Semaphore in os

While creating a process the operating system performs several operations. To identify the processes, it assigns a process identification number (PID) to each process. As the operating system supports multi-programming, it needs to keep track of all the processes. For this task, the process control block (PCB) is used to track the process’s execution status. Each block of memory contains information about the process state, program counter, stack pointer, status of opened files, scheduling algorithms, etc. All this information is required and must be saved when the process is switched from one state to another. When the process makes a transition from one state to another, the operating system must update information in the process’s PCB. A process control block (PCB) contains information about the process, i.e. registers, quantum, priority, etc. The process table is an array of PCBs, that means logically contains a PCB for all of the current processes in the system. • Pointer – It is a stack pointer which is required to be saved when the process is switched from one state to another to retain the current position of the process. • Process state – It stores the respective state of the process. • Process number – Every process is assigned with a unique id known as process ID or PID which stores the process identifier. • Program counter – It stores the counter which contains the address of the next instruction that is to be executed for the process. • Register – These are th...

In the operating system, mutexes and semaphores are kernel resources that provide synchronization services (also known as synchronizationprimitives).Synchronization is required when multiple processes are executing concurrently, to avoid conflicts between processes using shared resources. Mutex Mutex is a specific kind of binary semaphore that is used to provide a locking mechanism. It stands for Mutex uses a priority inheritance mechanism to avoid priority inversion issues. The priority inheritance mechanism keeps higher priority processes in the blocked state for the minimum possible time. However, this cannot avoid the priority inversion problem, but it can reduce its effect up to an extent. Advantages of Mutex • No race condition arises, as only one process is in the critical section at a time. • Data remains consistent and it helps in maintaining integrity. • It’s a simple locking mechanism that can be obtained by a process before entering into a critical section and released while leaving the critical section. Disadvantages of Mutex • If after entering into the critical section, the thread sleeps or gets preempted by a high priority process, no other thread can enter into the critical section. This can lead to starvation. • When the previous thread will leave the critical section, then only other processes can enter into it, there is no other mechanism to lock or unlock the critical section. • Implementation of mutex can lead to busy waiting that leads to the wastage...

I've started programming in Python a few weeks ago and was trying to use import threading sem = threading.Semaphore() def fun1(): while True: sem.acquire() print(1) sem.release() def fun2(): while True: sem.acquire() print(2) sem.release() t = threading.Thread(target = fun1) t.start() t2 = threading.Thread(target = fun2) t2.start() But it keeps printing just 1's. How can I intercale the prints? It is working fine, its just that its printing too fast for you to see . Try putting a time.sleep() in both functions (a small amount) to sleep the thread for that much amount of time, to actually be able to see both 1 as well as 2. Example - import threading import time sem = threading.Semaphore() def fun1(): while True: sem.acquire() print(1) sem.release() time.sleep(0.25) def fun2(): while True: sem.acquire() print(2) sem.release() time.sleep(0.25) t = threading.Thread(target = fun1) t.start() t2 = threading.Thread(target = fun2) t2.start() @VictorTurrisi Instead of while True if you put a big range and run your program, and then redirect the output to a file and then check the file, you may be able to see that 2 does get printed in between but its like , lots of 1 and then lots of 2 then again lots of 1, etc. This is because it is executing too fast and you need to put a time.sleep() between them to see them executing one after the other. Also, you can use Lock/mutex method as follows: import threading import time mutex = threading.Lock() # is equal to threading.Semaphore(1) def...

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Synchronization Primitives This chapter is not intended as an introduction to synchronization. It is assumed that you have some understanding of the basic concepts of locks and semaphores already. If you need additional background reading, synchronization is covered in most introductory operating systems texts. However, since synchronization in the kernel is somewhat different from locking in an application this chapter does provide a brief overview to help ease the transition, or for experienced kernel developers, to refresh your memory. As an OS X kernel programmer, you have many choices of synchronization mechanisms at your disposal. The kernel itself provides two such mechanisms: locks and semaphores. A lock A semaphore In addition to locks and semaphores, certain low-level synchronization primitives like test and set are also available, along with a number of other atomic operations libkern/gen/OSAtomicOperations.c in the kernel sources. Such atomic operations may be helpful if you do not need something as robust as a full-fledged lock or semaphore. Since they are not general synchronization mechanisms, however, they are beyond the scope of this chapter. Semaphores Semaphores and locks are similar, except that with semaphores, more than one thread can be doing a given operation at once. Semaphores are commonly used when protecting multiple indistinct resources. For example, you might use a semaphore to prevent a queue from overflowing its bounds. OS X uses traditional...

Semaphore in OS is an integer value that indicates whether the resource required by the process is available or not. The value of a semaphore is modified by wait() or signal() operation where the wait() operation decrements the value of semaphore and the signal() operation increments the value of the semaphore. The wait() operation is performed when the process wants to access the resources and the signal() operation is performed when the process want to release the resources. The semaphore can be binary semaphore or the counting semaphore. In this section, we will discuss the concept of semaphore in brief along with the types, advantages and disadvantages. Content: Semaphore in OS • • • • • What is Semaphore in OS? Semaphore is a process synchronization tool that prevents race condition that may occur when multiple cooperative processes try to access the same resources. Two or more process can synchronise by means of the signal. This means the process, trying to access the same shared resource can be forced to stop at a specific place until it is signalled to access the resource. Now you will be thinking how the semaphore signals the process in order to achieve synchronization? Actually, the semaphore is an integer variable whose value signals that if the process must access the resource or wait or wake up to access the shared resources. Apart from the initialization, the semaphore value can be modified using two operations wait() and signal(). The wait() operation decrem...