并发编程基础 Lecture Notes（四）

Monitor是另一种处理临界资源的机制。

信号量的缺点：

- 共享的变量对于所有进程来说是全局的，所有没有合理的保护机制，很容易误操作

- 并发代码散乱分布在整个程序中

- 一遇到mutual exclusion就要进行编码

管道的优点：

- 固定的共享变量对进程隐藏

- 通过管道procedure来访问管道

- 一次只允许一个进程在管道内进行操作，因为自动形成了mutual exclusion

- 条件变量可以提供同步

Condition variable c: an object defined inside a monitor which has its own queue of delayed processes

wait(c): add the process to c's queue of blocked processes

signal(c): move the front process from c's queue of blocked processes to a list of ready processes

如果阻塞队列为空的话，singal操作不会产生任何影响。

- Bounded Buffer Monitor

Monitor BBuffer {

buf[1:n]

int front = 1, rear = 1, count = 0;

cond not_full;

cond not_empty;

procedure deposit(item) {

while(count == n) wait(not_full);

put item into buf;

rear = (rear+1) % n;

count++;

signal(not_empty);

}

procedure fetch() {

while(count==0) wait(not_empty);

take item out of buf;

front = (front+1) % n;

count--;

singal(not_full);

}

}

- Readers and Writers

Monitor RWController {

int nr = 0, nw = 0;

cond oktoread;

cond oktowrite;

procedure request_read(){

while(nw > 0) wait(oktoread);

nr++;

}

procedure release_read(){

nr--;

while(nr == 0 && nw == 0) signal(oktowrite);

}

procedure request_write(){

while(nr > 0 && nw > 0) wait(oktowrite);

nw++;

}

procedure release_write(){

nw--;

while(nw == 0)

signal_all(oktoread);

}

}

- The Barber Shop 

Monitor BarberShop {

int chair = 0, barber = 0, open = 0;

cond bfree, chocc, doorop, cleft;

procedure get_cut(){

while(barber == 0) wait(bfree);

wake up the barber;

barber--;

sleep;

chair++;

signal(chocc);

hair cut;

while(open == 0) wait(doorop);

open--;

signal(cleft);

}

procedure get_customer(){

barber++;

signal(bfree);

while(chair == 0) wait(chocc);

chair--;

}

procedure finish_cut() {

open++;

signal(doorop);

while(open > 0) wait(cleft);

}

}