7.Dead Locks

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Senthilkanth,MCA.. The following ppt's full topic covers Operating System for BSc CS, BCA, MSc CS, MCA students.. 1.Introduction 2.OS Structures 3.Process 4.Threads 5.CPU Scheduling 6.Process Synchronization 7.Dead Locks 8.Memory Management 9.Virtual Memory 10.File system Interface 11.File system implementation 12.Mass Storage System 13.IO Systems 14.Protection 15.Security 16.Distributed System Structure 17.Distributed File System 18.Distributed Co Ordination 19.Real Time System 20.Multimedia Systems 21.Linux 22.Windows

Transcript of 7.Dead Locks

  • 1. Chapter 7: DeadlocksChapter 7: Deadlocks
  • 2. 7.2 Silberschatz, Galvin and GagneOperating System ConceptsChapter 7: DeadlocksChapter 7: DeadlocksThe Deadlock ProblemSystem ModelDeadlock CharacterizationMethods for Handling DeadlocksDeadlock PreventionDeadlock AvoidanceDeadlock DetectionRecovery from Deadlock
  • 3. 7.3 Silberschatz, Galvin and GagneOperating System ConceptsChapter ObjectivesChapter ObjectivesTo develop a description of deadlocks, which preventsets of concurrent processes from completing their tasksTo present a number of different methods for preventingor avoiding deadlocks in a computer system.
  • 4. 7.4 Silberschatz, Galvin and GagneOperating System ConceptsThe Deadlock ProblemThe Deadlock ProblemA set of blocked processes each holding a resource and waiting toacquire a resource held by another process in the set.ExampleSystem has 2 tape drives.P1 and P2 each hold one tape drive and each needs anotherone.Examplesemaphores A and B, initialized to 1P0 P1wait (A); wait(B)wait (B); wait(A)
  • 5. 7.5 Silberschatz, Galvin and GagneOperating System ConceptsBridge Crossing ExampleBridge Crossing ExampleTraffic 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 deadlockoccurs.Starvation is possible.
  • 6. 7.6 Silberschatz, Galvin and GagneOperating System ConceptsSystem ModelSystem ModelResource types R1, R2, . . ., RmCPU cycles, memory space, I/O devicesEach resource type Ri has Wi instances.Each process utilizes a resource as follows:requestuserelease
  • 7. 7.7 Silberschatz, Galvin and GagneOperating System ConceptsDeadlock CharacterizationDeadlock CharacterizationMutual exclusion: only one process at a time can use aresource.Hold and wait: a process holding at least one resource iswaiting to acquire additional resources held by otherprocesses.No preemption: a resource can be released onlyvoluntarily by the process holding it, after that process hascompleted its task.Circular wait: there exists a set {P0, P1, , P0} of waitingprocesses such that P0 is waiting for a resource that is heldby P1, P1 is waiting for a resource that is held byP2, , Pn1 is waiting for a resource that is held byPn, and P0 is waiting for a resource that is held by P0.Deadlock can arise if four conditions hold simultaneously.
  • 8. 7.8 Silberschatz, Galvin and GagneOperating System ConceptsResource-Allocation GraphResource-Allocation GraphV is partitioned into two types:P = {P1, P2, , Pn}, the set consisting of all theprocesses in the system.R = {R1, R2, , Rm}, the set consisting of all resourcetypes in the system.request edge directed edge P1 Rjassignment edge directed edge Rj PiA set of vertices V and a set of edges E.
  • 9. 7.9 Silberschatz, Galvin and GagneOperating System ConceptsResource-Allocation Graph (Cont.)Resource-Allocation Graph (Cont.)ProcessResource Type with 4 instancesPi requests instance of RjPi is holding an instance of RjPiPiRjRj
  • 10. 7.10 Silberschatz, Galvin and GagneOperating System ConceptsExample of a Resource AllocationExample of a Resource AllocationGraphGraph
  • 11. 7.11 Silberschatz, Galvin and GagneOperating System ConceptsResource Allocation Graph With AResource Allocation Graph With ADeadlockDeadlock
  • 12. 7.12 Silberschatz, Galvin and GagneOperating System ConceptsResource Allocation Graph With A Cycle But NoResource Allocation Graph With A Cycle But NoDeadlockDeadlock
  • 13. 7.13 Silberschatz, Galvin and GagneOperating System ConceptsBasic FactsBasic FactsIf 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 ofdeadlock.
  • 14. 7.14 Silberschatz, Galvin and GagneOperating System ConceptsMethods for Handling DeadlocksMethods for Handling DeadlocksEnsure that the system will never enter a deadlock state.Allow the system to enter a deadlock state and thenrecover.Ignore the problem and pretend that deadlocks never occurin the system; used by most operating systems, includingUNIX.
  • 15. 7.15 Silberschatz, Galvin and GagneOperating System ConceptsDeadlock PreventionDeadlock PreventionMutual Exclusion not required for sharable resources;must hold for nonsharable resources.Hold and Wait must guarantee that whenever aprocess requests a resource, it does not hold any otherresources.Require process to request and be allocated all itsresources before it begins execution, or allow processto request resources only when the process has none.Low resource utilization; starvation possible.Restrain the ways request can be made.
  • 16. 7.16 Silberschatz, Galvin and GagneOperating System ConceptsDeadlock Prevention (Cont.)Deadlock Prevention (Cont.)No Preemption If a process that is holding some resources requestsanother resource that cannot be immediately allocated toit, then all resources currently being held are released.Preempted resources are added to the list of resources forwhich the process is waiting.Process will be restarted only when it can regain its oldresources, 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 anincreasing order of enumeration.
  • 17. 7.17 Silberschatz, Galvin and GagneOperating System ConceptsDeadlock AvoidanceDeadlock AvoidanceSimplest and most useful model requires that each processdeclare the maximum number of resources of each typethat it may need.The deadlock-avoidance algorithm dynamically examinesthe resource-allocation state to ensure that there can neverbe a circular-wait condition.Resource-allocation state is defined by the number ofavailable and allocated resources, and the maximumdemands of the processes.Requires that the system has some additional a priori informationavailable.
  • 18. 7.18 Silberschatz, Galvin and GagneOperating System ConceptsSafe StateSafe StateWhen a process requests an available resource, system mustdecide if immediate allocation leaves the system in a safe state.System is in safe state if there exists a safe sequence of allprocesses.Sequence is safe if for each Pi, the resources that Pican still request can be satisfied by currently available resources +resources held by all the Pj, with j
  • 19. 7.19 Silberschatz, Galvin and GagneOperating System ConceptsBasic FactsBasic FactsIf 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 anunsafe state.
  • 20. 7.20 Silberschatz, Galvin and GagneOperating System ConceptsSafe, Unsafe , Deadlock StateSafe, Unsafe , Deadlock State
  • 21. 7.21 Silberschatz, Galvin and GagneOperating System ConceptsResource-Allocation Graph AlgorithmResource-Allocation Graph AlgorithmClaim edge Pi Rj indicated that process Pj may requestresource Rj; represented by a dashed line.Claim edge converts to request edge when a processrequests a resource.When a resource is