第五章死锁.ppt

太原科技大学 胡静 Concurrency: Deadlock and Starvation Chapter 6 6.1Deadlock 1、definition: Permanent blocking of a set of processes that either compete for system resources or communicate with each other 2、Permanent： No efficient solution 3、conflicting： needs for resources by two or more processes 6.1.1Reusable Resources 1、definition、Used by only one process at a time and not depleted by that use. Processes obtain resources that they later release for reuse by other processes Processors, I/O channels, main and secondary memory, devices, and data structures such as 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 200Kbytes, and the following sequence of events occur Deadlock occurs if both processes progress to their second request 6.1.2Consumable Resources 1、definition: Created (produced) and destroyed (consumed) Example: Interrupts, signals, messages, and information in I/O buffers Deadlock may occur if a Receive message is blocking May take a rare combination of events to cause deadlock Example of Deadlock Deadlock occurs if receive is blocking 6.1.3Resource Allocation Graphs Directed graph( 有向图） that depicts a state of the system of resources and processes Resource Allocation Graphs 6.1.4Conditions for Deadlock Mutual exclusion Only one process may use a resource at a time Hold-and-wait A process may hold allocated resources while awaiting assignment of others No preemption No resource can be forcibly removed form a process holding it Conditions for Deadlock Circular wait A closed chain of processes exists, such that each process holds at least one resource needed by the next process in the chain Possibility of Deadlock Mutual Exclusion No preemption Hold and wait Existence of Deadlock Mutual Exclusion No preemption Hold and wait Circular wait 6.2 Deadlock Prevention 1、Mutual Exclusion Must be supported by the op