Objecttives To develop a description of deadlocks, which prevent sets of concurrent processes from completing their tasks To present a number of diffe

Size: px

Start display at page:

Download "Objecttives To develop a description of deadlocks, which prevent sets of concurrent processes from completing their tasks To present a number of diffe"

倾起戎
6 years ago
Views:

1 操作系统原理与设计第 7 章 Deadlocks( 死锁 ) 陈香兰中国科学技术大学计算机学院 April 16, 2014 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

2 Objecttives To develop a description of deadlocks, which prevent sets of concurrent processes from completing their tasks To present a number of different methods for preventing or avoiding deadlocks in a compuer system 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

3 提纲 1 Background and System Model 2 Deadlock Characterization Necessary Conditions Resource-Allocation Graph Methods for Handling Deadlocks 3 Deadlock Prevention ( 死锁预防 ) 4 Deadlock Avoidance ( 死锁避免 ) Safe State ( 安全状态 ) Resource-Allocation Graph Scheme Banker s Algorithm ( 银行家算法 ) 5 Deadlock Detection ( 死锁检测 ) and Recovery 6 小结和作业陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

4 Outline 1 Background and System Model 2 Deadlock Characterization 3 Deadlock Prevention ( 死锁预防 ) 4 Deadlock Avoidance ( 死锁避免 ) 5 Deadlock Detection ( 死锁检测 ) and Recovery 6 小结和作业陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

5 The Deadlock Problem deadlock situation A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set Example 1 System has 2 disk drives P 1 and P 2 each hold one disk drive and each needs another one Example 2 P 0 P 1 semaphores A and B, initialized to 1 wait (A); wait(b) wait (B); wait(a) 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

Bridge Crossing Example Traffic only in one direction Each section of a bridge can be viewed as a resource If a deadlock occurs, it can be resolved if one car backs up

6 Bridge Crossing Example Traffic only in one direction Each section of a bridge can be viewed as a resource If a deadlock occurs, it can be resolved if one car backs up (preempt resources and rollback) Several cars may have to be backed up if a deadlock occurs Starvation is possible 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

7 System Model A system consists of a finite number of resources The resources are partitioned into several types, each consisting of some number of identical instance physical resources: CPU cycles, memory space, I/O devices logical resources: files, semaphores, and monitors System model Resource types R 1, R 2,, R m Each resource type R i has W i instances Each process utilizes a resource as follows: request: may wait until it can acquire the resource use release 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

8 Outline 1 Background and System Model 2 Deadlock Characterization Necessary Conditions Resource-Allocation Graph Methods for Handling Deadlocks 3 Deadlock Prevention ( 死锁预防 ) 4 Deadlock Avoidance ( 死锁避免 ) 5 Deadlock Detection ( 死锁检测 ) and Recovery 6 小结和作业陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

9 Deadlock Characterization: Necessary Conditions Deadlock can arise if four conditions hold simultaneously 1 Mutual exclusion( 互斥 ): only one process at a time can use a resource 2 Hold and wait( 持有并等待 ): a process holding at least one resource is waiting to acquire additional resources held by other processes 3 No preemption( 不剥夺 ): a resource can be released only voluntarily by the process holding it, after that process has completed its task 4 Circular wait( 循环等待 ): there exists a set {P 0, P 1,, P 0 } of waiting processes such that P 0 is waiting for a resource that is held by P 1, P 1 is waiting for a resource that is held by P 2,, P n 1 is waiting for a resource that is held by P n, and P n is waiting for a resource that is held by P 0 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

Deadlock Characterization: Resource-Allocation Graph System resource-allocation graph: A directed graph A set of vertices V and a set of edges E V is partitioned into two types: P = {P 1, P 2,, P n},

10 Deadlock Characterization: Resource-Allocation Graph System resource-allocation graph: A directed graph A set of vertices V and a set of edges E V is partitioned into two types: P = {P 1, P 2,, P n}, the set consisting of all the processes in the system R = {R 1, R 2,, R m}, the set consisting of all resource types in the system request edge( 请求边 ) directed edge P i R j assignment edge( 分配边 ) directed edge R j P i Process Resource Type with 4 instances P i requests instance of R j P i is holding an instance of R j 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

11 Example of a Resource Allocation Graph 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

12 Example of a resource Allocation Graph With A Deadlock 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

13 Graph With A Cycle But No Deadlock 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

14 Basic Facts If graph contains no cycles no deadlock If graph contains a cycle if only one instance per resource type, then deadlock if several instances per resource type, possibility of deadlock 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

15 Methods for Handling Deadlocks 1 Ensure that the system will never enter a deadlock state 1 Deadlock prevention 2 Deadlock avoidance 2 Allow the system to enter a deadlock state and then recover 1 Deadlock detection and recovery from deadlock 3 Ignore the problem and pretend that deadlocks never occur in the system; used by most operating systems, including UNIX 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

16 Outline 1 Background and System Model 2 Deadlock Characterization 3 Deadlock Prevention ( 死锁预防 ) 4 Deadlock Avoidance ( 死锁避免 ) 5 Deadlock Detection ( 死锁检测 ) and Recovery 6 小结和作业陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

17 Deadlock Prevention ( 死锁预防 ) Deadlock prevention provides a set of methods for ensuring that at least one of the necessary conditions cannot hold Restrain the ways request can be made 1 Mutual Exclusion not required for sharable resources (read-only files); must hold for nonsharable resources (printer) In general, therefore, we cannot deny the mutual-exclusion condition 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

18 Deadlock Prevention ( 死锁预防 ) Deadlock prevention provides a set of methods for ensuring that at least one of the necessary conditions cannot hold Restrain the ways request can be made 2 Hold and Wait must guarantee that whenever a process requests a resource, it does not hold any other resources 1 Require process to request and be allocated all its resources before it begins execution, or 2 allow process to request resources only when the process has none Disadvantage: 1 Low resource utilization; 2 starvation possible 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

19 Deadlock Prevention ( 死锁预防 ) Deadlock prevention provides a set of methods for ensuring that at least one of the necessary conditions cannot hold Restrain the ways request can be made 3 No Preemption 1 If a process that is holding some resources requests another resource that cannot be immediately allocated to it, then all resources currently being held are preempted Preempted resources are added to the list of resources for which the process is waiting Process will be restarted only when it can regain its old resources, as well as the new ones that it is requesting 2 preempt the desired resources from the waiting process and allocate them to the requesting process if the resource are neither available nor held by a waiting process, the requesting process must wait While waiting, some of its resources may be preempted by other requesting process a process can be restarted only when it is allocated the new resources it is requesting and recovers any resources that were preempted 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

20 Deadlock Prevention ( 死锁预防 ) Deadlock prevention provides a set of methods for ensuring that at least one of the necessary conditions cannot hold Restrain the ways request can be made 4 Circular Wait impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration 1 always in an increasing order 2 may release some higher ordered resource before requesting lower ordered resource 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

21 Outline 1 Background and System Model 2 Deadlock Characterization 3 Deadlock Prevention ( 死锁预防 ) 4 Deadlock Avoidance ( 死锁避免 ) Safe State ( 安全状态 ) Resource-Allocation Graph Scheme Banker s Algorithm ( 银行家算法 ) 5 Deadlock Detection ( 死锁检测 ) and Recovery 6 小结和作业陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

22 Deadlock Avoidance ( 死锁避免 ) Requires that the system has some additional a priori information available Simplest and most useful model requires that each process declare the maximum number of resources of each type that it may need The deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that there can never be a circular-wait condition Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

23 Safe State ( 安全状态 ) When a process requests an available resource, system must decide if immediate allocation leaves the system in a safe state System is in safe state if there exists a (safe) sequence ( 安全序列 ) of ALL the processes in the systems such that for each P i, the resources that P i can still request can be satisfied by currently available resources + resources held by all the P j, with j < i That is: If P i resource needs are not immediately available, then P i can wait until all P j have finished When P j is finished, P i can obtain needed resources, execute, return allocated resources, and terminate When P i terminates, P i+1 can obtain its needed resources, and so on 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

24 Basic Facts: Safe, Unsafe, Deadlock State I If a system is in safe state no deadlocks If a system is in unsafe state possibility of deadlock Avoidance ensure that a system will never enter an unsafe state 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

25 Basic Facts: Safe, Unsafe, Deadlock State II Example, 12 tape drives and 3 processes, at T 0 MaxNeeds current P P P if at T 2, P 2 request and is allocated one more tape drive,? 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

26 Avoidance algorithms 1 Single instance of a resource type Use a resource-allocation graph 2 Multiple instances of a resource type Use the banker s algorithm ( 银行家算法 ) 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

27 1 Resource-Allocation Graph Scheme Resource-Allocation Graph Claim edge ( 需求边 ) P i R j indicated that process P j may request resource R j ; represented by a dashed line Claim edge converts to request edge when a process requests a resource Request edge converted to an assignment edge when the resource is allocated to the process When a resource is released by a process, assignment edge reconverts to a claim edge Resources must be claimed a priori in the system 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

28 1 Resource-Allocation Graph Scheme Example: Safe State safe sequence: 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

29 1 Resource-Allocation Graph Scheme Example: Unsafe State In Resource-Allocation Graph 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

30 1 Resource-Allocation Graph Scheme Resource-Allocation Graph Algorithm Suppose that process P i requests a resource R j The request can be granted only if converting the request edge to an assignment edge does not result in the formation of a cycle in the resource allocation graph 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

31 2 Banker s Algorithm ( 银行家算法 ) Banker s Algorithm ( 银行家算法 ) Multiple instances Each process must a priori claim maximum use When a process requests a resource it may have to wait When a process gets all its resources it must return them in a finite amount of time 1 Data stuctures 2 safety algorithm 3 resource-request algorithm 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

32 2 Banker s Algorithm ( 银行家算法 ): Data Structures Let n = number of processes m = number of resources types Available: Vector of length m If available[j] = k, there are k instances of resource type R j available Max: n m matrix If Max[i, j] = k, then process P i may request at most k instances of resource type R j Allocation: n m matrix If Allocation[i, j] = k then P i is currently allocated k instances of R j Need: n m matrix If Need[i, j] = k, then P i may need k more instances of R j to complete its task Need[i, j] = Max[i, j] Allocation[i, j] 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

33 2 Banker s Algorithm ( 银行家算法 ): Safety Algorithm 1 Let Work and Finish be vectors of length m and n, respectively Initialize: Work = Available Finish[i] = false for i = 0, 1,, n 1 2 Find an i such that both: 1 Finish[i] = false 2 Need i Work If no such i exists, go to step 4 3 Work = Work + Allocation i, Finish[i] = true, go to step 2 4 If Finish[i] == true for all i, then the system is in a safe state 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

34 2 Banker s Algorithm: Resource-Request Algorithm for Process P i Request = request vector for process P i If Request i [j] = k then process P i wants k instances of resource type R j 1 If Request i Need i go to step 2 Otherwise, raise error condition, since process has exceeded its maximum claim 2 If Request i Available, go to step 3 Otherwise P i must wait, since resources are not available 3 Pretend to allocate requested resources to P i by modifying the state as follows: Available = Available Request; Allocation i = Allocation i + Request i ; Need i = Need i Request i ; If safe the resources are allocated to P i If unsafe P i must wait, and the old resource-allocation state is restored 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

35 2 Banker s Algorithm: Example 5 processes: P 0 P 4 ; 3 resource types: A (10 instances), B (5 instances), and C (7 instances) Snapshot at time T0: Need = Max Allocation Allocation Max Available A B C A B C A B C Need P A B C P P P P P P P P P The system is in a safe state since the sequence satisfies safety criteria 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

36 2 Banker s Algorithm: Example: P 1 Request (1,0,2) Check that Request Available (that is, (1,0,2) (3,3,2) true Allocation Need Available A B C A B C A B C P P P P P Executing safety algorithm shows that sequence satisfies safety requirement Can request for (3,3,0) by P 4 be granted? Can request for (0,2,0) by P 0 be granted? 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

37 Outline 1 Background and System Model 2 Deadlock Characterization 3 Deadlock Prevention ( 死锁预防 ) 4 Deadlock Avoidance ( 死锁避免 ) 5 Deadlock Detection ( 死锁检测 ) and Recovery 6 小结和作业陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

38 Deadlock Detection ( 死锁检测 ) and Recovery Allow system to enter deadlock state Detection algorithm 1 single instance 2 several instances Recovery scheme 1 Process termination 2 Resource preemption 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

1 Single Instance of Each Resource Type Maintain wait-for graph: Nodes are processes P i P j, if P i is waiting for P j Periodically invoke an algorithm that searches for a cycle in the graph If

39 1 Single Instance of Each Resource Type Maintain wait-for graph: Nodes are processes P i P j, if P i is waiting for P j Periodically invoke an algorithm that searches for a cycle in the graph If there is a cycle, there exists a deadlock COST: An algorithm to detect a cycle in a graph requires an order of n 2 operations, where n is the number of vertices in the graph 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

40 2 Several Instances of a Resource Type 1 Data structures: Available: A vector of length m indicates the number of available resources of each type Allocation: An n m matrix defines the number of resources of each type currently allocated to each process Request: An n m matrix indicates the current request of each process If Request[i][j] = k, then process P i is requesting k more instances of resource type R j 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

41 2 Several Instances of a Resource Type 2 Detection Algorithm 1 Let Work and Finish be vectors of length m and n, respectively Initialize: Work = Available For i = 1, 2,, n, if Allocation i 0, then Finish[i] = false;otherwise, Finish[i] = true 2 Find an i such that both: Finish[i] == false Request i Work If no such i exists, go to step 4 3 Work = Work + Allocation i, Finish[i] = true, go to step 2 4 If Finish[i] == false, for some i, 1 i n, then the system is in deadlock state Moreover, if Finish[i] == false, then P i is deadlocked Algorithm requires an order of O(m n 2 ) operations to detect whether the system is in deadlocked state 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

42 2 Several Instances of a Resource Type 3 Example of Detection Algorithm Five processes: P 0 P 4 ; three resource types: A (7 instances), B (2 instances), and C (6 instances) Snapshot at time T 0 : Allocation Request Available A B C A B C A B C P P P P P Sequence will result in Finish[i] = true for all i 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

43 2 Several Instances of a Resource Type 3 Example of Detection Algorithm If P 2 requests an additional instance of type C State of system? Request A B C P P P P P Can reclaim resources held by process P 0, but insufficient resources to fulfill other processes requests Deadlock exists, consisting of processes P 1, P 2, P 3, and P 4 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

44 2 Several Instances of a Resource Type 3 Example of Detection Algorithm 4 Detection-Algorithm Usage When, and how often, to invoke depends on: How often a deadlock is likely to occur? How many processes will need to be rolled back? one for each disjoint cycle If detection algorithm is invoked arbitrarily, there may be many cycles in the resource graph and so we would not be able to tell which of the many deadlocked processes caused the deadlock 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

45 Recovery from Deadlock: 1 Process Termination 1 Abort all deadlocked processes 2 Abort one process at a time until the deadlock cycle is eliminated To minimize cost: in which order should we choose to abort? Priority of the process How long process has computed, and how much longer to completion Resources the process has used Resources process needs to complete How many processes will need to be terminated Is process interactive or batch? 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

46 Recovery from Deadlock: 2 Resource Preemption Three issues need to be addressed: 1 Selecting a victim minimize cost 2 Rollback return to some safe state, restart process for that state 3 Starvation same process may always be picked as victim, include number of rollback in cost factor 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

47 Outline 1 Background and System Model 2 Deadlock Characterization 3 Deadlock Prevention ( 死锁预防 ) 4 Deadlock Avoidance ( 死锁避免 ) 5 Deadlock Detection ( 死锁检测 ) and Recovery 6 小结和作业陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

48 小结 1 Background and System Model 2 Deadlock Characterization Necessary Conditions Resource-Allocation Graph Methods for Handling Deadlocks 3 Deadlock Prevention ( 死锁预防 ) 4 Deadlock Avoidance ( 死锁避免 ) Safe State ( 安全状态 ) Resource-Allocation Graph Scheme Banker s Algorithm ( 银行家算法 ) 5 Deadlock Detection ( 死锁检测 ) and Recovery 6 小结和作业陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

49 作业参见课程主页陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

50 谢谢! 陈香兰 ( 中国科学技术大学计算机学院 ) 操作系统原理与设计 April 16, / 40

穨control.PDF

穨control.PDF TCP congestion control yhmiu Outline Congestion control algorithms Purpose of RFC2581 Purpose of RFC2582 TCP SS-DR 1998 TCP Extensions RFC1072 1988 SACK RFC2018 1996 FACK 1996 Rate-Halving 1997 OldTahoe