Principles of operating system

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Principles of Operating Systems
Text Book:
Operating System Concepts -
Avi Silberschatz,
Peter BaerGalvin
and Greg Gagne
Anilsatyadas Dharmapuri
Department of Computer Sciences
Pondicherry University
anilsatyadas@gmail.com

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Why Study Operating Systems?
Because OS hacking will make you a better
programmer and a better thinker.
 The OS is really large (Windows Vista is 50 million lines+).
 The OS manages concurrency.
 Concurrency leads to interesting programming challenges.
 (Interesting programming challenges can lead to wisdom.)
 OS code manages raw hardware.
 Programming raw hardware is challenging: timing dependent
behavior, undocumented behavior, HW bugs.
 OS code must be efficient, low CPU, memory, disk use.
 OS fails  machine fails. OS must fail less than user
programs.
 OS provides services that enable application programs
 … knowledge of OS will make you a better computer user
 OS basis of system security.

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Why Won’t Operating Systems Go Away?
Hardware needs an OS to be useful.
 Multiplex resources for efficiency and security.
OS is cornerstone of what makes computing fun.
The design of an MP3 player involves many OS
issues:
 Its OS implements a file system which is specially
designed to store music files.
 The OS manages communication with your PC.
 The OS reads music from the disc and buffers it in
memory.
 The OS controls the volume of the output device.
 The OS controls the display.
 The OS controls wireless network access.
Cell phone, mp3 player, DVD player, PDA, iPhone,
peer-to-peer file sharing
Systems/Theory/Artificial intelligence

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Operating Systems:
Basic Concepts and History

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Introduction to Operating Systems
An operating system is the interface between the user and
the architecture.
OS as juggler: providing the illusion of a dedicated machine
with infinite memory and CPU.
OS as government: protecting users from each other,
allocating resources efficiently and fairly, and providing
secure and safe communication
OS as complex system: keeping OS design and
implementation as simple as possible is the key to getting the
OS to work
User Applications
Operating System
Hardware
Virtual Machine Interface
Physical Machine Interface

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What is an Operating System?
Any code that runs with the hardware kernel bit set
 An abstract virtual machine
 A set of abstractions that simplify application design
 Files instead of “bytes on a disk”
Core OS services, written by “pros”
 Processes, process scheduling
 Address spaces
 Device control
 ~30% of Linux source code. Basis of stability and security
Device drivers written by “whoever”
 Software run in kernel to manages a particular vendor’s
hardware
E.g. Homer Simpson doll with USB
 ~70% of Linux source code
 OS is extensible
 Drivers are the biggest source of OS instability

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For any OS area (CPU scheduling, file systems, memory
management), begin by asking two questions
 What’s the hardware interface? (The Physical Reality)
 What is the application interface? (The Nicer Interface for
programmer producivity)
Key questions:
 Why is the application interface defined the way it is?
 Should we push more functionality into applications, the OS, or
the hardware?
 What are the tradeoffs between programmability, complexity,
and flexibility?

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Operating System Functions
Service provider
 Provide standard facilities
 File system
 Standard libraries
 Window system
 …
Coordinator: three aspects
 Protection: prevent jobs from interfering with each other
 Communication: enable jobs to interact with each other
 Resource management: facilitate sharing of resources across
jobs.
Operating systems are everywhere
 Single-function devices (embedded controllers, Nintendo, …)
 OS provides a collection of standard services
 Sometimes OS/middleware distinction is blurry
 Multi-function/application devices (workstations and servers)
 OS manages application interactions

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Why do we need operating systems?
Convenience
 Provide a high-level abstraction of physical resources.
 Make hardware usable by getting rid of warts & specifics.
 Enable the construction of more complex software systems
 Enable portable code.
 MS-DOS version 1 boots on the latest 3+ GHz Pentium.
 Would games that ran on MS-DOSv1 work well today?
Efficiency
 Share limited or expensive physical resources.
 Provide protection.

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Computer Architecture & Processes
CPU - the processor that performs the actual computation
I/O devices - terminal, disks, video board, printer, etc.
Memory - RAM containing data and programs used by the CPU
System bus - the communication medium between the CPU,
memory, and peripherals

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Evolution of Operating Systems
Why do operating systems change?
 Key functions: hardware abstraction and coordination
 Principle: Design tradeoffs change as technology changes.
Comparing computing systems from 1981 and 2007
1981 2007 Factor
MIPS 1 57,000 57,000
$/SPECInt $100K $2 50,000
DRAM size 128KB 2GB 16,000
Disk size 10MB 1TB 100,000
Net BW 9600 bps 100 Mb/s 10,000
Address bits 16 64 4
Users/machine 100 <1 100
Energy efficiency and parallelism loom on the horizon.
Data centers projected to consume 3% of US energy by next year
No more single-core CPUs

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From Architecture to OS to Application, and Back
Hardware Example OS Services User Abstraction
Processor Process management,
Scheduling, Traps,
Protections, Billing,
Synchronization
Process
Memory Management, Protection,
Virtual memory
Address space
I/O devices Concurrency with CPU,
Interrupt handling
Terminal, Mouse,
Printer, (System
Calls)
File system Management, Persistence Files
Distributed
systems
Network security,
Distributed file system
RPC system calls,
Transparent file
sharing

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From Architectural to OS to Application, and Back
OS Service Hardware Support
Protection Kernel / User mode
Protected Instructions
Base and Limit Registers
Interrupts Interrupt Vectors
System calls Trap instructions and trap vectors
I/O Interrupts or Memory-Mapping
Scheduling, error
recovery, billing
Timer
Synchronization Atomic instructions
Virtual Memory Translation look-aside buffers
Register pointing to base of page table

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Interrupts - Moving from Kernel to User Mode
User processes may not:
address I/O directly
use instructions that
manipulate OS memory
(e.g., page tables)
set the mode bits that
determine user or kernel
mode
disable and enable
interrupts
halt the machine
but in kernel mode, the OS does all these things
a status bit in a protected processor register indicates the
mode
Protected instructions can only be executed in kernel mode.
On interrupts (e.g., time slice) or system calls

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History of Operating Systems: Phases
Phase 1: Hardware is expensive, humans are cheap
 User at console: single-user systems
 Batching systems
 Multi-programming systems
Phase 2: Hardware is cheap, humans are expensive
 Time sharing: Users use cheap terminals and share servers
Phase 3: Hardware is very cheap, humans are very expensive
 Personal computing: One system per user
 Distributed computing: lots of systems per user
Phase 4: Ubiquitous computing
 Computers will be sewn into our clothes, but I hope not
implanted in our skin
 Cell phone, mp3 player, DVD player, TIVO, PDA, iPhone

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A Brief History of Operating Systems
Hand programmed machines (‘45-‘55)
Single user systems
OS = loader + libraries of common subroutines
Problem: low utilization of expensive components
= % utilization
Execution time
Execution time +
Card reader time

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Batch/Off-line processing (‘55-‘65)
Batching v. sequential execution of jobs
Card Reader:
CPU:
Printer:
Read Batch 1
Execute Batch 1 Batch 2 Batch 3
Batch 2 Batch 3
Print Batch 1 Batch 2 Batch 3
Card Reader:
CPU:
Printer:
Read Job 1
Execute Job 1 Job 2 Job 3
Job 2 Job 3
Print Job 1 Job 2 Job 3

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TapeTape
Batch processing (‘55-‘65)
Operating system = loader + sequencer + output processor
Input
Compute
Output
Card
Reader
Printer
Tape Tape
Operating System
“System Software”
User Program
User Data

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Multiprogramming (‘65-‘80)
Keep several jobs in memory and multiplex CPU
between jobs
Operating System
User Program 1
User Program 2User Program 2
User Program n
...
program P
begin
:
Read(var)
:
end P
system call Read()
begin
StartIO(input device)
WaitIO(interrupt)
EndIO(input device)
:
end Read
Simple, “synchronous” input:
What to do while we wait
for the I/O device?

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between jobs
Operating System
User Program 1
User Program n
...
Program 1
I/O
Device
k: read()
k+1:
endio()
interrupt
main{
}
}
OS
read{
startIO()
waitIO()

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between jobs
Operating System
User Program 1
User Program n
...
Program 1 Program 2OS
I/O
Device
k: read()
startIO()
interrupt
main{
read{
endio{
}
schedule()
main{
k+1:
}
}
schedule()

23

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Timesharing (‘70- )
A timer interrupt is used to multiplex CPU among
jobs
Operating System
User Program 1
User Program n
...
Program 1 Program 2OS
k+1:
schedule{
timer
interrupt
schedule{
timer
interrupt
k:
main{
}
main{
}
timer
interrupt
schedule{

25

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Operating Systems for PCs
Personal computing systems
 Single user
 Utilization is no longer a concern
 Emphasis is on user interface and API
 Many services & features not present
Evolution
 Initially: OS as a simple service provider
(simple libraries)
 Now: Multi-application systems with
support for coordination and
communication
 Growing security issues (e.g., online
commerce, medical records)

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Distributed Operating Systems
Typically support distributed services
 Sharing of data and coordination across multiple systems
Possibly employ multiple processors
 Loosely coupled v. tightly coupled systems
High availability & reliability requirements
 Amazon, CNN
OS
process
management
User
Program
CPU
LAN/WAN
OS
process management
memory management
User
Program
CPU
OS
file system
name services
mail services
CPU
Network

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Phase 4: Connecting people and their machines
 Intellectual property issues
 Information organization

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Course Overview
OS Structure, Processes and Process Management
CPU scheduling
Threads and concurrent programming
 Thread coordination, mutual exclusion, monitors
 Deadlocks
Virtual memory & Memory management
Disks & file systems
 Distributed file systems
Security

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A program that acts as an intermediary between a
user of a computer and the computer hardware.
Operating system goals:
Execute user programs
Make the computer system convenient to use.
Use the computer hardware in an efficient manner.

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Computer System Structure
Computer system can be divided into four components
 Hardware – provides basic computing resources
 CPU, memory, I/O devices
 Operating system
 Controls and coordinates use of hardware among
various applications and users
 Application programs – define the ways in which the
system resources are used to solve the computing
problems of the users
 Word processors, compilers, web browsers,
database systems, video games
 Users
 People, machines, other computers

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Four Components of a Computer System

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Operating System Definition
OS is a resource allocator
 Manages all resources
 Decides between conflicting requests for efficient and fair
resource use
OS is a control program
 Controls execution of programs to prevent errors and
improper use of the computer

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Operating System Structure
Multiprogramming needed for efficiency
 Single user cannot keep CPU and I/O devices busy at all times
 Multiprogramming organizes jobs (code and data) so CPU always has
one to execute
 A subset of total jobs in system is kept in memory
 One job selected and run via job scheduling
 When it has to wait (for I/O for example), OS switches to another
job
Timesharing (multitasking) is logical extension in which CPU switches
jobs so frequently that users can interact with each job while it is
running, creating interactive computing
 Response time should be < 1 second
 Each user has at least one program executing in memory process
 If several jobs ready to run at the same time  CPU scheduling
 If processes don’t fit in memory, swapping moves them in and out to
run
 Virtual memory allows execution of processes not completely in
memory

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Memory Layout for Multiprogrammed System

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Operating-System Operations
Interrupt driven by hardware
Software error or request creates exception or trap
 Division by zero, request for operating system service
Other process problems include infinite loop, processes modifying
each other or the operating system
Dual-mode operation allows OS to protect itself and other system
components
 User mode and kernel mode
 Mode bit provided by hardware
 Provides ability to distinguish when system is running user
code or kernel code
 Some instructions designated as privileged, only executable
in kernel mode
 System call changes mode to kernel, return from call resets
it to user

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Chapter 3: Processes
Process Concept
Process Scheduling
Operations on Processes
Cooperating Processes
Interprocess Communication
Communication in Client-Server Systems

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Process Concept
An operating system executes a variety of
programs:
 Batch system – jobs
 Time-shared systems – user programs or tasks
Textbook uses the terms job and process
almost interchangeably
Process – a program in execution; process
execution must progress in sequential
fashion
A process includes:
 program counter
 stack
 data section

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Process State
As a process executes, it changes state
new: The process is being created
running: Instructions are being executed
waiting: The process is waiting for some
event to occur
ready: The process is waiting to be
assigned to a processor
terminated: The process has finished
execution

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Process Control Block (PCB)
Information associated with each process
Process state
Program counter
CPU registers
CPU scheduling information
Memory-management information
Accounting information
I/O status information

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Process Control Block (PCB)

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CPU Switch From Process to Process

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Process Scheduling Queues
Job queue – set of all processes in the
system
Ready queue – set of all processes residing
in main memory, ready and waiting to
execute
Device queues – set of processes waiting
for an I/O device
Processes migrate among the various
queues

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Process Scheduling Queues
Job queue – set of all processes in the
system
Ready queue – set of all processes residing
in main memory, ready and waiting to
execute
Device queues – set of processes waiting
for an I/O device
Processes migrate among the various
queues

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Ready Queue And Various I/O Device Queues

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Representation of Process Scheduling

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Schedulers
Long-term scheduler (or job scheduler)
– selects which processes should be
brought into the ready queue
Short-term scheduler (or CPU
scheduler) – selects which process should
be executed next and allocates CPU

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Addition of Medium Term Scheduling

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Schedulers (Cont.)
Short-term scheduler is invoked very frequently
(milliseconds)  (must be fast)
Long-term scheduler is invoked very infrequently
(seconds, minutes)  (may be slow)
The long-term scheduler controls the degree of
multiprogramming
Processes can be described as either:
 I/O-bound process – spends more time doing I/O than
computations, many short CPU bursts
 CPU-bound process – spends more time doing
computations; few very long CPU bursts

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Context Switch
When CPU switches to another process, the
system must save the state of the old process
and load the saved state for the new process
Context-switch time is overhead; the system
does no useful work while switching
Time dependent on hardware support

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Process Creation
Parent process create children processes, which,
in turn create other processes, forming a tree of
processes
Resource sharing
 Parent and children share all resources
 Children share subset of parent’s resources
 Parent and child share no resources
Execution
 Parent and children execute concurrently
 Parent waits until children terminate

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Process Creation (Cont.)
Address space
 Child duplicate of parent
 Child has a program loaded into it
UNIX examples
 fork system call creates new process
 exec system call used after a fork to replace the
process’ memory space with a new program

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C Program Forking Separate Process
int main()
{
pid_t pid;
/* fork another process */
pid = fork();
if (pid < 0) { /* error occurred */
fprintf(stderr, "Fork Failed");
exit(-1);
}
else if (pid == 0) { /* child process */
execlp("/bin/ls", "ls", NULL);
}
else { /* parent process */
/* parent will wait for the child to complete */
wait (NULL);
printf ("Child Complete");
exit(0);
}
}

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A tree of processes on a typical Solaris

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Process Termination
Process executes last statement and asks the
operating system to delete it (exit)
 Output data from child to parent (via wait)
 Process’ resources are deallocated by operating system
Parent may terminate execution of children
processes (abort)
 Child has exceeded allocated resources
 Task assigned to child is no longer required
 If parent is exiting
Some operating system do not allow child to
continue if its parent terminates
 All children terminated - cascading termination

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Cooperating Processes
Independent process cannot affect or be
affected by the execution of another process
Cooperating process can affect or be affected by
the execution of another process
Advantages of process cooperation
 Information sharing
 Computation speed-up
 Modularity
 Convenience

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Producer-Consumer Problem
Paradigm for cooperating processes,
producer process produces information
that is consumed by a consumer process
 unbounded-buffer places no practical limit on
the size of the buffer
 bounded-buffer assumes that there is a fixed
buffer size

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Bounded-Buffer – Shared-Memory Solution
Shared data
#define BUFFER_SIZE 10
typedef struct {
. . .
} item;
item buffer[BUFFER_SIZE];
int in = 0;
int out = 0;
Solution is correct, but can only use
BUFFER_SIZE-1 elements

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Bounded-Buffer – Insert() Method
while (true) {
/* Produce an item */
while (((in = (in + 1) % BUFFER SIZE count) == out)
; /* do nothing -- no free buffers */
buffer[in] = item;
in = (in + 1) % BUFFER SIZE;
}

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Bounded Buffer – Remove() Method
while (true) {
while (in == out)
; // do nothing -- nothing to
consume
// remove an item from the buffer
item = buffer[out];
out = (out + 1) % BUFFER SIZE;
return item;
}

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Interprocess Communication (IPC)
Mechanism for processes to communicate and to
synchronize their actions
Message system – processes communicate with
each other without resorting to shared variables
IPC facility provides two operations:
 send(message) – message size fixed or variable
 receive(message)
If P and Q wish to communicate, they need to:
 establish a communication link between them
 exchange messages via send/receive
Implementation of communication link
 physical (e.g., shared memory, hardware bus)
 logical (e.g., logical properties)

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Implementation Questions
How are links established?
Can a link be associated with more than two
processes?
How many links can there be between every pair of
communicating processes?
What is the capacity of a link?
Is the size of a message that the link can
accommodate fixed or variable?
Is a link unidirectional or bi-directional?

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Indirect Communication
Messages are directed and received from
mailboxes (also referred to as ports)
 Each mailbox has a unique id
 Processes can communicate only if they share a
mailbox
Properties of communication link
 Link established only if processes share a common
mailbox
 A link may be associated with many processes
 Each pair of processes may share several
communication links
 Link may be unidirectional or bi-directional

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Operations
 create a new mailbox
 send and receive messages through mailbox
 destroy a mailbox
Primitives are defined as:
send(A, message) – send a message to
mailbox A
receive(A, message) – receive a message
from mailbox A

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Mailbox sharing
 P1, P2, and P3 share mailbox A
 P1, sends; P2 and P3 receive
 Who gets the message?
Solutions
 Allow a link to be associated with at most two processes
 Allow only one process at a time to execute a receive
operation
 Allow the system to select arbitrarily the receiver.
Sender is notified who the receiver was.

70
Synchronization
Message passing may be either blocking or non-
blocking
Blocking is considered synchronous
 Blocking send has the sender block until the message is
received
 Blocking receive has the receiver block until a message is
available
Non-blocking is considered asynchronous
 Non-blocking send has the sender send the message and
continue
 Non-blocking receive has the receiver receive a valid
message or null

71
Synchronization
blocking
received
 Blocking receive has the receiver block until a message
is available
continue
message or null

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Synchronization
blocking
received
 Blocking receive has the receiver block until a message
is available
continue
message or null

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Buffering
Queue of messages attached to the link;
implemented in one of three ways
1. Zero capacity – 0 messages
Sender must wait for receiver (rendezvous)
2. Bounded capacity – finite length of n messages
Sender must wait if link full
3. Unbounded capacity – infinite length
Sender never waits

75
Chapter 5: CPU Scheduling
Basic Concepts
Scheduling Criteria
Scheduling Algorithms
Multiple-Processor Scheduling
Real-Time Scheduling
Algorithm Evaluation

76
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
CPU burst distribution

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Alternating Sequence of CPU And I/O Bursts

78
Histogram of CPU-burst Times

79
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
All other scheduling is preemptive

80
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

81
Scheduling Criteria
CPU utilization – keep the CPU as busy as
possible
Throughput – # of processes that
complete their execution per time unit
Turnaround time – amount of time to
execute a particular process
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
(for time-sharing environment)

82
Optimization Criteria
Max CPU utilization
Max throughput
Min turnaround time
Min waiting time
Min response time

83
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:
Waiting time for P1 = 0; P2 = 24; P3 = 27
Average waiting time: (0 + 24 + 27)/3 = 17
P1 P2 P3
24 27 300

84
FCFS Scheduling (Cont.)
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
Convoy effect short process behind long process
P1P3P2
63 300

85
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

86
Process Arrival Time Burst Time Remaining time
P1 0.0 7 7
P2 2.0 4 5
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
73 160
P4
8 12

87
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 P3P2
42 110
P4
5 7
P2 P1
16

88
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
:Define4.
10,3.
burstCPUnexttheforvaluepredicted2.
burstCPUoflenghtactual1.
1





n
th
n nt
  .11 nnn t  

89
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
  .11 nnn t  

90
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

91
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

92
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

93
Time Quantum and Context Switch Time

94
Multilevel Queue
Ready queue is partitioned into separate queues:
foreground (interactive)
background (batch)
Each queue has its own scheduling algorithm
 foreground – RR
 background – FCFS
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

95
Multilevel Queue Scheduling

96
Multilevel Feedback Queue
A process can move between the various
queues; aging can be implemented 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

97
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 FCFS. 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 FCFS and receives 16 additional
milliseconds. If it still does not complete, it is
preempted and moved to queue Q2.

99
Multiple-Processor Scheduling
CPU scheduling more complex when
multiple CPUs are available
Homogeneous processors within a
multiprocessor
Load sharing
Asymmetric multiprocessing – only one
processor accesses the system data
structures, alleviating the need for data
sharing

100
Algorithm Evaluation
Deterministic modeling – takes a
particular predetermined workload and
defines the performance of each
algorithm for that workload
Queueing models
Implementation

Principles of operating system

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Principles of operating system