On the basis of synchronization, processes are categorized as one of the following two types:
- Independent Process : Execution of one process does not affects the execution of other processes.
- Cooperative Process : Execution of one process affects the execution of other processes.
Process synchronization problem arises in the case of Cooperative process also because resources are shared in Cooperative processes.
Critical Section Problem
Critical section is a code segment that can be accessed by only one process at a time. Critical section contains shared variables which need to be synchronized to maintain consistency of data variables.
In the entry section, the process requests for entry in the
Critical Section.
Any solution to the critical section problem must satisfy three requirements:
- Mutual Exclusion : If a process is executing in its critical section, then no other process is allowed to execute in the critical section.
- Progress : If no process is in the critical section, then no other process from outside can block it from entering the critical section.
- Bounded Waiting : A limit on the number of times a process can request to enter the critical section.
Peterson's Solution
Peterson's Solution is a classical software based solution to the critical section problem.
In Peterson's solution, we have two shared variables:
- boolean flag[i] :Initialized to FALSE, initially no one is interested in entering the critical section
- int turn : The process whose turn is to enter the critical section.
Peterson's Solution preserves all three conditions :
- Mutual Exclusion is assured as only one process can access the critical section at any time.
- Progress is also assured, as a process outside the critical section does not blocks other processes from entering the critical section.
- Bounded Waiting is preserved as every process gets a fair chance.
Disadvantages of Peterson’s Solution
- It involves Busy waiting
- It is limited to 2 processes.
TestAndSet
TestAndSet is a hardware solution to the synchronization problem. In TestAndSet, we have a shared lock variable which can take either of the two values, 0 or 1.
0 Unlock
1 Lock
Before entering into the critical section, a process inquires about the lock. If it is locked, it keeps on waiting till it become free and if it is not locked, it takes the lock and executes the critical section.
In TestAndSet, Mutual exclusion and progress are preserved but bounded waiting cannot be preserved.
Question : The enter_CS() and leave_CS() functions to implement critical section of a process are realized using test-and-set instruction as follows:
void enter_CS(X)
{
while test-and-set(X) ;
}
void leave_CS(X)
{
X = 0;
}
In the above solution, X is a memory location associated with the CS and is initialized to 0. Now, consider the following statements:
I. The above solution to CS problem is deadlock-free
II. The solution is starvation free.
III. The processes enter CS in FIFO order.
IV. More than one process can enter CS at the same time.
Which of the above statements is TRUE?
(A) I
(B) II and III
(C) II and IV
(D) IV
Click here for the Solution.
Semaphores
A Semaphore is an integer variable, which can be accessed only through two operations
wait() and
signal().
There are two types of semaphores : Binary Semaphores and Counting Semaphores
- Binary Semaphores : They can only be either 0 or 1. They are also known as mutex locks, as the locks can provide mutual exclusion. All the processes can share the same mutex semaphore that is initialized to 1. Then, a process has to wait until the lock becomes 0. Then, the process can make the mutex semaphore 1 and start its critical section. When it completes its critical section, it can reset the value of mutex semaphore to 0 and some other process can enter its critical section.
- Counting Semaphores : They can have any value and are not restricted over a certain domain. They can be used to control access a resource that has a limitation on the number of simultaneous accesses. The semaphore can be initialized to the number of instances of the resource. Whenever a process wants to use that resource, it checks if the number of remaining instances is more than zero, i.e., the process has an instance available. Then, the process can enter its critical section thereby decreasing the value of the counting semaphore by 1. After the process is over with the use of the instance of the resource, it can leave the critical section thereby adding 1 to the number of available instances of the resource.
References
http://www.csee.wvu.edu/~jdmooney/classes/cs550/notes/tech/mutex/Peterson.html
http://iit.qau.edu.pk/books/OS_8th_Edition.pdf
Similar Reads
Operating System Tutorial An Operating System(OS) is a software that manages and handles hardware and software resources of a computing device. Responsible for managing and controlling all the activities and sharing of computer resources among different running applications.A low-level Software that includes all the basic fu
4 min read
Types of Operating Systems Operating Systems can be categorized according to different criteria like whether an operating system is for mobile devices (examples Android and iOS) or desktop (examples Windows and Linux). Here, we are going to classify based on functionalities an operating system provides.8 Main Operating System
11 min read
What is an Operating System? An Operating System is a System software that manages all the resources of the computing device. Acts as an interface between the software and different parts of the computer or the computer hardware. Manages the overall resources and operations of the computer. Controls and monitors the execution o
9 min read
CPU Scheduling in Operating Systems CPU scheduling is a process used by the operating system to decide which task or process gets to use the CPU at a particular time. This is important because a CPU can only handle one task at a time, but there are usually many tasks that need to be processed. The following are different purposes of a
8 min read
Introduction of Deadlock in Operating System A deadlock is a situation where a set of processes is blocked because each process is holding a resource and waiting for another resource acquired by some other process. In this article, we will discuss deadlock, its necessary conditions, etc. in detail.Deadlock is a situation in computing where two
11 min read
Page Replacement Algorithms in Operating Systems In an operating system that uses paging for memory management, a page replacement algorithm is needed to decide which page needs to be replaced when a new page comes in. Page replacement becomes necessary when a page fault occurs and no free page frames are in memory. in this article, we will discus
7 min read
Paging in Operating System Paging is the process of moving parts of a program, called pages, from secondary storage (like a hard drive) into the main memory (RAM). The main idea behind paging is to break a program into smaller fixed-size blocks called pages.To keep track of where each page is stored in memory, the operating s
8 min read
Disk Scheduling Algorithms Disk scheduling algorithms are crucial in managing how data is read from and written to a computer's hard disk. These algorithms help determine the order in which disk read and write requests are processed, significantly impacting the speed and efficiency of data access. Common disk scheduling metho
12 min read
Memory Management in Operating System Memory is a hardware component that stores data, instructions and information temporarily or permanently for processing. It consists of an array of bytes or words, each with a unique address. Memory holds both input data and program instructions needed for the CPU to execute tasks.Memory works close
7 min read
Banker's Algorithm in Operating System Banker's Algorithm is a resource allocation and deadlock avoidance algorithm used in operating systems. It ensures that a system remains in a safe state by carefully allocating resources to processes while avoiding unsafe states that could lead to deadlocks.The Banker's Algorithm is a smart way for
8 min read