Process Synchronization | Race Condition in OS

Process Synchronization-

 

When multiple processes execute concurrently sharing system resources, then inconsistent results might be produced.

• Process Synchronization is a mechanism that deals with the synchronization of processes.
• It controls the execution of processes running concurrently to ensure that consistent results are produced.

 

Need of Synchronization-

 

Process synchronization is needed-

• When multiple processes execute concurrently sharing some system resources.
• To avoid the inconsistent results.

 

Critical Section-

 

Critical section is a section of the program where a process access the shared resources during its execution.

 

Example-

 

The following illustration shows how inconsistent results may be produced if multiple processes execute concurrently without any synchronization.

 

Consider-

• Two processes P₁ and P₂ are executing concurrently.
• Both the processes share a common variable named “count” having initial value = 5.
• Process P1 tries to increment the value of count.
• Process P2 tries to decrement the value of count.

 

 

In assembly language, the instructions of the processes may be written as-

 

 

Now, when these processes execute concurrently without synchronization, different results may be produced.

 

Case-01:

 

The execution order of the instructions may be-

P₁(1), P₁(2), P₁(3), P₂(1), P₂(2), P₂(3)

In this case,

Final value of count = 5

 

Case-02:

 

The execution order of the instructions may be-

P₂(1), P₂(2), P₂(3), P₁(1), P₁(2), P₁(3)

In this case,

Final value of count = 5

 

Case-03:

 

The execution order of the instructions may be-

P₁(1), P₂(1), P₂(2), P₂(3), P₁(2), P₁(3)

In this case,

Final value of count = 6

 

Case-04:

 

The execution order of the instructions may be-

P₂(1), P₁(1), P₁(2), P₁(3), P₂(2), P₂(3)

In this case,

Final value of count = 4

 

Case-05:

 

The execution order of the instructions may be-

P₁(1), P₁(2), P₂(1), P₂(2), P₁(3), P₂(3)

In this case,

Final value of count = 4

 

It is clear from here that inconsistent results may be produced if multiple processes execute concurrently without any synchronization.

 

Race Condition-

 

Race condition is a situation where-

• The final output produced depends on the execution order of instructions of different processes.
• Several processes compete with each other.

The above example is a good illustration of race condition.

 

PRACTICE PROBLEM BASED ON PROCESS SYNCHRONIZATION-

 

Problem-

 

The following two functions P1 and P2 that share a variable B with an initial value of 2 execute concurrently-

 

 

The number of distinct values that B can possibly take after the execution is-

1. 3
2. 2
3. 5
4. 4

 

Solution-

 

Different execution order of the instructions of P1 and P2 produce different results.

 

Case-01:

 

The execution order of the instructions may be-

P₁(1), P₁(2), P₂(1), P₂(2)

In this case,

Final value of B = 3

 

Case-02:

 

The execution order of the instructions may be-

P₂(1), P₂(2), P₁(1), P₁(2)

In this case,

Final value of B = 4

 

Case-03:

 

The execution order of the instructions may be-

P₁(1), P₂(1), P₂(2), P₁(2)

In this case,

Final value of B = 2

 

Case-04:

 

The execution order of the instructions may be-

P₂(1), P₁(1), P₁(2), P₂(2)

In this case,

Final value of B = 3

 

Case-05:

 

The execution order of the instructions may be-

P₁(1), P₂(1), P₁(2), P₂(2)

In this case,

Final value of B = 3

 

Case-06:

 

The execution order of the instructions may be-

P₂(1), P₁(1), P₂(2), P₁(2)

In this case,

Final value of B = 2

 

From here,

• Distinct values that may be produced are 2, 3 and 4.
• Number of distinct values that may be produced = 3

Thus, Option (A) is correct.

 

To gain better understanding of Process Synchronization,

Watch this Video Lecture

 

Next Article- Synchronization Mechanisms

 

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