Processes Synchronization

⏪Previous Next⏩

Process Synchronization means sharing system resources by processes in a such a way that, Concurrent access to shared data is handled thereby minimizing the chance of inconsistent data.

In the Operating System, there are a number of processes present in a particular state. At the same time, we have a limited amount of resources present, so those resources need to be shared among various processes. But you should make sure that no two processes are using the same resource at the same time because this may lead to data inconsistency. So, synchronization of process should be there in the Operating System. These processes that are sharing resources between each other are called Cooperative Processes and the processes whose execution does not affect the execution of other processes are called Independent Processes.

In this blog, we will learn about Process Synchronization in Operating System. We will learn the two important concepts that are related to process synchronization i.e. Race Condition and Critical Section. So, let's get started.

Race Condition

In an Operating System, we have a number of processes and these processes require a number of resources. Now, think of a situation where we have two processes and these processes are using the same variable "a". They are reading the variable and then updating the value of the variable and finally writing the data in the memory.

SomeProcess(){
    ...
    read(a) //instruction 1
    a = a + 5 //instruction 2
    write(a) //instruction 3
    ...        
}

In the above, you can see that a process after doing some operations will have to read the value of "a", then increment the value of "a" by 5 and at last write the value of "a" in the memory.
Now, we have two processes P1 and P2 that needs to be executed.
Let's take the following two cases and also assume that the value of "a" is 10 initially.

In this case, process P1 will be executed fully (i.e. all the three instructions) and after that, the process P2 will be executed. So, the process P1 will first read the value of "a" to be 10 and then increment the value by 5 and make it to 15. Lastly, this value will be updated in the memory. So, the current value of "a" is 15. Now, the process P2 will read the value i.e. 15, increment with 5(15+5 = 20) and finally write it to the memory i.e. the new value of "a" is 20. Here, in this case, the final value of "a" is 20.
In this case, let's assume that the process P1 starts executing. So, it reads the value of "a" from the memory and that value is 10(initial value of "a" is taken to be 10). Now, at this time, context switching happens between process P1 and P2(learn more about context switching from here). Now, P2 will be in the running state and P1 will be in the waiting state and the context of the P1 process will be saved. As the process P1 didn't change the value of "a", so, P2 will also read the value of "a" to be 10. It will then increment the value of "a" by 5 and make it to 15 and then save it to the memory. After the execution of the process P2, the process P1 will be resumed and the context of the P1 will be read. So, the process P1 is having the value of "a" as 10(because P1 has already executed the instruction 1). It will then increment the value of "a" by 5 and write the final value of "a" in the memory i.e. a = 15. Here, the final value of "a" is 15.

In the above two cases, after the execution of the two processes P1 and P2, the final value of "a" is different i.e. in 1st case it is 20 and in 2nd case, it is 15. What's the reason behind this?

The processes are using the same resource here i.e. the variable "a". In the first approach, the process P1 executes first and then the process P2 starts executing. But in the second case, the process P1 was stopped after executing one instruction and after that the process P2 starts executing. And here both the processes are dealing on the same resource i.e. variable "a" at the same time.

Here, the order of execution of processes changes the output. All these processes are in a race to say that their output is correct. This is called a race condition.

Critical Section Problem:

Consider a system consisting of n processes (P₀, P₁, ………P_n _-1) each process has a segment of code which is known as critical section in which the process may be changing common variable, updating a table, writing a file and so on. The important feature of the system is that when the process is executing in its critical section no other process is to be allowed to execute in its critical section. The execution of critical sections by the processes is a mutually exclusive. The critical section problem is to design a protocol that the process can use to cooperate each process must request permission to enter its critical section. The section of code implementing this request is the entry section. The critical section is followed on exit section. The remaining code is the remainder section.

Example:

While (1)

{

Entry Section;
Critical Section;
Exit Section;

Remainder Section;

}

A solution to the critical section problem must satisfy the following three conditions.

1.Mutual Exclusion: If process P_i is executing in its critical section then no any other process can be executing in their critical section.

2.Progress: If no process is executing in its critical section and some process wish to enter their critical sections then only those process that are not executing in their remainder section can enter its critical section next.
3.Bounded waiting: There exists a bound on the number of times that other processes are allowed to enter their critical sections after a process has made a request.