SOCSAMS e-learning
INTERPROCESS
COMMUNICATION
AND
SYNCHRONIZATION
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
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 user’s code)
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Concurrency - Basic Concept
A simple batch operating system can be viewed as 3
processes :
• a reader process
• an executor process
Input
Buffer
Process
• a printer process
Output
Buffer
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Inter-Process Communication
Basic Concepts of IPC & Synchronization:
In
order
processes
to
cooperate,
must
concurrently
communicate
and
executing
synchronize.
Interprocess communication is based on the use of
shared variables (variables that can be referenced by
more than one process) or message passing.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Inter-Process Communication
Basic Concepts of IPC & Synchronization:
Synchronization 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.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
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.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Inter-Process Communication
• Critical Resource
• Critical Section
• Mutual Exclusion
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
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
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Inter-Process Communication
• Critical Resource
• Critical Section
It is code that accesses a critical resource.
• Mutual Exclusion
Dept. of Computer Applications, MES College Marampally
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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.
Dept. of Computer Applications, MES College Marampally
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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.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
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)
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
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.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Message-Passing System
Methods for logically implementing a communication link
and the send/receive operations are classified into:
• Naming
• Buffering
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
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.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Message-Passing System
Methods for logically implementing a communication link
and the send/receive operations are classified into:
• Naming
• Buffering
Consisting of capacity
message properties.
and
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
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
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
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.
Dept. of Computer Applications, MES College Marampally
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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.
Dept. of Computer Applications, MES College Marampally
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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.
Dept. of Computer Applications, MES College Marampally
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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
Dept. of Computer Applications, MES College Marampally
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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.
Dept. of Computer Applications, MES College Marampally
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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.
Dept. of Computer Applications, MES College Marampally
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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.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
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.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Interprocess Synchronization
Race Condition
Process A
4
5
6
.
.
abc
Prog.c
Prog.n
next file
to be printed
Out = 4
In = 7
7
Process B
.
.
next free slot
in the directory
Value depends on which of the processes wins the race to
update the variable.
Dept. of Computer Applications, MES College Marampally
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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
Dept. of Computer Applications, MES College Marampally
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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.
Dept. of Computer Applications, MES College Marampally
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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.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Critical Section
Do {
entry section
critical section
exit section
remainder section
} while(1);
General structure of a typical process
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Critical Section
Do {
entry section
critical section
exit section
Section of code that request
permission to enter its critical
section.
remainder section
} while(1);
General structure of a typical process
Dept. of Computer Applications, MES College Marampally
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Critical Section
Do {
entry section
critical section
exit section
It is a part of code in which it is
necessary to have exclusive
access to shared data.
remainder section
} while(1);
General structure of a typical process
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Critical Section
Do {
entry section
critical section
Code for tasks just after exiting
from the critical section.
exit section
remainder section
} while(1);
General structure of a typical process
Dept. of Computer Applications, MES College Marampally
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Critical Section
Do {
entry section
critical section
The remaining code.
exit section
remainder section
} while(1);
General structure of a typical process
Dept. of Computer Applications, MES College Marampally
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Mutexes : Mutual Exclusion
var P1busy, P2busy : boolean;
{parent process}
P1busy:=false;
P2busy:=false;
initiate P1, P2;
end; {mutex}
Dept. of Computer Applications, MES College Marampally
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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;
P1busy:=false;
other_P1busy_processing;
end{while}
End; {P1}
{parent process}
P1busy:=false;
P2busy:=false;
initiate P1, P2;
end; {mutex}
Dept. of Computer Applications, MES College Marampally
process P2
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begin
while true do
begin
P2busy := true;
while P1busy do {keep
testing};
critical-section;
P2busy:=false;
other_P2busy_processing;
end{while}
End; {P2}
Mutexes : Mutual Exclusion
var P1busy, P2busy : boolean;
process P1
begin
while true do
begin
P1busy := true;
while P2busy do {keep testing};
critical-section;
P1busy:=false;
other_P1busy_processing;
end{while}
End; {P1}
{parent process}
P1busy:=false;
P2busy:=false;
initiate P1, P2;
end; {mutex}
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Critical-Section Problem
boolean flag[2];
int turn;
do {
flag[i] = true;
turn = j;
while (flag[j] && turn == j);
Critical-section
flag[i] = false;
remainder-section
} while(1);
Initially flag[0]=flag[1]=false
Turn = 0 or 1.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Bakery Algorithm
boolean choosing[n];
int number[n];
do {
choosing[i] = true;
number[i] = max(number[0], number[1], …, number[n-1]) + 1;
choosing[i] = flase;
for (j=0; j<n; j++){
while (choosing[j]);
while((number[j]!=0) && ((number[j],j) < (number[i],j)));
}
critical-section
number[i] = 0;
remainder-section
} while(1);
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Thank You
Murugan R
Dept. Computer Applications
MES College Marampally
Dept. of Computer Applications, MES College Marampally