Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts Operating Systems Lecture 30 Handling Deadlock II Read Ch. 8.6

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts Review Methods of handling deadlocks:  Ensure system never enters deadlock state  allow system to enter deadlock, then recover  Ignore the problem Deadlock prevention  Make sure at least one of 4 necessary conditions for deadlock cannot occur. Deadlock avoidance  Each process declares maximum number of resources of each type it may need.  Keep the system in a safe state: In which we can allocate resources to each process in some order and avoid deadlock.  Check for safe state by finding a safe sequence: where resources that Pi needs can be satisfied by available resources plus resources held by Pj where j < i.  Resource allocation graph algorithm uses claim edges to check for a safe state.

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts Unsafe State In Resource-Allocation Graph

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts Banker’s Algorithm The Banker's algorithm is a method of deadlock avoidance when there are multiple instances of resource types. Requirements:  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.

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts Data Structures for the Banker’s Algorithm 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]. Let n = number of processes, and m = number of resources types.

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts Example of Data Structures 2 processes P 0 and P 1 ; 3 resource types A (7 instances), B (3 instances), and C (6 instances). Snapshot at time T 0 : AllocationMaxNeedAvailable A B CA B C P P

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts 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 i ; 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

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts Safety Algorithm Idea: Find a safe sequence. If none exists, then in unsafe state. 1. Let Work and Finish be vectors of length m and n, respectively. Initialize: Work = Available Finish [i] = false for i - 1,3, …, n. 2.Find and i such that both: (a) Finish [i] = false (b) Need i  Work If no such i exists, go to step 4. 3.Work = Work + Allocation i Finish[i] = true go to step If Finish [i] == true for all i, then the system is in a safe state. Otherwise, the system is in an unsafe state.

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts Example of Banker’s Algorithm 5 processes P 0 through P 4 ; 3 resource types A (10 instances), B (5 instances), and C (7 instances). Snapshot at time T 0 : AllocationMaxNeedAvailable A B CA B C P P P P P Is the system safe? Yes! Sequence

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts Example: P 1 Request (1,0,2) Check that Request  Available (that is, (1,0,2)  (3,3,2)  true.) AllocationNeedAvailable A B CA B CA 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?

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts Deadlock Detection Allow system to enter deadlock state Detection algorithm Recovery scheme

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts 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. 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. For several instances of each resource type, an algorithm similar to the Banker's algorithm is used to detect deadlocks (see textbook).

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts Resource-Allocation Graph and Wait-for Graph Resource-Allocation GraphCorresponding wait-for graph

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts 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.

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts 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.  Resources the process has used.  Resources process needs to complete.  How many processes will need to be terminated.  Is process interactive or batch?

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts 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.

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts Combined Approach to Deadlock Handling Combine the three basic approaches  prevention  avoidance  detection allowing the use of the optimal approach for each of resources in the system.