06 lcd slides 1 - PROCESS SYNCHRONIZATION POWERPOINT

Process Synchronization
 Race Conditions
 The Critical Section
 Semaphores
 Classic Synchronization Problems

*Property of STI J0024
 Race conditions usually occur if two or more processes are allowed to modify
the same shared variable at the same time.
 To prevent race conditions, the operating system must perform process
synchronization to guarantee that only one process is updating a shared variable
at any one time.

 part of the process that contains the instruction or instructions that access a
shared variable or resource
Process P1
.
.
.
mov ax, counter
inc ax
mov counter, ax
.
.
.
Process P2
.
.
.
mov ax, counter
inc ax
mov counter, ax
.
.
.
Critical
Section of
Process
P1
Critical
Section of
Process
P2
Figure 6.1 Critical Sections for Process P1 and P2

Software solutions to the critical section problem:
SOLUTION 1
Process P0
.
.
.
while (TurnToEnter != 0) { };
TurnToEnter = 1;
.
.
.
critical section
Process P1
.
.
.
while (TurnToEnter != 1) { };
TurnToEnter = 0;
.
.
.
critical section
Figure 6.2 Critical Sections of Processes P1 and P2 for Solution 1

SOLUTION 2
Process P0
.
.
.
WantToEnter[0] = true;
while (WantToEnter[1] == true) { };
WantToEnter[0] = false;
.
.
.
critical section
Process P1
.
.
.
while (WantToEnter[0] == true) { };
.
.
.
critical section
Figure 6.3 Critical Section P1 and P2 for Solution 2

SOLUTION 3 (Peterson’s Algorithm)
Process P0
.
.
.
TurnToEnter = 1;
while (WantToEnter[1] == true && TurnToEnter == 1) { };
.
.
.
critical section
Process P1
.
.
.
TurnToEnter = 0;
while (WantToEnter[0] == true && TurnToEnter == 0) { };
.
.
.
critical section
Figure 6.4 Critical Sections of Processes P1 and P2 for Solution 3

SOLUTION TO THE CRITICAL SECTION PROBLEM INVOLVING SEVERAL
PROCESS ( Bakery Algorithm)
Process Pi
.
.
.
choosing[i] = true;
number[i] = max(number[0], ... , number[n – 1]) + 1;
choosing[i] = false;
for (j = 1; j < n; j++) {
while (choosing[j] == true) { }
while ((number[j] != 0) &&
((number[j] < number[i]) || (number[j] == number[i]) && (j < i))) { };
}
number[i] = 0;
.
.
.
critical section
Figure 6.5 Critical Section of Process P1 for the Bakery Algorithm

Hardware solutions to the critical section problem:
 Disabling interrupts
 Special hardware instructions
boolean test_and_set (boolean &i)
{
boolean val;
if (*i == false) {
val = false;
*i = true;
} else
val = true;
return val;
}
These instructions are
executed atomically
(uninterruptible)
Figure 6.6 Definition of test_and set instruction

.
.
.
while (test_and_set (&lock) == true) { };
*lock := false;
.
.
.
critical section
Process Pi
Figure 6.7 Using the test_and_set instruction to solve
the critical section problem

Semaphore is a tool that can easily be used to solve more complex synchronization
problems and does not use busy waiting.
.
.
.
wait(mutex);
signal(mutex);
.
.
.
critical section
Process Pi
Figure 6.8 Using Semaphores to Solve the
Critical Section Problem

The Dining Philosophers Problem
Restrictions:
1. A philosopher cannot start eating unless he has both forks.
2. A philosopher cannot pick up both forks at the same time. He has to do it one
at a time.
3. He cannot get the fork that is being used by the philosopher to his right or to
his left.
Figure 6.9 The Dining Philosophers Problem

.
.
.
wait(LeftFork);
wait(RightFork);
eat;
signal(RightFork);
signal(LeftFork);
.
.
.
Figure 6.10 Solution to the Dining Philosophers Problem

The Readers and Writers Problem
.
.
.
wait(wrt);
signal(wrt);
.
.
.
start writing data
Writer Process
Figure 6.11 Using Semaphores to Implement a Writer Process

.
.
.
wait(mutex);
readcount++;
if readcount ==1
wait(wrt);
signal(mutex);
wait(mutex);
readcount--;
If readcount == 0
signal(wrt);
signal(mutex);
.
.
.
start reading data
Reader Process
Figure 6.12 Using Semaphores to Implement a Reader Process

06 lcd slides 1 - PROCESS SYNCHRONIZATION POWERPOINT

More Related Content

What's hot

Viewers also liked

Similar to 06 lcd slides 1 - PROCESS SYNCHRONIZATION POWERPOINT

More from Anne Lee

Recently uploaded

06 lcd slides 1 - PROCESS SYNCHRONIZATION POWERPOINT