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1、Chapter 7:Deadlocks,Chapter Objectives,To develop a description of deadlocks,which prevent sets of concurrent processes from completing their tasksTo present a number of different methods for preventing or avoiding deadlocks in a computer system.,Content Overview,The Deadlock ProblemSystem ModelDead
2、lock CharacterizationMethods for Handling DeadlocksDeadlock PreventionDeadlock AvoidanceDeadlock Detection Recovery from Deadlock,The Deadlock Problem,A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set.Example System has 2 disk dri
3、ves.P1 and P2 each hold one disk drive and each needs another one.Example semaphores A and B,initialized to 1 P0 P1wait(A);wait(B)wait(B);wait(A),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
4、car backs up(preempt resources and rollback).Several cars may have to be backed up if a deadlock occurs.Starvation is possible.,7.1 System Model,Resource types R1,R2,.,RmCPU cycles,memory space,I/O devicesEach resource type Ri has Wi instances.Each process utilizes a resource as follows:request use
5、release,7.2 Deadlock Characterization,Mutual exclusion:only one process at a time can use a resource.Hold and wait:a process holding at least one resource is waiting to acquire additional resources held by other processes.No preemption:a resource can be released only voluntarily by the process holdi
6、ng it,after that process has completed its task.Circular wait:there exists a set P0,P1,P0 of waiting processes such that P0 is waiting for a resource that is held by P1,P1 is waiting for a resource that is held by P2,Pn1 is waiting for a resource that is held by Pn,and P0 is waiting for a resource t
7、hat is held by P0.,Deadlock can arise if four conditions hold simultaneously.,Resource-Allocation Graph,V is partitioned into two types:P=P1,P2,Pn,the set consisting of all the processes in the system.R=R1,R2,Rm,the set consisting of all resource types in the system.request edge directed edge P1 Rja
8、ssignment edge directed edge Rj Pi,A set of vertices V and a set of edges E.,Resource-Allocation Graph(Cont.),ProcessResource Type with 4 instancesPi requests instance of RjPi is holding an instance of Rj,Pi,Pi,Rj,Rj,Example of a Resource Allocation Graph,Resource Allocation Graph With A Deadlock,Gr
9、aph With A Cycle But No Deadlock,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.,7.3 Methods for Handling Deadlocks,Ensure that the system will never ent
10、er a deadlock state.Allow the system to enter a deadlock state and then recover.Ignore the problem and pretend that deadlocks never occur in the system;used by most operating systems,including UNIX.,7.4 Deadlock Prevention,Mutual Exclusion not required for sharable resources;must hold for non-sharab
11、le resources.Hold and Wait must guarantee that whenever a process requests a resource,it does not hold any other resources.Require process to request and be allocated all its resources before it begins execution,or allow process to request resources only when the process has none.Low resource utiliz
12、ation;starvation possible.,Restrain the ways request can be made.,Deadlock Prevention(Cont.),No Preemption 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 released.Preempted resources are add
13、ed 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.Circular Wait impose a total ordering of all resource types,and require that each process requests resources in an increasing
14、 order of enumeration.,7.5 Deadlock Avoidance,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 circul
15、ar-wait condition.Resource-allocation state is defined by the number of available and allocated resources,and the maximum demands of the processes.,Requires that the system has some additional a priori information available.,Safe State,When a process requests an available resource,system must decide
16、 if immediate allocation leaves the system in a safe state.System is in safe state if there exists a sequence of ALL the processes is the systems such that for each Pi,the resources that Pi can still request can be satisfied by currently available resources+resources held by all the Pj,with j i.That
17、 is:If Pi resource needs are not immediately available,then Pi can wait until all Pj have finished.When Pj is finished,Pi can obtain needed resources,execute,return allocated resources,and terminate.When Pi terminates,Pi+1 can obtain its needed resources,and so on.,Basic Facts,If a system is in safe
18、 state no deadlocks.If a system is in unsafe state possibility of deadlock.Avoidance ensure that a system will never enter an unsafe state.,Safe,Unsafe,Deadlock State,Avoidance algorithms,Single instance of a resource type.Use a resource-allocation graphMultiple instances of a resource type.Use the
19、bankers algorithm,Resource-Allocation Graph Scheme,Claim edge Pi Rj indicated that process Pj may request resource Rj;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
20、 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.,Resource-Allocation Graph,Unsafe State In Resource-Allocation Graph,Resource-Allocation Graph Algorithm,Suppose that process Pi requests a resource RjThe req
21、uest 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,Bankers Algorithm,Multiple instances.Each process must a priori claim maximum use.When a process requests a resource it may have to wait.When a p
22、rocess gets all its resources it must return them in a finite amount of time.,Data Structures for the Bankers Algorithm,Available:Vector of length m.If available j=k,there are k instances of resource type Rj available.Max:n x m matrix.If Max i,j=k,then process Pi may request at most k instances of r
23、esource type Rj.Allocation:n x m matrix.If Allocationi,j=k then Pi is currently allocated k instances of Rj.Need:n x m matrix.If Needi,j=k,then Pi may need k more instances of Rj to complete its task.Need i,j=Maxi,j Allocation i,j.,Let n=number of processes,and m=number of resources types.,Safety Al
24、gorithm,1.Let Work and Finish be vectors of length m and n,respectively.Initialize:Work=AvailableFinish i=false for i=0,1,n-1.2.Find and i such that both:(a)Finish i=false(b)Needi WorkIf no such i exists,go to step 4.3.Work=Work+AllocationiFinishi=truego to step 2.4.If Finish i=true for all i,then t
25、he system is in a safe state.,Resource-Request Algorithm for Process Pi,Request=request vector for process Pi.If Requesti j=k then process Pi wants k instances of resource type Rj.1.If Requesti Needi go to step 2.Otherwise,raise error condition,since process has exceeded its maximum claim.2.If Reque
26、sti Available,go to step 3.Otherwise Pi must wait,since resources are not available.3.Pretend to allocate requested resources to Pi by modifying the state as follows:Available=Available Request;Allocationi=Allocationi+Requesti;Needi=Needi Requesti;If safe the resources are allocated to Pi.If unsafe
27、Pi must wait,and the old resource-allocation state is restored,Example of Bankers Algorithm,5 processes P0 through P4;3 resource types:A(10 instances),B(5instances),and C(7 instances).Snapshot at time T0:AllocationMaxAvailableA B CA B C A B CP00 1 07 5 3 3 3 2 P12 0 0 3 2 2 P23 0 2 9 0 2 P32 1 1 2 2
28、 2 P40 0 24 3 3,Example(Cont.),The content of the matrix Need is defined to be Max Allocation.NeedA B C P07 4 3 P11 2 2 P26 0 0 P30 1 1 P44 3 1 The system is in a safe state since the sequence satisfies safety criteria.,Example:P1 Request(1,0,2),Check that Request Available(that is,(1,0,2)(3,3,2)tru
29、e.AllocationNeedAvailableA B CA B CA B C P00 1 0 7 4 3 2 3 0P13 0 20 2 0 P23 0 1 6 0 0 P32 1 1 0 1 1P40 0 2 4 3 1 Executing safety algorithm shows that sequence satisfies safety requirement.Can request for(3,3,0)by P4 be granted?Can request for(0,2,0)by P0 be granted?,7.6 Deadlock Detection,Allow sy
30、stem to enter deadlock state Detection algorithmRecovery scheme,Single Instance of Each Resource Type,Maintain wait-for graphNodes are processes.Pi Pj if Pi is waiting for Pj.Periodically invoke an algorithm that searches for a cycle in the graph.If there is a cycle,there exists a deadlock.An algori
31、thm to detect a cycle in a graph requires an order of n2 operations,where n is the number of vertices in the graph.,Resource-Allocation Graph and Wait-for Graph,Resource-Allocation Graph,Corresponding wait-for graph,Several Instances of a Resource Type,Available:A vector of length m indicates the nu
32、mber of available resources of each type.Allocation:An n x m matrix defines the number of resources of each type currently allocated to each process.Request:An n x m matrix indicates the current request of each process.If Request ij=k,then process Pi is requesting k more instances of resource type.R
33、j.,Detection Algorithm,1.Let Work and Finish be vectors of length m and n,respectively Initialize:(a)Work=AvailableFor i=1,2,n,if Allocationi 0,then Finishi=false;otherwise,Finishi=true.2.Find an index i such that both:(a)Finishi=false(b)Requesti WorkIf no such i exists,go to step 4.3.Work=Work+Allo
34、cationiFinishi=true go to step 2.If Finishi=false,for some i,1 i n,then the system is in deadlock state.Moreover,if Finishi=false,then Pi is deadlocked.,Algorithm requires an order of O(m x n2)operations to detect whether the system is in deadlocked state.,Example of Detection Algorithm,5 processes
35、P0 through P4;3 resource types A(7 instances),B(2 instances),and C(6 instances).Snapshot at time T0:AllocationRequestAvailableA B C A B C A B CP00 1 0 0 0 0 0 0 0P12 0 0 2 0 2P23 0 30 0 0 P32 1 1 1 0 0 P40 0 2 0 0 2Sequence will result in Finishi=true for all i.,Example(Cont.),P2 requests an additio
36、nal instance of type C.RequestA B C P00 0 0 P12 0 2P20 0 1P31 0 0 P40 0 2State of system?Can reclaim resources held by process P0,but insufficient resources to fulfill other processes requests.Deadlock exists,consisting of processes P1,P2,P3,and P4.,Detection-Algorithm Usage,When,and how often,to in
37、voke depends on:How often a deadlock is likely to occur?How many processes will need to be rolled back?one for each disjoint cycleIf 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“ca
38、used”the deadlock.,7.7 Recovery from Deadlock:Process Termination,Abort all deadlocked processes.Abort one process at a time until the deadlock cycle is eliminated.In which order should we choose to abort?Priority of the process.How long process has computed,and how much longer to completion.Resourc
39、es the process has used.Resources process needs to complete.How many processes will need to be terminated.Is process interactive or batch?,Recovery from Deadlock:Resource Preemption,Selecting a victim minimize cost.Rollback return to some safe state,restart process for that state.Starvation same process may always be picked as victim,include number of rollback in cost factor.,End of Chapter 7homework:1 2 6 7 10 11 14,
