Index

Chapter Eight

Process Synchronisation

Index

Objectives• Concurrent processes and

Asynchronous concurrent processes

• Process synchronisation

• Mutual exclusion

Index

Terminology• Critical Section– when it is accessing shared data in a

system.

• Semaphore– Used to implement the idea of mutual

exclusion due to multiple access

• Concurrent process– Processes are said to be concurrent if they

exist at the same time.

Index

Software implementationIdea of process synchronization

While gate = closed : DO null operation

(gate is open)

gate := closed : so that other cannot use it

………..

………..

Critical section

……….

gate := open : let other to use it

Above is impractical for multi-processes

Index

Process Type• Concurrent processes– processes exist at the same time– if on a single CPU system, then context

switching are required

• Asynchronous concurrent processes– concurrent processes function

independently– require occasional synchronization or

interprocess communication

IndexAsymmetric and Symmetric Processing

Index

Mutual Exclusion• To ensure the integrity of data,

shared data cannot be used by more than ONE process at the same time

• Otherwsie data will be updated incorrectly

Index

Critical Sections• When a process is accessing shared

modifiable data, it is in “Critical Section”

Index

Features of Critical Sections• when a process is in the critical section,

other processes may continue execution outside the critical region.

• when a process leaves its critical section, one other process waiting to enter the critical section may proceed.

• Mutual exclusion can be implemented by means of SEMAPHORES.

Index

Synchronization• A process may not be able to

continue until a service is provided.

• It involves the signaling between two or more processes.

• Process synchronization can be achieved by semaphores.

Index

Process Synchronization

Index

Two processes Compilation

Index

Semaphore• Semaphore was introduced by

Dijksta in 1965.

• A semaphore is a protected variable S.

• The value can be accessed and altered by three operations, namely, wait (P(S)), signal (V(S)) and initialization operations only.

Index

Characteristics of semaphores• Binary semaphores can assume only

the value 0 or 1

• Counting semaphores can assume only non-negative integer values

• P(S) and V(S) are the initial Dutch words. P(S) refers to wait; while V(S) is signal.

Index

Code for binary semaphoreP operation on variable S, P(S) is

• if S > 0

• then S := S - 1 (means no process is using)

• Else (wait on S to change value)

V operation (V means to signify), V(S) is

• If (one or more processes is/are waiting on S to change.)

• then (let one of theses processes proceed )

• else S:= S + 1 (no process in the semaphore queue)

Index

Two-process synchonization

Index

Software ExampleProgram example-one var active: semaphore;

Procedure process-A

begin

while true do

begin

P(active);

critical-sec;

:

V(active);

:

end

end;

Procedure process-B

begin

while true do

begin

:

P(active);

critical-sec;

:

V(active);

:

end

end;

Index

Software Example (Continue…)Begin

semaphore-init(active,1); /* initialise the semaphore variable active */

parbeginprocess-A;

process-B;

parend

end.

Index

Mutual Exclusion• When a resource can be accessed by at

least two processes, it can only be exclusively used by one of them until it is completed.

• In a system providing multiple processes, it is desirable to co-ordinate the activities of them

• The relationship can be described by a set of processes called producer-consumer.

Index

Producer and Consumer

IndexImplementation for mutual exclusion • Dekker’s Primitives

• Interrupt masking

• locks

IndexExample for Mutual ExclusionProcedure producer

While true do

{

....

produce an item

....

P(empty);

P(mutex);

....

add item to buffer;

....

V(mutex);

V(full);

}

Procedure consumer

While true do

{

P(full);

P(mutex);

....

remove an item from buffer to “local”

....

V(mutex);

V(empty);

....

consume the item in “local”

}

Index

Summary• Process synchronisation used to solve

synchronisation by semaphore.

• Those waiting to be served is at semaphore queue.

• To avoid deadlock, processes should ensure that the resources are exclusively used

• Producer and consumer model is used for mutual exclusion.