Deadlock in Operating SystemSystem Model Deadlock Characterization

Methods for Handling Deadlocks
Deadlock Prevention
Deadlock Avoidance
Deadlock Detection

1. Deadlock in Operating System By Mrs.R.SABITHA ., M.Sc.,M.Phil ., Assistant Professor, Department of Computer Science(SF) V.V.Vanniaperumal College for Women, Virudhunagar .

2. Contents System Model Deadlock Characterization Methods for Handling Deadlocks Deadlock Prevention Deadlock Avoidance Deadlock Detection

3. Problem of Deadlock 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 drives. P 1 and P 2 each hold one disk drive and each needs another one. Example semaphores A and B , initialized to 1 P P 1 wait (A); wait(B) wait (B); wait(A)

4. 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.

5. System Model Resource types R 1 , R 2 , . . ., R m CPU cycles, memory space, I/O devices Each resource type R i has W i instances. Each process utilizes a resource as follows: request use release

6. Deadlock Characterization Deadlock can arise if four conditions hold simultaneously. 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 holding it, after that process has completed its task . Circular wait: there exists a set { P , P 1 , …, P } of waiting processes such that P 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 is waiting for a resource that is held by P .

7. Resource-Allocation 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 – directed edge P 1  R j assignment edge – directed edge R j  P i

8. Resource-Allocation Graph Process Resource Type with 4 instances P i requests instance of R j P i is holding an instance of R j P i P i

9. Example

10. Resource Allocation Graph With A Deadlock

11. Graph With A Cycle But No Deadlock

12. 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.

13. Methods for Handling Deadlocks Deadlock Prevention - Ensure that the system will never enter a deadlock state. Deadlock Avoidance - Allow the system to enter a deadlock state and then recover. Deadlock Detection -Ignore the problem and pretend that deadlocks never occur in the system

14. 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 circular-wait condition. Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes.

15. Safe State System is in safe state if there exists a sequence < P 1 , P 2 , …, P n > of ALL the processes is 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.

16. Basic Facts 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

17. Safe, Unsafe , Deadlock State

18. 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 Banker’s Algorithm

19. Steps for Banker’s Algorithm Let n = number of processes, and 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 x m matrix. If Max [ i,j ] = k , then process P i may request at most k instances of resource type R j . Allocation : n x m matrix. If Allocation[ i,j ] = k then P i is currently allocated k instances of R j . Need : n x 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 ].

20. 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 Pi. If unsafe  Pi must wait, and the old resource-allocation state is restored

21. Example of Banker’s Algorithm 5 processes P through P 4 ; 3 resource types: A (10 instances), B (5instances), and C (7 instances). Snapshot at time T : Allocation Max Available A B C A B C A B C P 0 1 0 7 5 3 3 3 2 P 1 2 0 0 3 2 2 P 2 3 0 2 9 0 2 P 3 2 1 1 2 2 2 P 4 0 0 2 4 3 3

22. Example The content of the matrix Need is defined to be Max – Allocation . Need A B C P 7 4 3 P 1 1 2 2 P 2 6 0 0 P 3 0 1 1 P 4 4 3 1 The system is in a safe state since the sequence < P 1 , P 3 , P 4 , P 2 , P > satisfies safety criteria.

23. Recovery from Deadlock: Process Terminatio n Abort all deadlocked processes. Abort one process at a time until the deadlock cycle is eliminated. 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

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