![]() We looked at using locks to provide mutual exclusion Locks work, but they have some drawbacks when critical regions are long Spinlocks – inefficient Disabling interrupts – can miss or delay important events Instead, we want synchronization mechanisms that Block waiters Leave interrupts enabled inside the critical section Look at two common high-level mechanisms Semaphores: binary (mutex) and counting Monitors: mutexes and condition variables Use them to solve common synchronization problems Ding Yuan, ECE344 Operating SystemĤ Semaphores Semaphores are an abstract data type that provide mutual exclusion to critical region Semaphores can also be used as atomic counters More later Semaphores are integers that support two operations: wait(semaphore): decrement, block until semaphore is open Also P(), after the Dutch word for test, or down() signal(semaphore): increment, allow another thread to enter Also V() after the Dutch word for increment, or up() That's it! No other operations – not even just reading its value – exist P and V are probably the most unintuitive names you encounter in this course and you have Edsger W. ![]() Operating Systems ECE344 Lecture 6: Synchronization (II) – Semaphores and Monitors Ding YuanĢ Review of last lecture Goal: Use mutual exclusion to protect critical sections of code that access shared resources Method: Use locks (spinlocks or disable interrupts) Problem: Critical sections can be long Spinlocks: Threads waiting to acquire lock spin in test-and-set loop Wastes CPU cycles Longer the CS, the longer the spin Greater the chance for lock holder to be interrupted Disabling Interrupts: Should not disable interrupts for long periods of time Can miss or delay important events (e.g., timer, I/O) acquire(lock) … Critical section release(lock) Ding Yuan, ECE344 Operating System Monitor contains special memory location which allows storing a thread’s data and handles one at a time.1 Lecture 6: Synchronization (II) – Semaphores and Monitors Synchronization allows the threads to execute one after another without interrupting the thread that is running. Cooperation is enabling threads work together for a common goal. Cooperation between threads is performed with the usage of wait() and notify() methods of Object class. Multiple threads will work independently on shared data without interfering each other with the help of object locks concept. Thread monitor supports the threads which are synchronized and mutual exclusion and co-operation. It can also be used to guard critical operations such as dead locks. It is a construct that can be used to send signals between synchronized threads in order to avoid missing signals. Semaphore:Ī semaphore is a construct in multithreading. Explain the concepts of semaphore and monitors in Java threading. ![]() A monitor allows only one thread to lock an object at once. The monitor uses the lock to ensure that only a single thread is active in the monitor code at any time.Ī semaphore is a generalization of a monitor. The drawback of semaphores is that there is no control or guarantee of proper usage.Ī Monitor defines a lock and condition variables for managing concurrent access to shared data. A semaphore is a flag variable used to check whether a resource is currently being used by another thread or process. Explain semaphore and monitors in java threading.
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