What Is The Meaning Semaphore ?

1.	What Is The Meaning Semaphore ?
Answer» A semaphore is like a nightclub: it has a certain capacity, enforced by a bouncer. Once it’s full, no more people can enter, and a queue builds up outside. Then, for each person that leaves, one person enters from the head of the queue. The constructor requires a minimum of two arguments: the number of places currently available in the nightclub and the CLUB’s total capacity. A semaphore with a capacity of one is similar to a Mutex or lock, except that the semaphore has no “owner”—it’s thread-agnostic. Any thread can call Release on a Semaphore, whereas with Mutex and lock, only the thread that obtained the lock can release it. Semaphores can be USEFUL in limiting concurrency—preventing too many threads from EXECUTING a particular piece of code at once. In the following example, five threads try to enter a nightclub that allows only three threads in at once: class TheClub // No door lists! { static SemaphoreSlim _sem = new SemaphoreSlim (3); // Capacity of 3 static void Main() { for (int i = 1; i <= 5; i++) new Thread (Enter).Start (i); } static void Enter (object id) { Console.WriteLine (id + " wants to enter"); _sem.Wait(); Console.WriteLine (id + " is in!"); // Only three threads Thread.Sleep (1000 * (int) id); // can be here at Console.WriteLine (id + " is leaving"); // a time. _sem.Release(); } } Out Put: 1 wants to enter 1 is in! 2 wants to enter 2 is in! 3 wants to enter 3 is in! 4 wants to enter 5 wants to enter 1 is leaving 4 is in! 2 is leaving 5 is in! If the Sleep statement was instead performing intensive DISK I/O, the Semaphore WOULD improve overall performance by limiting excessive concurrent hard-drive activity. A Semaphore, if named, can span processes in the same way as a Mutex. A semaphore is like a nightclub: it has a certain capacity, enforced by a bouncer. Once it’s full, no more people can enter, and a queue builds up outside. Then, for each person that leaves, one person enters from the head of the queue. The constructor requires a minimum of two arguments: the number of places currently available in the nightclub and the club’s total capacity. A semaphore with a capacity of one is similar to a Mutex or lock, except that the semaphore has no “owner”—it’s thread-agnostic. Any thread can call Release on a Semaphore, whereas with Mutex and lock, only the thread that obtained the lock can release it. Semaphores can be useful in limiting concurrency—preventing too many threads from executing a particular piece of code at once. In the following example, five threads try to enter a nightclub that allows only three threads in at once: class TheClub // No door lists! { static SemaphoreSlim _sem = new SemaphoreSlim (3); // Capacity of 3 static void Main() { for (int i = 1; i <= 5; i++) new Thread (Enter).Start (i); } static void Enter (object id) { Console.WriteLine (id + " wants to enter"); _sem.Wait(); Console.WriteLine (id + " is in!"); // Only three threads Thread.Sleep (1000 * (int) id); // can be here at Console.WriteLine (id + " is leaving"); // a time. _sem.Release(); } } Out Put: 1 wants to enter 1 is in! 2 wants to enter 2 is in! 3 wants to enter 3 is in! 4 wants to enter 5 wants to enter 1 is leaving 4 is in! 2 is leaving 5 is in! If the Sleep statement was instead performing intensive disk I/O, the Semaphore would improve overall performance by limiting excessive concurrent hard-drive activity. A Semaphore, if named, can span processes in the same way as a Mutex.

Answer»

A semaphore is like a nightclub: it has a certain capacity, enforced by a bouncer. Once it’s full, no more people can enter, and a queue builds up outside. Then, for each person that leaves, one person enters from the head of the queue. The constructor requires a minimum of two arguments: the number of places currently available in the nightclub and the CLUB’s total capacity.

A semaphore with a capacity of one is similar to a Mutex or lock, except that the semaphore has no “owner”—it’s thread-agnostic. Any thread can call Release on a Semaphore, whereas with Mutex and lock, only the thread that obtained the lock can release it.

Semaphores can be USEFUL in limiting concurrency—preventing too many threads from EXECUTING a particular piece of code at once. In the following example, five threads try to enter a nightclub that allows only three threads in at once:

class TheClub // No door lists!

{

static SemaphoreSlim _sem = new SemaphoreSlim (3); // Capacity of 3

static void Main()

{

for (int i = 1; i <= 5; i++) new Thread (Enter).Start (i);

}

static void Enter (object id)

{

Console.WriteLine (id + " wants to enter");

_sem.Wait();

Console.WriteLine (id + " is in!"); // Only three threads

Thread.Sleep (1000 * (int) id); // can be here at

Console.WriteLine (id + " is leaving"); // a time.

_sem.Release();

}

Out Put:

1 wants to enter

1 is in!

2 wants to enter

2 is in!

3 wants to enter

3 is in!

4 wants to enter

5 wants to enter

1 is leaving

4 is in!

2 is leaving

5 is in!

If the Sleep statement was instead performing intensive DISK I/O, the Semaphore WOULD improve overall performance

by limiting excessive concurrent hard-drive activity.

A Semaphore, if named, can span processes in the same way as a Mutex.

Semaphores can be useful in limiting concurrency—preventing too many threads from executing a particular piece of code at once. In the following example, five threads try to enter a nightclub that allows only three threads in at once: