INTERPROCESS COMMUNICATION

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INTERPROCESS COMMUNICATION AND SYNCHRONIZATION
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Concurrency - Basic Concept

• In a multi-programming environment, there will be
  concurrent processes which are of two types
• Operating system processes (those that execute
  system code)
• User processes (those that execute users code)

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Concurrency - Basic Concept

• A simple batch operating system can be viewed as
  3 processes
• a reader process
• an executor process
• a printer process

Input Buffer
Process
Output Buffer
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Inter-Process Communication
Basic Concepts of IPC Synchronization In order
to cooperate, concurrently executing processes
must communicate and synchronize. Interprocess
communication is based on the use of shared
variables (variables that can be referenced by
more than one process) or message passing.
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Inter-Process Communication
Basic Concepts of IPC Synchronization Synchroni
zation is often necessary when processes
communicate. Processes are executed with
unpredictable speed. Yet to communicate one
process must perform some action such as setting
the value of a variable or sending a message that
the other detects. This only works if the events
perform an action or detect an action are
constrained to happen in that order. Thus one can
view synchronization as a set of constraints on
the ordering of events. The programmer employs a
synchronization mechanism to delay execution of a
process in order to satisfy such constraints.
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Inter-Process Communication
IPC is a capability supported by operating system
that allows one process to communicate with
another process. The processes can be running on
the same computer or on different computers
connected through a network. IPC enables one
application to control another application, and
for several applications to share the same data
without interfering with one another.
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Inter-Process Communication

• Critical Resource
• Critical Section
• Mutual Exclusion

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Inter-Process Communication

• Critical Resource
• It is a resource shared with constraints on its
  use(e.g., memory, files, printers, etc)
• Critical Section
• Mutual Exclusion

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Inter-Process Communication

• Critical Resource
• Critical Section
• It is code that accesses a critical resource.
• Mutual Exclusion

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Inter-Process Communication

• Critical Resource
• Critical Section
• Mutual Exclusion
• At most one process may be executing a critical
  section with respect to a particular critical
  resource simultaneously.

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Inter-Process Communication

• IPC can be possible in two different ways
• Shared-Memory System
• Message-Passing System

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Shared-Memory System
Shared-memory system require communication
processes to share some variables. The processes
are expected to exchange information through the
use of these shared variables. Responsibility for
providing communication rests with the
application programmer. The OS only needs to
provide shared memory.
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Message-Passing System

• MPS allow communication processes to exchange
  messages responsibility for providing
  communication rest with OS.
• The function of a MPS is to allow processes to
  communicate with each other without the need to
  resort to shared variables.
• An IPC facility basically provides two
  operations
• send(message)
• receive(message)

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Message-Passing System

• MPS allow communication processes to exchange
  messages responsibility for providing
  communication rest with OS.
• The function of a MPS is to allow processes to
  communicate with each other without the need to
  resort to shared variables.
• An IPC facility basically provides two
  operations
• send(message)
• receive(message)

In order to send and to receive messages, a
communication link must exist between the two
involved processes.
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Message-Passing System

• Methods for logically implementing a
  communication link and the send/receive
  operations are classified into
• Naming
• Buffering

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Message-Passing System

• Methods for logically implementing a
  communication link and the send/receive
  operations are classified into
• Naming
• Buffering

Consisting of direct and indirect communication.
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Message-Passing System

• Methods for logically implementing a
  communication link and the send/receive
  operations are classified into
• Naming
• Buffering

Consisting of capacity and message properties.
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Direct Communication

• Each process that wants to communicate must
  explicitly name the recipient or sender of the
  communication. In this scheme the send and
  receive primitives are
• send(P, message) send a message to process P
• receive(Q, message) receive a message from
  process Q

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Direct Communication

• Each process that wants to communicate must
  explicitly name the recipient or sender of the
  communication. In this scheme the send and
  receive primitives are
• send(P, message) send a message to process P
• receive(Q, message) receive a message from
  process Q

Here a link is established automatically between
every pair of processes that want to communicate.
Exactly one link exists between each pair of
processes.
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Direct Communication

• Symmetry in addressing
• send(P, message) send a message to process P
• receive(Q, message) receive a message from
  process Q
• This scheme shows the symmetry in addressing
  that is both the sender and the receiver
  processes must name the other to communicate.

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Direct Communication

• Asymmetry in addressing
• send(P, message) send a message to process P
• receive(id, message) receive a message from
  any process the variable id is set to the name
  of the process with which communication has
  taken place.
• Only the sender names the recipient the
  recipient is not required to name the sender.

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Indirect Communication

• With indirect communication, the message are sent
  to and receive from a mailbox. It is an object
  into which messages may be placed and from which
  messages may be removed by processes. Each
  mailbox owns a unique identification. A process
  may communicate with some other process by a
  number of different mailboxes.
• send(A, message) send a message to mailbox A
• receive(A, message) receive a message from
  mailbox A

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Indirect Communication
Mailbox owned by process Mailboxes may be owned
by either by a process or by the system. If the
mailbox is owned by a process, then the owner
who can only receive from this mailbox and the
user who can only send message to the mailbox are
to be distinguished. When a process that owns a
mailbox terminates, its mailbox disappears.
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Indirect Communication

• Mailbox owned by the OS
• It has an existence of its own, i.e., it is
  independent and not attached to any particular
  process.
• The OS provides a mechanism that allows a process
  to
• create a new mailbox
• send and receive message through the mailbox
• destroy a mailbox

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Buffering

• Whether the communication is direct or indirect,
  messages exchanged by communicating processes
  reside in a temporary queue. This queue can be
  implemented in three ways
• Zero capacity
• Bounded capacity
• Unbounded capacity

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Buffering - Capacity
Zero capacity The queue has maximum length 0
thus, the link cannot have any messages waiting
in it. In this case, the sender must block until
the recipient receives the message. The
zero-capacity link is referred to as a
message-passing system with no buffering.
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Buffering - Capacity
Bounded capacity The queue has finite length n
thus, at most n messages can reside in it. If a
new message is sent, and the queue is not full,
it is placed in the queue either by copying the
message or by keeping a pointer to the message
and the sender can continue execution without
waiting. If the link is full, the sender must
block until space is available in the queue.
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Buffering - Capacity
Unbounded capacity The queue has potentially
infinite length thus, any number of messages can
wait in it. The sender never blocks. Bounded and
Unbounded capacity link is referred to as
message-passing system with automatic buffering.
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Interprocess Synchronization
next file to be printed
Race Condition Value depends on which of the
processes wins the race to update the variable.
. .
Out 4
4
abc
5
Prog.c
In 7
6
Prog.n
Process A
7
Process B
next free slot in the directory
. .
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Interprocess Synchronization

• Serialization
• Make an operating system not to perform several
  tasks in parallel.
• Two strategies to serializing processes in a
  multitasking environment
• The Scheduler can be disabled
• A Protocol can be introduced

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Interprocess Synchronization

• The Scheduler can be disabled
• Scheduler can be disabled for a short period of
  time, to prevent control being given to another
  process during a critical action like modifying
  shared data.
• Inefficient on multiprocessor machines, since all
  other processors have to be halted every time one
  wishes to execute a critical section.

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Interprocess Synchronization

• A Protocol can be introduced
• A protocol can be introduced which all programs
  sharing data must obey. The protocol ensures that
  processes have to queue up to gain access to
  shared data.

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Critical Section
Do entry section critical section
exit section remainder section
while(1) General structure of a typical process
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Critical Section
Do entry section critical section
exit section remainder section
while(1) General structure of a typical process
Section of code that request permission to enter
its critical section.
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Critical Section
Do entry section critical section
exit section remainder section
while(1) General structure of a typical process
It is a part of code in which it is necessary to
have exclusive access to shared data.
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Critical Section
Do entry section critical section
exit section remainder section
while(1) General structure of a typical process
Code for tasks just after exiting from the
critical section.
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Critical Section
Do entry section critical section
exit section remainder section
while(1) General structure of a typical process
The remaining code.
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Mutexes Mutual Exclusion
var P1busy, P2busy boolean
parent process P1busyfalse
P2busyfalse initiate P1, P2 end mutex
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Mutexes Mutual Exclusion
var P1busy, P2busy boolean
process P1 begin while true do begin
P1busy true while P2busy do keep
testing critical-section
P1busyfalse other_P1busy_processing
endwhile End P1
parent process P1busyfalse
P2busyfalse initiate P1, P2 end mutex
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Mutexes Mutual Exclusion
process P2 begin while true do begin
P2busy true while P1busy do keep
testing critical-section
P2busyfalse other_P2busy_processing
endwhile End P2
var P1busy, P2busy boolean
process P1 begin while true do begin
P1busy true while P2busy do keep
testing critical-section
P1busyfalse other_P1busy_processing
endwhile End P1
parent process P1busyfalse
P2busyfalse initiate P1, P2 end mutex
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Critical-Section Problem
boolean flag2 int turn do flagi
true turn j while (flagj
turn j) Critical-section flagi
false remainder-section while(1)
Initially flag0flag1false Turn 0 or 1.
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Bakery Algorithm
boolean choosingn int numbern do
choosingi true numberi
max(number0, number1, , numbern-1) 1
choosingi flase for (j0 jltn
j) while (choosingj)
while((numberj!0) ((numberj,j) lt
(numberi,j)))
critical-section numberi 0
remainder-section while(1)
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Thank You

• Murugan R
• Dept. Computer Applications
• MES College Marampally