操作系统原理6章节(1413KB).ppt

Condition Signal Consider the released process (from the signaled condition) and the process that signals. There are two processes executing in the monitor, and mutual exclusion is violated! There are two common and popular approaches to address this problem: The released process takes the monitor and the signaling process waits somewhere. The released process waits somewhere and the signaling process continues to use the monitor. Operating System Concepts Semaphore vs. Condition Operating System Concepts Monitor With Condition Variables Operating System Concepts Operating System Concepts Producer-Consumer with Monitors Operating System Concepts Dining Philosophers Example monitor dp { enum {thinking, hungry, eating} state[5]; condition self[5]; void pickup(int i) // following slides void putdown(int i) // following slides void test(int i) // following slides void init() { for (int i = 0; i 5; i++) state[i] = thinking; } } Operating System Concepts Dining Philosophers void pickup(int i) { state[i] = hungry; test[i]; if (state[i] != eating) self[i].wait(); } void putdown(int i) { state[i] = thinking; // test left and right neighbors test((i+4) % 5); test((i+1) % 5); } Operating System Concepts Dining Philosophers void test(int i) { if ( (state[(i + 4) % 5] != eating) (state[i] == hungry) (state[(i + 1) % 5] != eating)) { state[i] = eating; self[i].signal(); } } Operating System Concepts Monitor Implementation Using Semaphores Variables semaphore mutex; // (initially = 1) semaphore next; // (initially = 0) int next-count = 0; Each external procedure F will be replaced by wait(mutex); … body of F; … if (next-count 0) signal(next) else signal(mutex); Mutual exclusion within a monitor is ensured. Operating System Concepts Monitor Implementation For each condition variable x, we have: semaphore x-sem; // (initially = 0) int x-count = 0; Th