Transcript Module 6: CPU Scheduling

Lecture 8
Chapter 5: CPU Scheduling
Modified from Silberschatz, Galvin and Gagne ©2009
Chapter 5: CPU Scheduling
 Basic Concepts
 Scheduling Criteria
 Scheduling Algorithms
 Thread Scheduling
 Multiple-Processor Scheduling
 Operating Systems Examples
 Algorithm Evaluation
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Objectives
 To introduce CPU scheduling, which is the basis for multiprogrammed
operating systems
 To describe various CPU-scheduling algorithms
 To discuss evaluation criteria for selecting a CPU-scheduling algorithm for
a particular system
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Basic Concepts
 Maximum CPU utilization obtained with multiprogramming
 CPU–I/O Burst Cycle: Process execution
consists of a cycle of CPU execution and
I/O wait

Alternating Sequence of CPU And I/O Bursts
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Histogram of CPU-burst Times
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CPU Scheduler
 Selects from among the processes in memory that are ready to execute,
and allocates the CPU to one of them
 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
 Scheduling under 1 and 4 is nonpreemptive

Processes keep CPU until it releases either by terminating or I/O wait.
 All other scheduling is preemptive

Interrupts
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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
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Scheduling Criteria
 CPU utilization – keep the CPU as busy as possible

Typically between 40% to 90%
 Throughput – # of processes that complete their execution per time unit

Depends on the length of process
 Turnaround time – amount of time to execute a particular process

Sum of wait for memory, ready queue, execution, and I/O.
 Waiting time – amount of time a process has been waiting in the ready
queue

Sum of wait in ready queue
 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
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Scheduling Algorithm Optimization Criteria
 Max CPU utilization
 Max throughput
 Min turnaround time
 Min waiting time
 Min response time
 In most cases, systems optimize average measure
 It is important to minimize variance

Users prefer predictable response time to faster system with
high variances.
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First-Come, First-Served (FCFS) Scheduling
Process
P1
P2
P3
Burst Time
24
3
3
 Suppose that the processes arrive in the order: P1 , P2 , P3
The Gantt Chart for the schedule is:
P1
P2
0
24
 Waiting time for P1 = 0; P2 = 24; P3 = 27
 Average waiting time: (0 + 24 + 27)/3 = 17
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P3
27
30
FCFS Scheduling (Cont)
Suppose that the processes arrive in the order
P2 , P3 , P1
 The Gantt chart for the schedule is:
P2
0
P3
3
P1
6
30
 Waiting time for P1 = 6; P2 = 0; P3 = 3
 Average waiting time: (6 + 0 + 3)/3 = 3
 Much better than previous case
 Nonpreemtive
 Convoy effect short process behind long process
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Shortest-Job-First (SJF) Scheduling
 Associate with each process the length of its next CPU burst.

Use these lengths to schedule the process with the shortest time

shortest-next-CPU-burst
 SJF is optimal

Gives minimum average waiting time for a given set of processes

The difficulty is knowing the length of the next CPU request
 SFJ scheduling is preferred for long-term scheduling
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Example of SJF
Process
Arrival Time
Burst Time
P1
0.0
6
P2
2.0
8
P3
4.0
7
P4
5.0
3
 SJF scheduling chart
P4
0
P3
P1
3
9
P2
16
 Average waiting time = (3 + 16 + 9 + 0) / 4 = 7
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Determining Length of Next CPU Burst
 Can only estimate the length
 Can be done by using the length of previous CPU bursts

using exponential averaging
1. t n  actual length of n th CPU burst
2.  n 1  predicted value for the next CPU burst
3.  , 0    1
 n 1   t n  1    n .
4. Define :
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Examples of Exponential Averaging
  =0
n+1 = n
 Recent history does not count

  =1
n+1 =  tn
 Only the actual last CPU burst counts

 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
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Prediction of the Length of the Next CPU Burst
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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
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