SYSTEM OPERATING CONCEPTS
Avi Silberschatz
Department of Computer Sciences
University of Texas at Austin
Peter Galvin
Department of Computer Science
Brown University
Copyright 1994 Avi Silberschatz & Peter Galvin
CHAPTER 1: INTRODUCTION
What is an operating system?
Early Systems
Simple Batch Systems
Multiprogramming Batched Systems
Time-Sharing Systems
Personal-Computer Systems
Parallel Systems
Distributed Systems
Real-Time Systems
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Operating system a program that acts as an
intermediary between a user of a computer and the
computer hardware.
Operating system goals:
Execute user programs and make solving user
problems easier.
Make the computer system convenient to use.
Use the computer hardware in an efcient
manner.
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Computer System Components
1. Hardware provides basic computing resources
(CPU, memory, I/O devices).
2. Operating system controls and coordinates the
use of the hardware among the various application programs for the various users.
3. Applications programs dene the ways in
which the system resources are used to solve the
computing problems of the users (compilers,
database systems, video games, business programs).
4. Users (people, machines, other computers).
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user
1
user
2
user
3
...
user
n
compiler
assembler
text
editor
...
database
system
Application Programs
Operating System
Computer
Hardware
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Operating System Denitions
Resource allocator manages and allocates
resources.
Control program controls the execution of user
programs and operation of I/O devices.
Kernel the one program running at all times (all
else being application programs).
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Early Systems bare machine (early 1950s)
Structure
- Large machines run from console
- Single user system
- Programmer/User as operator
- Paper tape or punched cards
Early Software
- Assemblers
- Loaders
- Linkers
- Libraries of common subroutines
- Compilers
- Device drivers
Secure
Inefcient use of expensive resources
- Low CPU utilization
- Signicant amount of setup time
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Simple Batch Systems
Hire an operator
User operator
Add a card reader
Reduce setup time by batching similar jobs
Automatic job sequencing automatically
transfers control from one job to another. First
rudimentary operating system.
Resident monitor
- initial control in monitor
- control transfers to job
- when job completes control transfers back to
monitor
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Problems:
1) How does the monitor know about the nature of
the job (e.g., Fortran versus Assembly) or which
program to execute?
2) How does the monitor distinguish
a) job from job?
b) data from program?
Solution: introduce control cards
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Control Cards
Special cards that tell the resident monitor which
programs to run.
$JOB
$FTN
$RUN
$DATA
$END
Special characters distinguish control cards from
data or program cards:
$ in column 1
// in column 1 and 2
7-9 in column 1
Parts of resident monitor
- Control card interpreter responsible for reading and carrying out instructions on the cards.
- Loader loads systems programs and applications programs into memory.
- Device drivers know special characteristics
and properties for each of the systems I/O devices.
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Problem: Slow Performance since I/O and CPU
could not overlap, and card reader very slow.
Solution: Off-line operation speed up computation by loading jobs into memory from tapes and
card reading and line printing done off-line.
card
reader
satellite
processor
printer
system tapes
main
computer
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Advantage of off-line operation main computer
not constrained by the speed of the card readers
and line printers, but only by the speed of faster
magnetic tape units.
No changes need to be made to the application
programs to change from direct to off-line I/O
operation.
Real gain possibility of using multiple readerto-tape and tape-to-printer systems for one CPU.
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Spooling overlap the I/O of one job with the computation of another job.
disk
card
reader
printer
CPU
While executing one job, the operating system:
- reads the next job from the card reader into a
storage area on the disk (job queue).
- outputs the printout of previous job from disk
to the line printer.
Job pool data structure that allows the operating
system to select which job to run next, in order to
increase CPU utilization.
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Multiprogrammed Batch Systems several jobs are
kept in main memory at the same time, and the CPU
is multiplied among them.
OS
u1
u2
u3
u4
I/O
CPU
u2
u1
OS
L read ()
SIO
scheduler
L+1
M block
scheduler
interrupt
R
scheduler
R+1
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OS Features Needed for Multiprogramming
I/O routine supplied by the system.
Memory management the system must allocate
the memory to several jobs.
CPU scheduling the system must choose among
several jobs ready to run.
Allocation of devices.
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Time-Sharing Systems Interactive Computing
The CPU is multiplied among several jobs that are
kept in memory and on disk (the CPU is allocated
to a job only if the job is in memory).
A job is swapped in and out of memory to the
disk.
On-line communication between the user and the
system is provided; when the operating system
nishes the execution of one command, it seeks
the next control statement not from a card
reader, but rather from the users keyboard.
On-line le system must be available for users to
access data and code.
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Personal-Computer Systems
Personal computers computer system dedicated
to a single user.
I/O devices keyboards, mice, display screens,
small printers.
User convenience and responsiveness.
Can adopt technology developed for larger
operating systems; often individuals have sole use
of computer and do not need advanced CPU utilization or protection features.
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Parallel Systems multiprocessor systems with
more than one CPU in close communication.
Tightly coupled system processors share
memory and a clock; communication usually
takes place through the shared memory.
Advantages of parallel systems:
- Increased throughput
- Economical
- Increased reliability
graceful degradation
fail-soft systems
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Symmetric multiprocessing
- Each processor runs an identical copy of the
operating system.
- Many processes can run at once without performance deterioration.
Asymmetric multiprocessing
- Each processor is assigned a specic task;
master processor schedules and allocates work
to slave processors.
- More common in extremely large systems.
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Distributed Systems distribute the computation
among several physical processors.
Loosely coupled system each processor has its
own local memory; processors communicate with
one another through various communication lines,
such as high-speed buses or telephone lines.
Advantages of distributed systems:
- Resource sharing
- Computation speed up load sharing
- Reliability
- Communication
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Real-Time Systems
Often used as a control device in a dedicated
application such as controlling scientic experiments, medical imaging systems, industrial control systems, and some display systems.
Well-dened xed-time constraints.
Hard real-time system.
- Secondary storage limited or absent; data
stored in short-term memory, or read-only
memory (ROM).
- Conicts with time-sharing systems; not supported by general-purpose operating systems.
Soft real-time system.
- Limited utility in industrial control or robotics.
- Useful in applications (multimedia, virtual
reality) requiring advanced operating-system
features.
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CHAPTER 2: COMPUTER-SYSTEM STRUCTURES
Computer-System Operation
I/O Structure
Storage Structure
Storage Hierarchy
Hardware Protection
General System Architecture
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Computer-System Operation
Devices
D11
CPU
D12
DC1
Dn1
D21
DC2
...
Dn2
Dn3
DCn
Device
Controller
Memory controller
Memory
I/O devices and the CPU can execute concurrently.
Each device controller is in charge of a particular
device type.
Each device controller has a local buffer.
CPU moves data from/to main memory to/from
the local buffers.
I/O is from the device to local buffer of controller.
Device controller informs CPU that it has nished
its operation by causing an interrupt.
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Common Functions of Interrupts
Interrupt transfers control to the interrupt service
routine, generally, through the interrupt vector,
which contains the addresses of all the service
routines.
Interrupt architecture must save the address of the
interrupted instruction.
Incoming interrupts are disabled while another
interrupt is being processed to prevent a lost
interrupt.
A trap is a software-generated interrupt caused
either by an error or a user request.
An operating system is interrupt driven.
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Interrupt Handling
The operating system preserves the state of the
CPU by storing registers and the program counter.
Determines which type of interrupt has occurred:
- polling
- vectored interrupt system
Separate segments of code determine what action
should be taken for each type of interrupt.
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I/O Structure
After I/O starts, control returns to user program
only upon I/O completion.
- wait instruction idles the CPU until the next
interrupt.
- wait loop (contention for memory access).
- at most one I/O request is outstanding at a
time; no simultaneous I/O processing.
After I/O starts, control returns to user program
without waiting for I/O completion.
- System call request to the operating system
to allow user to wait for I/O completion.
- Device-status table contains entry for each I/O
device indicating its type, address, and state.
- Operating system indexes into I/O device table
to determine device status and to modify table
entry to include interrupt.
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Direct Memory Access (DMA) Structure
Schema
Memory
CPU
I/O devices
I/O instructions
Used for high-speed I/O devices able to transmit
information at close to memory speeds.
Device controller transfers blocks of data from
buffer storage directly to main memory without
CPU intervention.
Only one interrupt is generated per block, rather
than the one interrupt per byte.
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Storage Structure
Main memory only large storage media that the
CPU can access directly.
Secondary storage extension of main memory
that provides large nonvolatile storage capacity.
Magnetic disks rigid metal or glass platters
covered with magnetic recording material.
- Disk surface is logically divided into tracks,
which are subdivided into sectors.
- The disk controller determines the logical
interaction between the device and the computer.
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Storage Hierarchy
Storage systems organized in hierarchy:
- speed
- cost
- volatility
Caching copying information into faster storage
system; main memory can be viewed as a fast
cache for secondary memory.
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Hardware Protection
Dual-Mode Operation
I/O Protection
Memory Protection
CPU Protection
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Dual-Mode Operation
Sharing system resources requires operating system to ensure that an incorrect program cannot
cause other programs to execute incorrectly.
Provide hardware support to differentiate between
at least two modes of operations.
1. User mode execution done on behalf of a
user.
2. Monitor mode (also supervisor mode or system
mode) execution done on behalf of operating
system.
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Mode bit added to computer hardware to indicate
the current mode: monitor (0) or user (1).
When an interrupt or fault occurs hardware
switches to monitor mode
interrupt/fault
monitor
user
set user mode
Privileged instructions can be issued only in
monitor mode.
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I/O Protection
All I/O instructions are privileged instructions.
Must ensure that a user program could never gain
control of the computer in monitor mode (i.e., a
user program that, as part of its execution, stores
a new address in the interrupt vector).
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Memory Protection
Must provide memory protection at least for the
interrupt vector and the interrupt service routines.
In order to have memory protection, add two
registers that determine the range of legal
addresses a program may access:
- base register holds the smallest legal physical memory address.
- limit register contains the size of the range.
Memory outside the dened range is protected.
0
monitor
256000
job 1
300040
job 2
300040
base register
job 3
120900
limit register
420940
880000
job 4
1024000
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Protection hardware
base
CPU
address
base + limit
yes
no
yes
<
no
memory
trap to operating system
monitoraddressing error
When executing in monitor mode, the operating
system has unrestricted access to both monitor
and users memory.
The load instructions for the base and limit registers are privileged instructions.
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CPU Protection
Timer interrupts computer after specied period
to ensure operating system maintains control.
- Timer is decremented every clock tick.
- When timer reaches the value 0, an interrupt
occurs.
Timer commonly used to implement time sharing.
Timer also used to compute the current time.
Load-timer is a privileged instruction.
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General-System Architecture
Given that I/O instructions are privileged, how
does the user program perform I/O?
System call the method used by a process to
request action by the operating system.
- Usually takes the form of a trap to a specic
location in the interrupt vector.
- Control passes through the interrupt vector to a
service routine in the OS, and the mode bit is
set to monitor mode.
- The monitor veries that the parameters are
correct and legal, executes the request, and
returns control to the instruction following the
system call.
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CHAPTER 3: OPERATING-SYSTEM STRUCTURES
System Components
Operating-System Services
System Calls
System Programs
System Structure
Virtual Machines
System Design and Implementation
System Generation
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Most operating systems support the following types
of system components:
Process Management
Main-Memory Management
Secondary-Storage Management
I/O System Management
File Management
Protection System
Networking
Command-Interpreter System
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Process Management
A process is a program in execution. A process
needs certain resources, including CPU time,
memory, les, and I/O devices, to accomplish its
task.
The operating system is responsible for the following activities in connection with process
management:
- process creation and deletion.
- process suspension and resumption.
- provision of mechanisms for:
process synchronization
process communication
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Main-Memory Management
Memory is a large array of words or bytes, each
with its own address. It is a repository of quickly
accessible data shared by the CPU and I/O devices.
Main memory is a volatile storage device. It
loses its contents in the case of system failure.
The operating system is responsible for the following activities in connection with memory
management:
- Keep track of which parts of memory are
currently being used and by whom.
- Decide which processes to load when memory
space becomes available.
- Allocate and deallocate memory space as
needed.
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Secondary-Storage Management
Since main memory (primary storage) is volatile
and too small to accommodate all data and programs permanently, the computer system must
provide secondary storage to back up main
memory.
Most modern computer systems use disks as the
principle on-line storage medium, for both programs and data.
The operating system is responsible for the following activities in connection with disk management:
- Free-space management
- Storage allocation
- Disk scheduling
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I/O System Management
The I/O system consists of:
- A buffer-caching system
- A general device-driver interface
- Drivers for specic hardware devices
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File Management
A le is a collection of related information
dened by its creator. Commonly, les represent
programs (both source and object forms) and
data.
The operating system is responsible for the following activities in connection with le management:
- File creation and deletion.
- Directory creation and deletion.
- Support of primitives for manipulating les
and directories.
- Mapping les onto secondary storage.
- File backup on stable (nonvolatile) storage
media.
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Protection System
Protection refers to a mechanism for controlling
access by programs, processes, or users to both
system and user resources.
The protection mechanism must:
- distinguish between authorized and unauthorized usage.
- specify the controls to be imposed.
- provide a means of enforcement.
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Networking (Distributed Systems)
A distributed system is a collection of processors
that do not share memory or a clock. Each processor has its own local memory.
The processors in the system are connected
through a communication network.
A distributed system provides user access to various system resources.
Access to a shared resource allows:
- Computation speed-up
- Increased data availability
- Enhanced reliability
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Command-Interpreter System
Many commands are given to the operating system by control statements which deal with:
- process creation and management
- I/O handling
- secondary-storage management
- main-memory management
- le-system access
- protection
- networking
The program that reads and interprets control
statements is called variously:
- control-card interpreter
- command-line interpreter
- shell (in UNIX)
Its function is to get and execute the next command statement.
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Operating-System Services
Program execution system capability to load a
program into memory and to run it.
I/O operations since user programs cannot execute I/O operations directly, the operating system
must provide some means to perform I/O.
File-system manipulation program capability to
read, write, create, and delete les.
Communications exchange of information
between processes executing either on the same
computer or on different systems tied together by
a network. Implemented via shared memory or
message passing.
Error detection ensure correct computing by
detecting errors in the CPU and memory
hardware, in I/O devices, or in user programs.
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Additional operating-system functions exist not for
helping the user, but rather for ensuring efcient system operation.
Resource allocation allocating resources to multiple users or multiple jobs running at the same
time.
Accounting keep track of and record which
users use how much and what kinds of computer
resources for account billing or for accumulating
usage statistics.
Protection ensuring that all access to system
resources is controlled.
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System Calls
System calls provide the interface between a running program and the operating system.
- Generally available as assembly-language
instructions.
- Languages dened to replace assembly
language for systems programming allow system calls to be made directly (e.g., C, Bliss,
PL/360).
Three general methods are used to pass parameters between a running program and the operating
system:
- Pass parameters in registers.
- Store the parameters in a table in memory, and
the table address is passed as a parameter in a
register.
- Push (store) the parameters onto the stack by
the program, and pop off the stack by the
operating system.
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System Programs
System programs provide a convenient environment for program development and execution.
They can be divided into:
- File manipulation
- Status information
- File modication
- Programming-language support
- Program loading and execution
- Communications
- Application programs
Most users view of the operation system is
dened by system programs, not the actual system calls.
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System Structure Simple Approach
MS-DOS written to provide the most functional-
ity in the least space; it was not divided into
modules. MS-DOS has some structure, but its
interfaces and levels of functionality are not well
separated.
UNIX limited by hardware functionality, the
original UNIX operating system had limited structuring. The UNIX OS consists of two separable
parts:
- the systems programs.
- the kernel, which consists of everything below
the system-call interface and above the physical hardware. Provides the le system, memory CPU
scheduling, management, and other
operating-system functions; a large number of
functions for one level.
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System Structure Layered Approach
The operating system is divided into a number of
layers (levels), each built on top of lower layers.
The bottom layer (layer 0) is the hardware; the
highest (layer N) is the user interface.
With modularity, layers are selected such that
each uses functions (operations) and services of
only lower-level layers.
A layered design was rst used in the THE operating system. Its six layers are as follows:
Level 5:
user programs
Level 4:
buffering for input and output devices
Level 3:
operator-console device driver
Level 2:
memory management
Level 1:
CPU scheduling
Level 0:
hardware
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Virtual Machines
A virtual machine takes the layered approach to
its logical conclusion. It treats hardware and the
operating system kernel as though they were all
hardware.
A virtual machine provides an interface identical
to the underlying bare hardware.
The operating system creates the illusion of multiple processes, each executing on its own processor with its own (virtual) memory.
The resources of the physical computer are shared
to create the virtual machines.
- CPU scheduling can create the appearance that
users have their own processor.
- Spooling and a le system can provide virtual
card readers and virtual line printers.
- A normal user time-sharing terminal serves as
the virtual machine operators console.
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Advantages and Disadvantages of Virtual Machines
The virtual-machine concept provides complete
protection of system resources since each virtual
machine is isolated from all other virtual
machines. This isolation, however, permits no
direct sharing of resources.
A virtual-machine system is a perfect vehicle for
operating-systems research and development.
System development is done on the virtual
machine, instead of on a physical machine and so
does not disrupt normal system operation.
The virtual machine concept is difcult to implement due to the effort required to provide an
exact duplicate of the underlying machine.
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System Design Goals
User goals operating system should be convenient to use, easy to learn, reliable, safe, and
fast.
System goals operating system should be easy
to design, implement, and maintain, as well as
exible, reliable, error-free, and efcient.
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Mechanisms and Policies
Mechanisms determine how to do something; policies decide what will be done.
The separation of policy from mechanism is a
very important principle; it allows maximum
exibility if policy decisions are to be changed
later.
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System Implementation
Traditionally written in assembly language,
operating systems can now be written in higherlevel languages.
Code written in a high-level language:
- can be written faster.
- is more compact.
- is easier to understand and debug.
An operating system is far easier to port (move to
some other hardware) if it is written in a highlevel language.
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System Generation (SYSGEN)
Operating systems are designed to run on any of a
class of machines; the system must be congured
for each specic computer site.
SYSGEN program obtains information concerning
the specic conguration of the hardware system.
Booting starting a computer by loading the kernel.
Bootstrap program code stored in ROM that is
able to locate the kernel, load it into memory, and
start its execution.
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CHAPTER 4: PROCESSES
Process Concept
Process Scheduling
Operation on Processes
Cooperating Processes
Threads
Interprocess Communication
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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 a sequential fashion.
A process includes:
- program counter
- stack
- data section
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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 nished execution.
Diagram of process state:
new
admitted
interrupt
terminated
running
ready
I/O or event
exit
scheduler dispatch
completion
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I/O or event
waiting
4.2
wait
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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 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 a
particular I/O device.
Process migration between the various queues.
ready queue
CPU
end
job queue
I/O
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I/O waiting
queue(s)
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Schedulers
- Long-term scheduler (job scheduler) selects
which processes should be brought into the
ready queue.
- Short-term scheduler (CPU scheduler)
selects which process should be executed next
and allocates CPU.
long term
short term
end
CPU
ready queue
I/O
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I/O waiting
queue(s)
4.5
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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 creates 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 parents resources.
- Parent and child share no resources.
Execution
- Parent and children execute concurrently.
- Parent waits until children terminate.
Address space
- Child duplicate of parent.
- Child has a program loaded into it.
UNIX examples
- fork system call creates new process.
- execve system call used after a fork to replace
the process memory space with a new program.
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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 fork).
- 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.
- Parent is exiting.
Operating system does not allow child to
continue if its parent terminates.
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 xed
buffer size.
Shared-memory solution:
- Shared data
var n;
type item = ... ;
var buffer: array [0..n1] of item;
in, out: 0..n1;
in := 0;
out := 0;
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- Producer process
repeat
...
produce an item in nextp
...
while in+1 mod n = out do no-op;
buffer[in] := nextp;
in := in+1 mod n;
until false;
- Consumer process
repeat
while in = out do no-op;
nextc := buffer[out];
out := out+1 mod n;
...
consume the item in nextc
...
until false;
- Solution is correct, but can only ll up n 1
buffer.
Operating System Concepts, Addison-Wesley 1994
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Threads
A thread (or lightweight process) is a basic unit
of CPU utilization; it consists of:
- program counter
- register set
- stack space
A thread shares with its peer threads its:
- code section
- data section
- operating-system resources
collectively known as a task.
A traditional or heavyweight process is equal to a
task with one thread.
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In a task containing multiple threads, while one
server thread is blocked and waiting, a second
thread in the same task could run.
- Cooperation of multiple threads in same job
confers higher throughput and improved performance.
- Applications that require sharing a common
buffer (producerconsumer problem) benet
from thread utilization.
Threads provide a mechanism that allows sequential processes to make blocking system calls
while also achieving parallelism.
Kernel-supported threads (Mach and OS/2).
User-level threads; supported above the kernel,
via a set of library calls at the user level (Project
Andrew from CMU).
Hybrid approach implements both user-level and
kernel-supported threads (Solaris 2).
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Solaris 2 version of UNIX with support for threads
at the kernel and user levels, symmetric multiprocessing, and real-time scheduling.
LWP intermediate level between user-level
threads and kernel-level threads.
Resource needs of thread types:
- Kernel thread small data structure and a
stack; thread switching does not require changing memory access information, and therefore
is relatively fast.
- LWP PCB with register data, accounting
information, and memory information; switching between LWPs is relatively slow.
- User-level thread needs only a stack and a
program counter. Switching is fast since kernel is not involved. Kernel only sees the LWPs
in the process that support user-level threads.
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Interprocess Communication (IPC) provides a
mechanism to allow 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) messages can be of either
xed or variable size.
- receive(message)
If P and Q wish to communicate, they need to:
- establish a communication link between them
- exchange messages via send/receive
Communication link
- physical implementation (e.g., shared memory,
hardware bus)
- logical implementation (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 xed or variable?
Is a link unidirectional or bidirectional?
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Direct Communication
Processes must name each other explicitly:
- send(P, message) send a message to
process P
- receive(Q, message) receive a message from
process Q
Properties of communication link
- Links are established automatically.
- A link is associated with exactly one pair of
communicating processes.
- Between each pair there exists exactly one
link.
- The link may be unidirectional, but is usually
bidirectional.
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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 the two processes
share a mailbox in common.
- A link may be associated with many processes.
- Each pair of processes may share several communication links.
- Link may be unidirectional or bidirectional.
Operations
- create a new mailbox
- send and receive messages through mailbox
- destroy a mailbox
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Indirect Communication (Continued)
Mailbox sharing
- P 1, P 2, and P 3 share mailbox A.
- P 1 sends; P 2 and P 3 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 notied who the receiver
was.
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Buffering queue of messages attached to the link;
implemented in one of three ways.
Zero capacity 0 messages
Sender must wait for receiver (rendezvous).
Bounded capacity nite length of n messages
Sender must wait if link full.
Unbounded capacity innite length
Sender never waits.
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Exception Conditions error recovery
Process terminates
Lost messages
Scrambled Messages
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CHAPTER 5: CPU SCHEDULING
Basic Concepts
Scheduling Criteria
Scheduling Algorithms
Multiple-Processor Scheduling
Real-Time Scheduling
Algorithm Evaluation
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Basic Concepts
Maximum CPU utilization obtained with multiprogramming.
CPUI/O Burst Cycle Process execution consists of a cycle of CPU execution and I/O wait.
CPU burst distribution
frequency
burst time
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Short-term 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.
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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
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 rst
response is produced, not output (for timesharing environment)
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Optimization
- Max CPU utilization
- Max throughput
- Min turnaround time
- Min waiting time
- Min response time
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First-Come, First-Served (FCFS) Scheduling
Example:
Process
Burst time
P1
P2
P3
24
3
3
Suppose that the processes arrive in the order:
P 1, P 2, P 3.
The Gantt chart for the schedule is:
P1
P2
0
Waiting time for:
24
27
30
P1 = 0
P 2 = 24
P 3 = 27
Average waiting time:
Operating System Concepts, Addison-Wesley 1994
P3
(0 + 24 + 27)/3 = 17
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Suppose that the processes arrive in the order:
P 2, P 3, P 1.
The Gantt chart for the schedule is:
P2
0
P3
3
P1
6
Waiting time for:
30
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
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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.
Two schemes:
a) nonpreemptive once CPU given to the process it cannot be preempted until it completes
its CPU burst.
b) 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-RemainingTime-First (SRTF).
SJF is optimal gives minimum average waiting
time for a given set of processes.
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Example of SJF
Process
Arrival time
CPU time
P1
P2
P3
P4
0
2
4
5
7
4
1
4
SJF (non-preemptive)
P1
P3
0
7
P4
P2
8
12
16
Average waiting time = (0 + 6 + 3 + 7)/4 = 4
SRTF (preemptive)
P2
P1
0
2
P3
4
5
P1
P4
P2
7
11
16
Average waiting time = (9 + 1 + 0 + 2)/4 = 3
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How do we know the length of the next CPU burst?
Can only estimate the length.
Can be done by using the length of previous CPU
bursts, using exponential averaging.
1. Tn = actual length of n th CPU burst
2. n = predicted value of n th CPU burst
3. 0 W 1
4. Dene:
n
+1
= W * Tn + (1 W ) n
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Examples:
W =0
n + 1 = n
Recent history does not count.
W =1
n + 1 = Tn
Only the actual last CPU burst counts.
If we expand the formula, we get:
n
+1
= W Tn + (1 W ) W Tn 1 +
(1 W )2 W Tn 2 + ... + (1 W )q
W Tn q
So if W = 1/2 each successive term has less
and less weight.
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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).
a) preemptive
b) nonpreemptive
SJN 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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Round Robin (RR)
Each process gets a small unit of CPU time (time
quantum), usually 10100 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.
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Example of RR with time quantum = 20
Process
CPU times
P1
P2
P3
P4
53
17
68
24
The Gantt chart is:
P1 P2
0
20
P4
P3
37
57
P3
P1
77
97
P4 P1
P3
P3
117 121 134 154 162
Typically, higher average turnaround than SRT,
but better response.
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Multilevel Queue
Ready queue is partitioned into separate queues.
Example: foreground (interactive)
background (batch)
Each queue has its own scheduling algorithm.
Example: foreground RR
background FCFS
Scheduling must be done between the queues.
- Fixed priority scheduling
Example: 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.
Example:
80% to foreground in RR
20% to background in FCFS
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Multilevel Feedback Queue
A process can move between the various queues;
aging can be implemented this way.
Multilevel-feedback-queue scheduler dened by
the following parameters:
- number of queues
- scheduling algorithm 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
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Example of multilevel feedback queue
Three queues:
- Q 0 time quantum 8 milliseconds
- Q 1 time quantum 16 milliseconds
- Q 2 FCFS
Scheduling
A new job enters queue Q 0 which is served FCFS.
When it gains CPU, job receives 8 milliseconds.
If it does not nish 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.
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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.
Real-Time Scheduling
- Hard real-time systems required to complete
a critical task within a guaranteed amount of
time.
- Soft real-time computing requires that critical processes receive priority over less fortunate ones.
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Algorithm Evaluation
Deterministic modeling takes a particular
predetermined workload and denes the performance of each algorithm for that workload.
Queueing models
Implementation
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