The document discusses process scheduling in operating systems. It covers basic concepts like scheduling criteria, algorithms like FCFS, SJF, priority and round robin scheduling. It explains key process states and scheduling techniques like preemptive and non-preemptive. Examples are provided to illustrate different scheduling algorithms and how they optimize criteria like waiting time, response time and CPU utilization.

1. Chapter 5: Process Scheduling

2. Chapter 5: Process Scheduling
 Basic Concepts
 Scheduling Criteria
 Scheduling Algorithms (6)
 Multiple-Processor Scheduling
 Thread Scheduling
 OS Examples
 Algorithm Evaluation

3. Basic Concepts
 In a multiprogramming system, multiple processes exist c
oncurrently in main memory.
 Each process alternates between using a processor and
waiting for some event to occur, such as the completion o
f an I/O operation.
 The processor is kept busy by executing one process whil
e the others wait.
 The key to multiprogramming is scheduling.
 Process Scheduling is
 the basis of multi-programmed operating system
 a fundamental function of operating-system.

4. Basic Concepts
 In a single-processor system
 Only one process can run at a time
 Any others must wait until the CPU is free and can be rescheduled.
 When the running process goes to the waiting state,
 the OS may select another process to assign CPU to improve
CPU utilization.
 Every time one process has to wait, another process can take over
use of the CPU
 Process scheduling is
 to select a process from the ready queue and assign the CPU

5. Diagram of Process State from ch.3
 It is important to realize that only one process can be running on any
processor at any instant.
 Many processes may be ready and waiting states.

6. Process Scheduling from ch.3
• The process selection is carried out by the short-term scheduler (or
CPU scheduler).
• The scheduler selects a process from the processes in memory that are
ready to execute and allocates the CPU to that process.

7. CPU - I/O burst Cycle
 Process execution consists of
 a cycle of CPU execution (CPU burst) and I/O wait (I/O burst)
 Process alternate between these two states
 Process execution begins with a CPU burst, which is followed by
an I/O burst, and so on.
 Eventually, the final CPU burst ends with an system call to
terminate execution.
 CPU burst distribution of a process
 varies greatly from process to process and from computer to
computer

8. Alternating Sequence of CPU & I/O Bursts
CPU burst time
I/O burst time
CPU burst time
CPU burst time

9. Histogram of CPU-burst Times
 CPU burst distribution is generally characterized
 as exponential or hyper-exponential
 with large number of short CPU burst and small number of long CPU burst
 I/O bound process has many short CPU bursts
 CPU bound process might have a few long CPU bursts.

10. Process Scheduler
 selects one of the processes in memory that are ready to
execute, and allocates the CPU to the selected process.
 CPU scheduling decisions may take place when a
process:
1. switches from running to waiting state: I/O request, invocation of
wait() for the termination of other process
2. switches from running to ready state: when interrupt occurs
3. switches from waiting to ready: at completion of I/O
4. terminates

11. Process Scheduler(cont.)
 Scheduling under 1 and 4 is non-preemptive
 Scheduling under 2 and 3 is preemptive

12. Non-preemptive vs. Preemptive
 Non-preemptive scheduling
 Once the CPU has been allocated to a process, the process keeps the
CPU until it releases the CPU
 either by terminating or by switching to the waiting state.
 used by Windows 3.x
 Preemptive scheduling
 Current running process can be switched with another at any time
 because interrupt can occur at any time
 Most of modern OS provides this scheme. (Windows XP, Max OS, UNIX)

13. 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
CPU-scheduling function
? ?

14. Context Switch from ch. 3

15. Scheduling Criteria
 Based on the scheduling criteria, the performance of various scheduling
algorithm could be evaluated.
 Different scheduling algorithms have different properties.
 CPU utilization –keep the CPU as busy as possible. i.e., ratio (%) of the
amount of time while the CPU was busy per time unit.
 Throughput – # of processes that complete their execution per time unit.
 Turnaround time – the interval from the time of submission of a process
to the time of completion. Sum of the periods spent waiting to get into
memory, waiting in the ready queue, executing on the CPU, and doing
I/O
 Waiting time – Amount of time a process has been waiting in the ready
queue, which is affected by scheduling algorithm
 Response time – In an interactive system, amount of time it takes from
when a request was submitted until the first response is produced, not
output (for time-sharing environment)

16. Optimization Criteria
 It is desirable to maximize:
 The CPU utilization
 The throughput
 It is desirable to minimize:
 The turnaround time
 The waiting time
 The response time
 However in some circumstances, it is desirable to
optimize the minimum or maximum values rather than
the average.
 Interactive systems, it is more important to minimize the variance
in the response time than minimize the average response time.

17. Process Scheduling Algorithms
 First-Come, First-Served Scheduling (FCFS)
 Shortest-Job-First Scheduling (SJF)
 Priority Scheduling
 Round-Robin Scheduling
 Our measure of comparison is the average waiting time.

18. First-Come, First-Served (FCFS) Scheduling
 The process that request the CPU first is allocated the
CPU first.
Process Burst Time(ms)
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 = 17ms
P1 P2 P3
24 27 30
0

19. FCFS Scheduling
Suppose that the processes arrive in the order
P2 , P3 , P1
 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
P1
P3
P2
6
3 30
0

20. FCFS Scheduling
 FCFS scheduling algorithm is non-preemptive
 Once the CPU has been allocated to a process, that process keeps
the CPU until it releases the CPU, either by terminating or by
requesting I/O.
 is particularly troublesome for time-sharing systems (response
time ).
 Convoy effect (short process behind long process)occurs:
 When one CPU-bound process with long CPU burst and many
I/O-bound process which short CPU burst.
 All I/O bound process waits for the CPU-bound process to get off
the CPU while I/O is idle
 All I/O- and CPU- bound processes executes I/O operation while
CPU is idle.
 results in low CPU and device utilization

21. Shortest-Job-First (SJF) Scheduling
 SJF associates with each process the length of its next
CPU burst.
 use these lengths to schedule the process with the
shortest time
 Two schemes:
 non-preemptive – 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 known as the Shortest-Remaining-Time-First (SRTF)
 SJF is optimal – gives minimum average waiting time for a
given set of processes

22. Example of Non-Preemptive SJF
Process Burst Time
P1 6
P2 8
P3 7
P4 0 3
 SJF scheduling chart (non-preemptive)
 Average waiting time = (3 + 16 + 9 + 0) / 4 = 7
P4
P3
P1
3 16
0 9
P2
24

23. Example of Preemptive SJF (SRTF)
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

24. How do we know the length of the next CPU
burst?
 by computing an approximation of the length of the next
CPU burst (estimate the length of the next CPU burst)
 can be done by using the length of previous CPU bursts,
using exponential averaging
 The value of tn contains our most recent information.
 n+1 stores the past history
 The parameter  controls the relative weight of recent and
past history in our prediction.
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25. Prediction of the Length of the Next CPU Burst
 In this example, 0 = 10,  = ½ , t1=6
 1 =  x t0 + (1- ) x 0 = ½ x 6 + ½ x 10 = 8
 2 =  x t2 + (1- ) x 1 = ½ x 4 + ½ x 8 = 6

26. Examples of Exponential Averaging
  = 0
 n+1 = n = n-1 = n-2 . … = 0
 Recent history does not count
  = 1
 n+1 = tn
 Only the actual last CPU burst counts i.e., the most recent CPU
burst
 If we expand the formula, we get:
n+1 =  tn + (1 - ) tn - 1 + …
+ (1 -  )j  tn -j + …
+ (1 -  )n +1 0
 Since both  and (1 - ) are less than or equal to 1, each
successive term has less weight than its predecessor

27. 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
 Non-preemptive

28. Process Burst Time Priority arrival time
P1 10 3 0
P2 1 1 0
P3 2 4 0
P4 1 5 0
P5 5 2 0
 Priority Scheduling (non-preemptive)
 Average waiting time = (0 + 1 + 6 + 16 + 18)/5 = 8.2
Example 1 of Non-Preemptive Priority
P2 P1 P3
16
1
0
P4
18
P5
6 19

29. Example 2 of Non-Preemptive Priority
Process Ready queue arrive time CPU burst time Priority
P1 0 3 ms 5
P2 1 7 ms 3
P3 4 2 ms 4
P4 2 3 ms 3
P5 6 4 ms 1
P1 P5
P2 P3
P4
0 3 17
14
10 19
scheduling chart (non-preemptive)
Average waiting time= 31/5 = 6.2 ms
Total Waiting time = (0-0) + (3-1) +(17-4) + (14-2) + (10-6) = 0+2+13+12+4=31 ms
Total Turnaround time= (3-0) + (10-1) + (19-4) + (17-2) + (14- 6) = 50 ms
Average Turnaround time= 50/5 = 10ms

30. Example of Preemptive Priority
Process Ready queue arrive time Cpu burst time Priority
P1 0 3 ms 5
P2 1 7 ms 3
P3 4 2 ms 4
P4 2 3 ms 3
P5 6 4 ms 1
P3
P5
P2
P1 P4 P2 P1
0 6 13
1 17
10 15 19
scheduling chart (preemptive)
Average waiting time= 42/5= 8.4 ms
Total Waiting time = (0-0+17-1) + (1-1+13-6) +(15-4) + (10-2) + (6-6) =42ms
Total Turnaround time= (19-0) + (15-1) + (17-4) + (13-2) + (10-6) = 61ms
Average Turnaround time= 61/5 = 12.2ms
Process P1 preempted by p2 because p2 has higher priority

31. Priority Scheduling
 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
127
0

32. Round Robin (RR) Scheduling
 It is similar to FCFS scheduling, but preemption is
added to enable the system to switch between
process.
 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.

33.  If CPU cycle (CPU burst time ) > time quantum
 Job is preempted and put at the end of the READY Q
 If CPU cycle < time quantum
 If job finished
 resources released
 If interrupted by I/O request
 Info saved in PCB
 Linked at end of appropriate I/O queue
 When I/O complete, job returns to READY Q

34. Example of RR with Time Quantum = 4
Process Burst Time Arrival Time
P1 24 0
P2 3 0
P3 3 0
 The Gantt chart is:
P1 P2 P3 P1 P1 P1 P1 P1
0 4 7 10 14 18 22 26 30
Average waiting time= 17/3= 5.66 ms
Total Waiting time = (0-0+10-4) + (4-0) +(7-0) =17ms
Total Turnaround time= (30-0) + (7-0) + (10-0) = 47ms
Average Turnaround time= 47/5= 9.4 ms
Context switch ???

35. Example of RR with Time Quantum = 10
 The Gantt chart is:
Process Ready queue arrive time CPU burst time
P1 0 7 ms
P2 1 13 ms
P3 5 25 ms
P4 6 22 ms
P5 8 40 ms
0 17 74
7 27 37 47 50 60 64
P2 P2
P1 P3 P4 P5 P3 P4 P5
79
P3 P5
89
P5
99
Average waiting time= 180/5= 36 ms
Total Waiting time = (0-0) + (7-1 + 47-17 )+(17-5+ 50-27+ 74- 60)+(27-6+60-37)+
(37-8+64-47+79-64 ) = 180 ms
Total Turnaround time= (7-0) + (50-1) + (79-5)+(64-6)+(99-8) = 279ms
Average Turnaround time= 279/5= 55.8 ms

36. Round Robin (RR) Scheduling
 Performance depends on the size of the time
quantum.
 If the time quantum is extremely large, the RR policy is
the same as the same as the FCFS policy
 If the time quantum is extremely small (say 1 millisec) ,
the RR approach is called processor sharing i.e.,
provides high concurrency: each of n processes has its
own processor running at 1/n the speed of the real
processor

37. Time Quantum and Context Switch Time
In (a) the job finishes before the time quantum expires.
In (b) and (c), the time quantum expires first, interrupting the job
 The effect of context switching on the performance of RR
scheduling, for example one process of 10 time quantum.
 quantum = 12 time units, finished in less than 1 time quantum
 quantum = 6 time units, requires 2 quanta, 1 context switch
 quantum = 1 time units, requires 10 quanta, 9 context switch
a
b
c

38. Round Robin (RR) Scheduling
 The time quantum q must be large with respect to context
switch, otherwise overhead is too high
 If the context switching time is 10% of the time quantum,
then about 10% of the CPU time will be spent in context
switching
 Most modern OS have time quanta ranging from 10 to 100
milliseconds,
 The time required for a context switch is typically less than
10 microseconds; thus the context-switch time is a small
fraction of the time quantum.

39. Turnaround Time varies with the Time Quantum
 The turnaround time depends on the size of
the time quantum
 The average turnaround
time of a set of processes
dose not necessarily improve
as the time quantum size
Increased.
 The average turnaround
time can be improved
if most processes finish
their next CPU burst
in a single time quantum.

40. Scheduling Algorithm with multi-Queues
 Multi-level Queue Scheduling
 Multi-level Feedback Queue Scheduling

41. Multilevel Queue
 Ready queue is partitioned into separate queues:
foreground (interactive)
background (batch)
 The processes are permanently assigned to one queue, generally
based on some property, or process type.
 Each queue has its own scheduling algorithm
 foreground – RR
 background – FCFS
 Scheduling must be done between the queues
 Fixed priority scheduling - serve all from foreground then from
background, Possibility of starvation.
 Time slice scheduling – 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

42. Multilevel Queue Scheduling
 No process in the batch queue could run unless the queues with high
priority were all empty.
 If an interactive editing process entered the ready queue while a batch
process was running, the batch process would be preempted.

43. Multilevel Feedback Queue
 A process can move between the various queues; aging
can be implemented in this way
 Multilevel-feedback-queue scheduler defined by the
following parameters:
 number of queues
 scheduling algorithms for each queue
 method used to determine when to upgrade a process
 method used to determine when to demote a process
 method used to determine which queue a process will enter when
that process needs service

44. Example of Multilevel Feedback Queue
 Three queues:
 Q0 – RR with time quantum 8 milliseconds
 Q1 – RR time quantum 16 milliseconds
 Q2 – FCFS
 Scheduling
 A new job enters queue Q0 which is served RR. When it gains CPU,
job receives 8 milliseconds. If it does not finish in 8 milliseconds,
job is moved to queue Q1.
 At Q1 job is again served RR and receives 16 additional
milliseconds. If it still does not complete, it is preempted and
moved to queue Q2.
 The job is serverd based on FCFS in queue Q2

45. Multilevel Feedback Queues

46. Summary
 CPU scheduling is the task of selecting a waiting process from the
ready queue and allocating the CPU to it.
 The CPU is allocated to the selected process by the dispatcher.
 FCFS scheduling is simple, cause short processes to wait for long time
 SJF scheduling is provably optimal, providing the shortest averaging
waiting time. But predicting the length of the next CPU bursts is difficult.
 Priority scheduling allocates the CPU to the heights priority process.
 Both priority and SJF may suffer from starvation. Aging is a technique to
prevent starvation.
 RR scheduling is more appropriate for a time-shared system.
 Major problem of RR scheduling is the selection of the time quantum.
 FCFS is non-preemptive, RR is preemptive, SJF and Priority may be
preemptive and non-preemptive.
 Multilevel queue allows different scheduling algorithm for each queue.
 Multilevel feedback queue allow process to move from one queue to
another.

47. End of Chapter 5