操作系统习题课_1.ppt

习题课 A computer system has 8 printers that were competed by K process, and each process can take up no more than 3 printers. What is the minimum value of K to must cause the system deadlock? What is the minimum value of K to may be cause the system deadlock? 先看一个简单的例子： 8个资源，K个进程，每个进程使用N ?3个资源 K=1 ? K=2 ? K=3 ? K=4： Suppose an infinite capacity warehouse can hold two type goods A and B. But the number of A and B must satisfy the inequality as follow: -M?the number of A-the number of B?N Here, M and N are positive integer. Please description A and B storage process by semaphores and PV operation. 问题分析： 已知条件 -M≤A物品数量－B物品数量≤N?可以拆成两个不等式，即 A物品数量－B物品数量≤N， B物品数量－A物品数量≤M。 这两个不等式的含义是：仓库中A物品可以比B物品多，但不能超过N个；Ｂ物品可以比A物品多，但不能超过M个。 若只放入A，而不放入B，则A最多可放N次后阻塞；若只放入B，不放入A，则B最多放M次后阻塞； 每放入一次A，就多一次放入B的机会。同理，每放入一次B，就对一次放入A的机会。 Semaphore s=?,pa=?,pb=? CoBegin Procedure A; Procedure B; While (true) { While (true) { P(sa); P(sb); P(s); P(s); //A产品入库 //B产品入库 V(s); V(s); V(sb); V(sa); } CoEnd; In an electronic funds transfer system, there are hundreds of identical processes that work as follows. Each process reads an input line specifying an amount of money, the account to be credited, and the account to be debited. Then it locks both accounts and transfers the money, releasing the locks when done. With many processes running in parallel, there is a very real danger that having locked account x it will be unable to lock y because y has been locked by a process now waiting for x. Devise a scheme that avoids deadlocks. Do not release an account record until you have completed the transactions. Two Approaches to avoid Deadlock Do not start a process if its demands might lead to deadlock Do not grant an incremental resource request to a process if this allocation might lead to deadlock To avoid circular wait, number the resourc