Module 1 Introduction.ppt

Operating System Concepts(Eighth Edition)
---Abraham Silberschatz
--- Peter B Galvin

MODULE 1
CHAPTER 1 INTRODUCTION TO OPERATING
SYSTEMS

What is an Operating System?
• Operating system is a program that manages computer hardware
and act as a intermediary between 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 efficient manner.

Four Components of a Computer System

Computer System Structure
Hardware – provides basic computing resources such as 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.

What Operating Systems do
User’s View – The user view of the computer varies according
to the interface being used.
• Some users may use PC’s. Here the system is designed so that
only one user can utilize the resource, as a result performance
will be good.
• Users sits at a terminal connected to mainframe. These users
share resources and exchange information.
• Operating system is designed in such a way that to have maximize
resource utilization.

What Operating Systems do
 System View
• OS is a resource allocator
– to allocate resources (software and hardware) of the computer
system and manage them efficiently. OS is a manager .
• OS is a control program
– Controls execution of programs to prevent errors and improper
use of the computer.
• Kernel
-- The program that executes forever.

Computer Startup
• When a computer is powered up ,it needs to have initial program to
run. This initial program is called bootstrap program stored in ROM
or EPROM, generally known as firmware.
• Initializes all aspects of system and loads operating system kernel
and starts executing the first process known as “init”.
• The occurrence of an event is usually signaled by an interrupt either
through hardware(using system bus) or software(using system call).
• Interrupts are signals sent to the CPU by I/O devices. They tell the
CPU to stop its current activities and execute appropriate part of the
operating system.

Interrupts
• When the CPU is interrupted, it stops what it is doing and
immediately transfers execution to a fixed location.
• The fixed location usually contains the starting address where the
service routine for the process is located.
• The interrupt service routine executes; on completion, the CPU
resumes the interrupted computation .

Computer System Organization
 Computer-system operation
– One or more CPUs, device controllers connect through common bus
providing access to shared memory.
– Concurrent execution of CPUs and devices competing for memory
cycles.

Computer-System Operation
• 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 local buffers.
• Device controller informs CPU that it has finished its operation by
causing an interrupt.

Storage Structure
• The CPU can load instructions only from memory, so any programs
to run must be stored in main memory (RAM).
• Main memory is usually too small to store all needed programs and
data permanently and volatile .
• Extension to main memory is Secondary memory(ROM) which
holds large quantities of data temporarily.
• The most common secondary-storage device is a magnetic disk
which provides storage for both programs and data.
• Most programs are stored on a disk until they are loaded into
memory. Many programs then use the disk as both the source and
the destination of their processing.

Storage-Device Hierarchy
• The wide variety of storage systems in a computer system can be
organized in a hierarchy according to speed and cost.
• The higher levels are expensive, but they are fast.
• As we move down the hierarchy, the cost per bit generally
decreases, whereas the access time generally increases.

How a Modern Computer Works
• Device controller transfers blocks of data from buffer storage
directly to main memory without CPU intervention.

Computer System Architecture
 Single Processor Systems
– One CPU capable of executing instructions comes from user
processes including PDA’s and main frames.
 Multiple Processor Systems(Tightly coupled system)
– More than one CPU or one CPU with multiple cores.
– Processors share memory and a clock, communication usually
takes place through the shared memory.
Advantages :
1. Increased throughput : By increasing the number of
processors more work will be done in less time. When multiple
process cooperate on task, a certain amount of overhead is
incurred in keeping all parts working correctly.

Multi Processing Systems (Cont.)
2. Economy of scale : Multi processor system can save more
money than multiple processor, since they share peripherals,
mass storage & power supplies.
3. Increased reliability :If a program is distributed properly on
several processors, then the failure of one processor will not halt
the system but it only slows down.
• Symmetric multiprocessing (SMP)
– Each processor runs an identical copy of the operating system.
– Many processes can run at once without performance
deterioration.
– Most modern operating systems support SMP.

Symmetric Multiprocessing Architecture
• Asymmetric multiprocessing
-- Each processor is assigned a specific task; master processor
schedules and allocates work to slave processors.
-- More common in extremely large systems.

Clustered Systems
• Clustered systems gather together multiple CPUs to accomplish
computational work.
• Clustering provides high availability service i.e. service will
continue even if one or more systems in the cluster fail.
• In asymmetric clustering, one machine is in hot stand by mode
which monitors the active server and the other is running the
applications.
• If that server fails, the hot-standby host becomes the active server.
• In symmetric clustering, two or more hosts are running
applications and are monitoring each other.

Clustered Systems
Memory Layout for Multiprogrammed System

Operating System Structure
 Batch Operating System
--In 1980, there was scarcity of computers, main frame
computers are used.
-- 4 Jobs (J1,J2,J3,J4)
-- Efficiency is less and not interactive.
 Multiprogramming (1 CPU)
-- Process requires CPU time and I/O time.
-- J1 goes to I/O device then CPU can take other jobs & process it.
-- CPU will not be idle and busy all the times as a result efficiency
will be improved .

Multitasking
-- 4 jobs (J1,J2,J3,J4) CPU will choose J1 for sometime, J2 for
sometime and so on switching to remaining jobs.
-- CPU will be multiplexing all the jobs without completing
anything.
-- Interactivity can be improved.
Multiprocessing (Multiple CPU’s)
-- Lots of CPU on a single computer. As a result many jobs can be
processed simultaneously hence parallelism can improved.
-- Makes the processing faster and provides reliability.
-- E.g. : Dual core , Quad core

Operating System Operations
• To ensure the proper execution of the operating system, we must
be able to distinguish between the execution of operating-system
code and user defined code.
• Two separate modes of operation : User mode and Kernel mode.
• A bit, called mode bit is added to the hardware of the computer to
indicate the current mode: kernel (0) or user (1).
• When the computer system is executing on behalf of a user
application, the system is in user mode.
• When a user application requests a service from the operating
system (via a system call), it must transition from user to kernel
mode to fulfill the request.

Timer
• Ensure that operating system maintains control over the CPU.
• We cannot allow a user program to get stuck in an infinite loop and
never return control to the operating system. To accomplish this goal ,
we use timer.
• A timer can be set by the operating system to interrupt the computer
after a specified period. Every time clock ticks, counter is
decremented. When counter reaches 0, an interrupt occurs.

Process Management
• A process is a program in execution. It is a unit of work within the
system. Program is a passive entity, process is an active entity.
• Process needs resources like CPU, memory, I/O files to accomplish
its task.
• The operating system is responsible for the following activities in
connection with process management:
--Creating and deleting both user and system processes
--Suspending and resuming processes
-- Providing mechanisms for process synchronization
-- Providing mechanisms for process communication
--Providing mechanisms for deadlock handling

Memory Management
• To execute a program, all (or part) of the instructions must be in
memory.
• All (or part) of the data that is needed by the program must be in
memory. Memory management determines what is in memory .
• Memory management activities
– Keeping track of which parts of memory are currently being
used and by whom.
– Deciding which processes (or parts thereof) and data to move
into and out of memory.
– Allocating and deallocating memory space as needed.

Storage Management
• OS provides uniform, logical view of information storage known as
file.
• File-System management
– Files usually organized into directories
– Access control on most systems to determine who can access
what.
– OS activities include
• Creating and deleting files and directories
• Primitives to manipulate files and directories
• Mapping files onto secondary storage
• Backup files onto stable (non-volatile) storage media

Mass-Storage Management
• Usually disks used to store data that does not fit in main memory
or data that must be kept for a “long” period of time.
• Proper management is of central importance.
• Entire speed of computer operation hinges on disk subsystem and
its algorithms.
• OS activities
– Free-space management
– Storage allocation
– Disk scheduling

Caching
• Caching is an important principle of computer systems.
• Information is normally kept in main memory . As it is used,
it is copied into a cache-on a temporary basis.
• When we need a particular piece of information, cache is checked
first to determine if information is there
-- If it is, information used directly from the cache (fast)
-- If not, data is copied to cache under the assumption that we
will need it again soon and used there.
Migration of integer A from disk to register

• In a multiprocessor environment , each of the CPUs also contains a
local cache .Copy of A must exist simultaneously in several caches.
• Since the various CPUs can all execute concurrently ,we must make
sure that update to the value of A in one cache is immediately
reflected to all other caches.
• This situation is called cache coherency.

Distributed System
• A distributed system is a collection of physically separate, possibly
heterogeneous, computer systems that are networked to provide
the users with access to various resources that the system
maintains.
• A network is a communication path between two or more
systems.
• A LAN connects computers within a room , a floor or a building.
• A WAN usually links buildings, cities or countries.
• A MAN could link buildings within a city.
• A NOS is an operating system that provides features like file
sharing across the network which allows different processes on
different computers to exchange messages.

 Real Time Embedded System
• Embedded computers are the most prevalent form of computers in
existence and it will have some specific task.
• These devices are found everywhere, from car engines and
manufacturing robots to DVDs and microwave ovens.
 Multimedia Systems
• Multimedia data consist of video and audio files such as MP3, DVD
movies, video conferencing, and short video clips of movie
previews or news stories downloaded over the internet.
 Handheld Systems
• It includes Pocket-Pc’s, cellular telephones which use special-
purpose embedded operating systems.
• Because of their size, most handheld devices have small amounts of
memory, slow processors, and small display screens.

Computing Environments
 Traditional Computing
• In a "typical office environment" consisted of PCs connected to a
network, with servers providing file and print services.
• Portability was achieved by the use of laptop computers.
• At home, most users had a single computer with a slow modem
connection to the office, the internet, or both.
• Today, network-connection speeds are available at great cost and
relatively inexpensive, giving home users more access to more data.

Client Server Computing
• Server systems can be of two types : Compute server ,File server
• Compute server system provides an interface to which a client can
send a request to perform an action (Eg: to read data) in response,
the server executes the action and sends back results to the client.
E g : A server running a database that responds to client requests
for data.
• File server system provides a file-system interface where clients
can create, update, read, and delete files.
E g : Webserver that delivers files to clients running Web browsers.

Peer to Peer Computing
• No distinguish between clients and servers . All the nodes are
treated as peers(either client or server).
• Here, services can be provided by several nodes distributed
throughout the network.
• To participate in a peer-to-peer system, a node must first join the
network of peers.
• Once a node has joined the network, it can begin providing services
to and requesting services from other nodes in the network.
• When a node joins a network, it registers its service with a
centralized lookup service on the network. Any node desiring a
specific service first contacts this centralized lookup service to
determine which node provides the service.

Operating System Services
• Operating systems provide an environment for execution of
programs and services to the users.

File-system manipulation- The file system is of particular interest.
Programs need to read , write files and directories, create and
delete them, search them, list file Information, permission
management.
Communication – Processes may exchange information, on the
same computer or between computers over a network . It can be
shared memory/message passing.
Error detection – Errors occur in the CPU, memory hardware, I/O
devices, user program.
• For each type of error, OS should take the appropriate action
to ensure correct and consistent operation.
• Debugging facilities can greatly enhance the users and
programmers abilities to efficiently use the system.

Operating System Services
Resource allocation - When multiple users or multiple jobs
running concurrently, resources must be allocated to each of
them.
Accounting - To keep track of which users use how much and
what kinds of computer resources.
Protection and security - The owners of information stored in a
networked computer system may want to control use of that
information, concurrent processes should not interfere with
each other.
• Protection involves ensuring that all access to system
resources is controlled.
• Security of the system from outsiders requires user
authentication, extends to defending external I/O devices
from invalid access attempts.

User Operating System Interface - CLI
• One set of operating-system services provides functions that are
helpful to the user:
– User interface - Almost all operating systems have a user
interface (UI).
• Varies between Command-Line(CLI), Graphics User
Interface (GUI), Batch.
 CLI or command interpreter allows direct command entry
– Primarily fetches a command from user and executes it.

User Operating System Interface - GUI
 User-friendly desktop metaphor interface
– Usually mouse, keyboard, and monitor.
– Icons represent files, programs, actions, etc.
– Invented at Xerox PARC.
 Touchscreen Screen Interfaces
-- Mouse not possible or not desired
-- Actions and selection based on gestures
-- Virtual keyboard for text entry
-- Voice commands.

System Calls
• Programming interface to the services provided by the OS.
• Typically written in a high-level language (C or C++).
• System call sequence to copy the contents of one file to
another file.

System Call Implementation
• Typically, a number associated with each system call
– System-call interface maintains a table indexed according to
these numbers.
• The system call interface invokes the intended system call in OS
kernel and returns status of the system call and return values.
• The caller need know nothing about how the system call is
implemented.
– Just needs to obey API and understand what OS will do as a
result of system call.
– Most details of OS interface hidden from programmer by API.
• Managed by run-time support library (set of functions built
into libraries included with compiler).

API – System Call – OS Relationship

Standard C Library Example
• C program invoking printf() library call, which calls write()
system call

Types of System Calls
• Process control
– create process, terminate process
– end, abort
– load, execute
– get process attributes, set process attributes
– wait for time
– wait event, signal event
– allocate and free memory
• Information maintenance
– get time or date, set time or date
– get system data, set system data
– get and set process, file, or device attributes

Types of System Calls
• File management
– create file, delete file
– open, close file
– read, write, reposition
• Device management
– request device, release device
– read, write, reposition
– get device attributes, set device attributes
– logically attach or detach devices
• Protection
– Control access to resources
– Get and set permissions
– Allow and deny user access

System Programs
• System programs provide a convenient environment for program
development and execution(Editors , Compilers). System programs
can be divided in to 6 categories .
 File management - Create, delete, copy, rename, print, list, and
generally manipulate files and directories.
 Status information
– Some ask the system for information - date, time, amount of
available memory, disk space, number of users.
– Some systems implement a registry - used to store and retrieve
configuration information.
 Application programs
- Run by users
– Not typically considered part of OS.
– Launched by command line, mouse click.

System Programs (Cont.)
 File modification
– Text editors to create and modify files.
– Special commands to search contents of files or perform
transformations of the text.
 Programming Language Support - Compilers, assemblers, debuggers
and interpreters are used.
 Communications - Provide the mechanism for creating virtual
connections among processes, users, and ‘n’ computer systems.
 Program loading and Execution
 Background Services
– Provide facilities like disk checking, process scheduling, error
logging, printing
– Run in user context not kernel context and also known as
services, subsystems.

Operating System Design and Implementation
• Design and implementation of OS not “solvable”, but some
approaches have proven successful.
• Internal structure of different operating systems can vary widely.
Start the design by defining goals and specifications .
• Affected by the choice of hardware, type of system(batch, single
user, multi user).
• User goals and System 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 flexible, reliable, error-free,
and efficient.

Operating System Design and Implementation
• Important principle is to separate mechanism and policy.
Policy: What will be done?
Mechanism: How to do something?
• Policies are likely to change across places or over time . Each
change in policy would require a change in mechanism.
• Timer construct is a mechanism for ensuring CPU protection but
deciding how long the timer is to be set for a particular user is a
policy decision.
• Specifying and designing an OS is highly creative task of software
engineering.

Implementation
• Traditionally OS’s have been written in assembly language.
• Master Control Program was the first system that was not written
in assembly language. It is a variant of ALGOL.
• MULTICS developed at MIT was written mainly in PL/1.
• Linux and Windows XP operating system are written in C.
• Use of higher-level language : Code can be written faster, more
compact , easier to understand ,debug and port.
• Emulators are the programs that duplicate the functionality of
one system with another system.
• Disadvantage of implementing in higher language : Reduced speed
and increased storage requirement.

Operating System Structure
 MS-DOS (Microsoft Disk Operating System )
• Do not have well defined structure and was written to provide the
most functionality in the least space so not divided into modules.
• Its interfaces and level of functionality are not well separated.
E.g. : Application programs are able to access the basic I/O routine to
write directly to the display . As a result when user program fails
entire system crashes.

Non Simple Structure -- UNIX
 UNIX – limited by hardware functionality, the original UNIX
operating system had limited structuring.
 The UNIX OS consists of two separable parts - system programs
and kernel.
Kernel :
• Consists of everything below the system-call interface and
above the physical hardware.
• Provides the file system, CPU scheduling, memory
management, and other operating-system functions, a large
number of functions for one level.

Traditional UNIX System Structure
Beyond simple but not fully layered

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 the functions
(operations) and services of only
lower-level layers.
 Layered approach involves defining
the various layers. Because a layer can
use only lower-level layers, hence
careful planning is necessary.

Advantages of Layered Approach
 E g : The device driver for the backing store must be at a lower
level than the memory-management routines, because memory
management requires the ability to use the backing store.
 Simplicity of construction and debugging : The layers are selected
so that each uses functions and services of only lower level layers.
 Simplicity of design and implementation :
• The first layer can be debugged without any concern for the rest
of the system.
• Once the first layer is debugged, its correct functioning can be
assumed while the second layer is debugged, and so on.
• If an error is found during the debugging of a particular layer, the
error must be on that layer, because the layers below it are
already debugged.

Virtual Machines
• The idea behind virtual machine is to abstract the hardware of a
single computer into several different execution environments,
thereby creating the illusion that each separate execution
environment is running its own private computer.

Benefits of virtual machines
• Host system is protected from the virtual machines. A virus inside a
guest operating system might damage that operating system but it
will not affect the host or the other guests.
• Multiple operating systems can be running on the developer's
workstation concurrently .
• Production of data-center use is system consolidation which involves
taking two or more separate systems and running them in virtual
machines on one system.
• A virtual-machine system is a perfect vehicle for operating-systems
research and development.

• A virtual machine is a program that acts as a virtual computer. It runs
on your current operating system – the “host” operating system –
and provides virtual hardware to “guest” operating systems.
• The guest operating systems run in windows on your host operating
system, just like any other program on your computer.
• The guest operating system runs normally, as if it were running on a
physical computer – from the guest operating system’s perspective,
the virtual machine appears to be a real, physical computer.

VM Ware
 In this scenario, Linux is running as the host operating system and
FreeBSD, Windows NT, and Windows XP are running as guest
operating systems.
 The virtualization layer is the heart of VMware, as it abstracts the
physical hardware into isolated virtual machines running as guest
operating systems.
 Each virtual machine has its own virtual CPU, memory, disk drives,
network interfaces, and so forth.

Operating System generation
• The system must then be generated for each specific computer
site, a process sometimes known as SYSGEN.
• SYSGEN program reads from a given file, or asks the operator of
the system for information concerning the specific configuration of
the hardware.
• The following kinds of information must be determined.
 What CPU is to be used?
 How much memory is available?
 How will the boot disk be formatted? How many sections, or
"partitions,“ will it be separated into, and what will go into each
partition?
 What operating-system options are desired? how many buffers of
which sizes should be used, what type of CPU-scheduling
algorithm is desired, what the maximum number of processes to be
supported.

Process Concept
• Process – Program in execution; process execution must
progress in sequential fashion.
• Program is passive entity stored on disk (executable file),
process is active entity with a program counter specifying
the next instruction to execute.
– Program becomes process when executable file loaded
into memory.

Process Concept (Cont.)
• Multiple parts
– The program code, also called text section
– Current activity including program counter,
processor registers.
– Stack containing temporary data
• Function parameters, return addresses, local
variables
– Data section containing global variables
– Heap containing memory dynamically allocated
during run time

Process State
• As a process executes, it changes state.
• State of a process is defined as current activity of the process.
– 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.

Threads
• Process is a program that performs a single thread of execution.
• E g : When a process is running a word-processor program, a
single thread of instructions is being executed. This single thread of
control allows the process to perform only one task at a time.
• The user cannot simultaneously type in characters and run the
spell checker within the same process.
• Many modern operating systems have extended the process
concept to allow a process to have multiple threads of execution
and thus to perform more than one task at a time.

Process Control Block (PCB)
• Each process is represented in operating system by a process
control block.
• Process state – running, waiting, etc.
• Program counter – Address of the next instruction to be executed
for this process.
• CPU registers – contents of all process-centric registers.
• CPU scheduling information- Process priority, pointers to
scheduling queues .
• Memory-management information – memory allocated to the
process
• Accounting information – CPU used, clock time elapsed since start,
time limits
• I/O status information – List of I/O devices allocated to process, list
of open files.

CPU Switch From Process to Process
PCB

Process Scheduling
• Multiprogramming :To have some process running at all times.
• Time Sharing : Switch the CPU among processes so frequently
that users can interact with each program while it is running.
• Process scheduler selects among available processes for next
execution on CPU.
• Scheduling queues: As processes enters the system , they are
put in to job queue.
– Job queue – set of all processes in the system
– Ready queue – set of all processes residing in main memory,
ready and waiting to execute are kept on a list.
– Device queues – set of processes waiting for an I/O device.
– Processes migrate among the various queues

Ready Queue
Ready queue is generally stored as a linked list.
 A ready-queue header contains pointers to the first and final
PCBs in the list.
Each PCB includes a pointer field that points to the next PCB
in the ready queue.

Representation of Process Scheduling
Queuing diagram :
Each rectangular box represents a queue. The circles represent the
resources that serve the queues.
The arrows indicate the flow of processes in the system.
A new process is initially put in the ready queue. Once the process is
allocated the CPU and is executing, one of several events could occur.

Schedulers
• Short-term scheduler (CPU scheduler) – selects which process
should be executed next and allocates CPU.
– Short-term scheduler is invoked frequently (milliseconds) 
(must be fast)
• Long-term scheduler (job scheduler) – selects which processes
should be brought into the ready queue.
– Long-term scheduler is invoked 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.

Addition of Medium Term Scheduling
 Medium-term scheduler can be added if degree of multiple
programming needs to decrease.
 Remove process from memory, store on disk, bring back in
from disk to continue execution: swapping
 It is necessary to improve process mix because change in
memory requirements has overcommitted the available
memory requiring memory to be freed up.

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 via a context switch.
• Context of a process represented in the PCB.
• Context-switch time is overhead; the system does no useful work
while switching.
– The more complex the OS and the PCB  the longer the
context switch.
• Operations on Process : System must provide mechanisms for
– process creation
– process termination

Process Creation
• Parent process create children processes, which, in turn create
other processes, forming a tree of processes.
• Generally, process identified and managed via a process
identifier (pid).
• Resource sharing options
– Parent and children share all resources.
– Children share subset of parents resources.
• Execution options
– Parent and children execute concurrently.
– Parent waits until children terminates.

Process Creation (Cont.)
 Address space
• The child process is a duplicate of the parent process (it has the
same program and data as the parent).
• The child process has a new 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.

C Program Forking Separate Process

Process Termination
• Process executes last statement and then asks the operating
system to delete it using the exit() system call.
– Returns status data from child to parent (via wait())
– Process resources are deallocated by operating system.
• Parent may terminate the execution of children processes using
the abort() system call. Some reasons for doing so:
– Child has exceeded allocated resources.
– Task assigned to child is no longer required.
– The parent is exiting and the operating system does not allow
a child to continue if its parent terminates.

Process Termination
• Some operating systems do not allow child to exists if its parent
has terminated. If a process terminates, then all its children must
also be terminated.
– cascading termination : All children, grandchildren, etc. are
terminated.
– The termination is initiated by the operating system.
• The parent process may wait for termination of a child process by
using the wait()system call.The call returns status information
and the pid of the terminated process
pid = wait(&status);
• If parent is waiting ( invoke wait()) process is a zombie.
• If parent terminated without invoking wait , process is an
orphan.

Interprocess Communication
• Processes within a system may be independent or cooperating.
• Cooperating process can affect or be affected by other processes,
including sharing data.
• Independent process cannot affect or be affected by the execution
of another process.
• Cooperating processes need interprocess communication (IPC)
• Two models of IPC
– Shared memory
– Message passing

Cooperating Processes
 Advantages of process cooperation
• Information sharing :Several users want to access same piece of
information (Eg: a shared file), we must provide an environment to
allow concurrent access to such information.
• Computation speed-up: If we want a particular task to run faster,
we must break it into subtasks, each of which will be executing in
parallel with the others.
• Modularity: We may want to construct the system in a modular
fashion, dividing the system functions into separate
processes/threads.
• Convenience :An individual user may work on many tasks at the
same time.

Communications Models
Message Passing : Communication takes place by means of
messages exchanged between the cooperating process.
Shared Memory : A region of memory is shared among the
cooperating process . Process can then exchange information by
reading and writing data to shared region.

Shared Memory : Producer-Consumer Problem
• Producer and consumer processes to run concurrently, we must
have available a buffer of items that can be filled by the producer
and emptied by the consumer.
• The buffer will reside in a region of memory that is shared by the
producer and consumer processes. A producer can produce one
item while the consumer is consuming another item.
• The producer and consumer must be synchronized, so that the
consumer does not try to consume an item that has not yet been
produced.

Bounded-Buffer – Shared-Memory Solution
• 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.
• Shared data
#define BUFFER_SIZE 10
typedef struct {
. . .
} item;
item buffer[BUFFER_SIZE];
int in = 0;
int out = 0;

Bounded-Buffer – Producer
item next_produced;
while (true) {
/* produce an item in next produced */
while (((in + 1) % BUFFER_SIZE) == out)
; /* do nothing */
buffer[in] = next_produced;
in = (in + 1) % BUFFER_SIZE;
}

Bounded Buffer – Consumer
item next_consumed;
while (true) {
while (in == out)
; /* do nothing */
next_consumed = buffer[out];
out = (out + 1) % BUFFER_SIZE;
/* consume the item in next consumed */
}

Message Passing
• If processes P and Q wish to communicate, they need to:
– Establish a communication link between them.
– Exchange messages via send/receive.
• Implementation issues:
– 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?

Message Passing (Cont.)
• Implementation of communication link
– Physical:
• Shared memory
• Hardware bus
• Network
– Logical:
• Direct or indirect
• Synchronous or asynchronous
• Automatic or explicit buffering

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 bi-
directional

Indirect Communication
• Operations
– create a new mailbox (port)
– 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

Indirect Communication
• 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 arbitarily the receiver. Sender is
notified who the receiver was.

Synchronization
• Message passing may be either blocking or non-blocking.
• Blocking is considered synchronous.
– Blocking send -- the sender is blocked until the message is
received
– Blocking receive -- the receiver is blocked until a message is
available.
• Non-blocking is considered asynchronous.
– Non-blocking send -- the sender sends the message and
continue.
– Non-blocking receive -- the receiver receives:
 A valid message, or Null message

Buffering
• Whether communication is direct or indirect, messages exchanged
by communicating processes reside in a temporary queue.
• Implemented in one of three ways
1.Zero capacity – Queue has maximum length zero.Sender must
block until recipient receives the message.
2.Bounded capacity – finite length of n messages. If the queue is not
full when a new message is sent, the message is placed in the queue
and the sender can continue execution without waiting.
3.Unbounded capacity – infinite length .Any number of messages can
wait in it. The sender never blocks.

Module 1 Introduction.ppt

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Module 1 Introduction.ppt