3
𝑃1 𝑃2
𝑃2
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
83ms
𝑃3 𝑃4
7ms 8ms
𝑃2
𝑃3
16ms
𝑃4
12ms 16ms
36ms
𝑃4
16ms
59ms67ms
43ms
7ms 15ms 23ms 31ms
8ms 8ms
Turnaround Time
𝑷𝟏 𝑷𝟐 𝑷𝟑 𝑷𝟒
7 − 0 67 − 1 43 − 2 83 − 3
= 𝟕 = 𝟔𝟔 = 𝟒𝟏 = 𝟖𝟎

75. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(24/32)
Process
P1 P
Arrival Time
2
P3
P4
0
1
2
3
𝑃1 𝑃2
𝑃2
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
83ms
𝑃3 𝑃4
7ms 8ms
𝑃2
𝑃3
16ms
𝑃4
12ms 16ms
36ms
𝑃4
16ms
59ms67ms
43ms
7ms 15ms 23ms 31ms
8ms 8ms
Response Time
𝑷𝟏 𝑷𝟐 𝑷𝟑 𝑷𝟒

76. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(24/32)
Process
P1 P
Arrival Time
2
P3
P4
0
1
2
3
𝑃1 𝑃2
𝑃2
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
83ms
𝑃3 𝑃4
7ms 8ms
𝑃2
𝑃3
16ms
𝑃4
12ms 16ms
36ms
𝑃4
16ms
59ms67ms
43ms
7ms 15ms 23ms 31ms
8ms 8ms
Response Time
𝑷𝟏 𝑷𝟐 𝑷𝟑 𝑷𝟒
0 − 0
= 𝟎

77. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(24/32)
Process
P1 P
Arrival Time
2
P3
P4
0
1
2
3
𝑃1 𝑃2
𝑃2
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
83ms
𝑃3 𝑃4
7ms 8ms
𝑃2
𝑃3
16ms
𝑃4
12ms 16ms
36ms
𝑃4
16ms
59ms67ms
43ms
7ms 15ms 23ms 31ms
8ms 8ms
Response Time
𝑷𝟏 𝑷𝟐 𝑷𝟑 𝑷𝟒
0 − 0 7 − 1
= 𝟎 = 𝟔

78. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(24/32)
Process
P1 P
Arrival Time
2
P3
P4
0
1
2
3
𝑃1 𝑃2
𝑃2
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
83ms
𝑃3 𝑃4
7ms 8ms
𝑃2
𝑃3
16ms
𝑃4
12ms 16ms
36ms
𝑃4
16ms
59ms67ms
43ms
7ms 15ms 23ms 31ms
8ms 8ms
Response Time
𝑷𝟏 𝑷𝟐 𝑷𝟑 𝑷𝟒
0 − 0 7 − 1 15 − 2
= 𝟎 = 𝟔 = 𝟏𝟑

79. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(24/32)
Process
P1 P
Arrival Time
2
P3
P4
0
1
2
3
𝑃1 𝑃2
𝑃2
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
83ms
𝑃3 𝑃4
7ms 8ms
𝑃2
𝑃3
16ms
𝑃4
12ms 16ms
36ms
𝑃4
16ms
59ms67ms
43ms
7ms 15ms 23ms 31ms
8ms 8ms
Response Time
𝑷𝟏 𝑷𝟐 𝑷𝟑 𝑷𝟒
0 − 0 7 − 1 15 − 2 23 − 3
= 𝟎 = 𝟔 = 𝟏𝟑 = 𝟐𝟎

80. Scheduling Algorithms
(25/32)
5. Multilevel Queue Scheduling
Now we add the concepts of varying arrival times and preemption to the
analysis
Process Arrival Time Burst Time
P1 0 7
P2 1 60
P3 2 20
P4 3 40
Multilevel Queue Fixed priority
non-preemptive
Using single-core
processor

81. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(26/32)
𝑄0
𝑄1
𝑄2
𝑄0
𝑄1
𝑄2
1
Process Arrival Time Burst Time
P 0 7
P2 1 60
P3 2 20
P4 3 40

82. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(27/32)
𝑄0
𝑄1
𝑄2
𝑃1
𝑄0
𝑄1
𝑄2
7ms
1
Process Arrival Time Burst Time
P 0 7
P2 1 60
P3 2 20
P4 3 40
7ms

83. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(27/32)
𝑄0
𝑄1
𝑄2
𝑃1
𝑄0
𝑄1
𝑄2
7ms
1
Process Arrival Time Burst Time
P 0 7
P2 1 60
P3 2 20
P4 3 40
7ms

84. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(27/32)
𝑄0
𝑄1
𝑄2
Process Arrival Time Burst Time
P1 0 7
P2 1 60 52
P3 2 20
P4 3 40
𝑃1 𝑃2
7ms 8ms
15ms
7ms

85. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(27/32)
𝑄0
𝑄1
𝑄2
Process Arrival Time Burst Time
P1 0 7
P2 1 60 52
P3 2 20 12
P4 3 40
7ms 8ms
𝑃1 𝑃2 𝑃3
15ms
7ms 23ms

86. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(27/32)
𝑄0
𝑄1
𝑄2
Process
P1 P2
P3 P4
Arrival Time Burst Time
0 7
1
2
3
60 52
20 12
40 32
𝑃1 𝑃2 𝑃3 𝑃4
15ms 31ms
7ms 23ms
7ms 8ms 8ms 8ms

87. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(27/32)
𝑄0
𝑄1
𝑄2
Process
P1 P2
P3 P4
Arrival Time Burst Time
0 7
1
2
3
60 52 36
20 12
40 32
7ms 23ms
𝑃1 𝑃2 𝑃3 𝑃4
15ms 31ms
7ms 8ms 8ms 8ms
𝑃2
16ms
47ms

88. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(27/32)
𝑄0
𝑄1
𝑄2
Process
P1 P2
P3 P4 60 52 36
20 12
40 32
7ms 23ms
𝑃1 𝑃2 𝑃3 𝑃4
15ms 31ms
7ms 8ms 8ms 8ms
𝑃2
16ms
47ms
𝑃3
12ms
Arrival Time Burst Time
0 7
1
2
3
59ms

89. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(27/32)
𝑄0
𝑄1
𝑄2
Process
P1 P2
P3 P4 60 52 36
20 12
40 32
7ms 23ms
𝑃1 𝑃2 𝑃3 𝑃4
15ms 31ms
7ms 8ms 8ms 8ms
𝑃2
16ms
47ms
𝑃3
12ms
Arrival Time Burst Time
0 7
1
2
3
59ms

90. 5. Multilevel Queue Scheduling
(27/32)
𝑄0
𝑄1
𝑄2
Process
P1 P2
P3 P4 60 52 36
20 12
40 32 16
7ms 23ms
𝑃1 𝑃2 𝑃3 𝑃4
15ms 31ms
7ms 8ms 8ms 8ms
𝑃2
16ms
47ms
𝑃3
12ms
Arrival Time Burst Time
0 7
1
2
3
59ms
𝑃4
16ms
75ms

91. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(27/32)
𝑄0
𝑄1
𝑄2
Process
P1 P2
P3 P4 60 52 36
20 12
40 32 16
7ms 23ms
𝑃1 𝑃2 𝑃3 𝑃4
15ms 31ms
7ms 8ms 8ms 8ms
𝑃2
16ms
47ms
𝑃3
12ms
Arrival Time Burst Time
0 7
1
2
3
59ms
𝑃4
16ms
75ms
𝑃2
36ms
111ms

92. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(27/32)
𝑄0
𝑄1
𝑄2
Process
P1 P2
P3 P4 60 52 36
20 12
40 32 16
7ms 23ms
𝑃1 𝑃2 𝑃3 𝑃4
15ms 31ms
7ms 8ms 8ms 8ms
𝑃2
16ms
47ms
𝑃3
12ms
Arrival Time Burst Time
0 7
1
2
3
59ms
𝑃4
16ms
75ms
𝑃2
36ms
111ms

93. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(27/32)
𝑄0
𝑄1
𝑄2
Process
P1 P2
P3 P4 60 52 36
20 12
40 32 16
7ms 23ms
𝑃1 𝑃2 𝑃3 𝑃4
15ms 31ms
7ms 8ms 8ms 8ms
𝑃2
16ms
47ms
𝑃3
12ms
Arrival Time Burst Time
0 7
1
2
3
59ms
𝑃4
16ms
75ms
𝑃2
36ms
111ms127ms
𝑃4
16ms

94. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(27/32)
𝑄0
𝑄1
𝑄2
Process
P1 P2
P3 P4 60 52 36
20 12
40 32 16
7ms 23ms
𝑃1 𝑃2 𝑃3 𝑃4
15ms 31ms
7ms 8ms 8ms 8ms
𝑃2
16ms
47ms
𝑃3
12ms
Arrival Time Burst Time
0 7
1
2
3
59ms
𝑃4
16ms
75ms
𝑃2
36ms
111ms127ms
𝑃4
16ms

95. 5. Multilevel Queue Scheduling
(28/32)
Process Arrival Time Burst Time
P1 0 7
P2 1 60 52 36
P3 2 20 12
P4 3 40 32 16
15ms
7ms 23ms
𝑃1 𝑃2 𝑃3 𝑃4
31ms
7ms 8ms 8ms 8ms
𝑃2
16ms
47ms
𝑃3
12ms
59ms
𝑃4
16ms
75ms
𝑃2
36ms
111ms127ms
𝑃4
16ms

96. 5. Multilevel Queue Scheduling
(29/32)
Process Arrival Time
P1 0
P2 1
P3 2
P4 3
15ms
7ms 23ms
𝑃1 𝑃2 𝑃3 𝑃4
31ms
7ms 8ms 8ms 8ms
𝑃2
16ms
47ms
𝑃3
12ms
59ms
𝑃4
16ms
75ms
𝑃2
36ms
111ms127ms
𝑃4
16ms
Waiting Time
𝑷𝟏 𝑷𝟐 𝑷𝟑 𝑷𝟒
0 6 + 16
+ 28
13 + 24 20 + 28
+ 36
= 𝟎 = 𝟓𝟎 = 𝟑𝟕 = 𝟖𝟒

97. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(30/32)
Process Arrival Time
P1 0
P2 1
P3 2
P4 3
15ms
7ms 23ms
𝑃1 𝑃2 𝑃3 𝑃4
31ms
7ms 8ms 8ms 8ms
𝑃2
16ms
47ms
𝑃3
12ms
59ms
𝑃4
16ms
75ms
𝑃2
36ms
111ms127ms
𝑃4
16ms
Turnaround Time
𝑷𝟏 𝑷𝟐 𝑷𝟑 𝑷𝟒
7 − 0 111 − 1 59 − 2 127 − 3
= 𝟕 = 𝟏𝟏𝟎 = 𝟓𝟕 = 𝟏𝟐𝟒

98. 5. Multilevel Queue Scheduling
Scheduling Algorithms
(31/32)
Process Arrival Time
P1 0
P2 1
P3 2
P4 3
15ms
7ms 23ms
𝑃1 𝑃2 𝑃3 𝑃4
31ms
7ms 8ms 8ms 8ms
𝑃2
16ms
47ms
𝑃3
12ms
59ms
𝑃4
16ms
75ms
𝑃2
36ms
111ms127ms
𝑃4
16ms
Response Time
𝑷𝟏 𝑷𝟐 𝑷𝟑 𝑷𝟒
0 − 0 7 − 1 15 − 2 23 − 3
= 𝟎 = 𝟔 = 𝟏𝟑 = 𝟐𝟎

99. Scheduling Algorithms
(32/32)
Student Example:
Using a Multilevel Queue Scheduling algorithm in Fig.1. Consider the
following processes with the relative CPU bursts.
Process Arrival Time Burst Time
P1 0 20
P2 1 60
P3 2 5
P4 2 40
Multilevel Queue Fixed priority
non-preemptive
Fig.1