产生死锁与饿死的原因; 解决死锁的方法;.pptVIP

CHAPTER 6Concurrency：deadlock And Starvation 6.1 Principles of Deadlock 死锁的定义: Permanent blocking of a set of processes that either compete for system resources or communicate with each other(一组竞争系统资源或互相通信的进程间相互的“永久”阻塞。 ) No efficient solution Involve conflicting needs for resources by two or more processes（所有死锁涉及到两个或更多的进程之间因对资源的需求所引起的冲突。 ） 现在考虑涉及进程和计算机资源的死锁的描述。 例, Process P and Q compete two resources, Their general forms are: Process P Process Q … … Get A Get B … … Get B Get A … … Release A Release B … … Release B Release A … … 死锁与进程的推进顺序有关。若修改P的代码，则不会产生死锁 Process P … Get A … Release A … Get B … Release B … Reusable Resources(可重用资源) 资源通常可分为两类：可重用的和可消费的。 Used by one process at a time and not depleted by that use ( 可重用资源是指一次只能供一个进程安全地使用，并且不会由于使用而耗尽。) 可重用资源释放之后供其他进程再次使用。 可重用资源的例子：Processors、 I/O channels, main and secondary memory（辅存）、files、 databases、 and semaphores Deadlock occurs if each process holds one resource and requests the other Example of Deadlock Another Example of Deadlock Space is available for allocation of 200K bytes, and the following sequence of events occur Deadlock occurs if both processes progress to their second request Consumable Resources(可消耗资源) Created (produced) and destroyed (consumed) by a process（可消费资源是指可以创建（生产）并且可以销毁（消费）的资源。当进程得到一个资源时，该资源就不再存在了。 ） 可消费资源的例子 ：Interrupts, signals, messages, and information in I/O buffers 可消费资源死锁的例子如下页 ： Example of Deadlock Deadlock occurs if receive is blocking 此类死锁是由于设计失误造成的，很难发现，且潜伏期较长 Conditions for Deadlock（死锁条件） Mutual exclusion(互斥) only one process may use a resource at a time Hold-and-wait(保持并等待) A process may hold allocated resources while awaiting　assignment of　other No preemption(不剥夺) Circular wait(环路等待) Conditions for Deadlock 条件Mutual exc