Chapter 5（Operating System）

Chapter 5 : Concurrency

并发会在三种context中出现

1. Multiple Applications (允许多个活动在应用程序间动态共享处理器时间)
2. Structured Applications：•extension of modular design and structured programming
3. Operating System Structure：•OS themselves implemented as a set of processes or threads

Special Machine Instruction

The only process that may enter its critical section is one that finds bolt equal to 0.

When a process leaves its critical section, it resets bolt to 0

• Compare and swap instruction

  等着的进程进入busy waiting或spin waiting, 只能等着permission。

  The choice of process depends on which process happens to execute the compare & swap instruction next.

• Exchange instruction

  Each process uses a local variable key that is initialized to 1.

  通过把bolt置为1来避免其他进程进入临界区

Advantages

1. share main memory
2. simple and easy to verify

Disadvantages

1. Busy-waiting consumes processor time
2. Starvation
3. Deadlock

Semaphore(信号量)

传送信号：semSignal(s)信号量加一

接收信号：semWait(s) 信号量减一

把信号量视为一个integer， 只定义了以下三个操作

1)May be initialized to a nonnegative integer value

2)The semWait operation decrements the value

3)The semSignal operation increments the value

二元信号