In my project of sina spider as well as the course of Operating System, I come up with the use of Semaphore and Mutex, I make a note here for my study and my work.
Definition
In computer science, a semaphore is a variable or abstract data type provides a simple but useful abstraction controlling access by multiple processes to a common resource in a parallel programming environment.
Semaphores are a useful tool in the prevention of race conditions; however, their use is by no means a guarantee that a program is free from these problems. Semaphores which allow an arbitrary resource count are called counting semaphores, while semaphores which are restricted to the values 0 and 1 (or locked/unlocked, unavailable/available) are called binary semaphores (same functionality that mutexes have).
P & V:
operation P:
//apply for a resource
Semaphore--;
//if allocation fails, the process sleeps
if(Semaphore < 0)
sleep();
operation V:
//release a resource
Semaphore++;
//if resource is avaliable, wake up a process
if(Semaphore > 0)
wakeupaprocess()
Semaphore vs. mutex
A mutex is essentially the same thing as a binary semaphore, and sometimes uses the same basic implementation. However, the term "mutex" is used to describe a construct which prevents two processes from accessing a shared resource concurrently. The term "binary semaphore" is used to describe a construct which limits access to a single resource.
In many cases a mutex has a concept of an "owner": the process which locked the mutex is the only process allowed to unlock it. In contrast, semaphores generally do not have this restriction, something the producer-consumer example above depends upon.
Example 1: A Mutex Example
Description:
There is a classroom , if a student enters it, variable 'conunt' increases by 1, and if a student exits it, variable 'conunt' decreases by 1.
Solution:
Initialize mutex = 1
Enter:
P(mutex)
count++
V(mutex)
Exit:
P(mutex)
count--
V(mutex)
Example 2: Use in my Crawler
Description：
In one of my Crawler for Sina microblog, I use parallel programming and there is a UID stack that need to be locked while one thread is visiting it, and when the top element of UID stack is poped out, the lock can be released. So I used the Semaphore to deal with it.
Solution:
Initialize mutex = 1
With the control of UID Stack:
P(mutex)
UIDStack.top()
UIDStack.pop()
V(mutex)