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# 12_Deadlocks_III - CSC 4103 Operating Systems Fall 2009...

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CSC 4103 - Operating Systems Fall 2009 Tevfik Ko ! ar Louisiana State University October 6 th , 2009 Lecture - XII Deadlocks - III 2 Deadlock Detection Allow system to enter deadlock state Detection algorithm Recovery scheme 3 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 . Resource-Allocation Graph Corresponding wait-for graph 4 Single Instance of Each Resource Type 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. Only good for single-instance resource allocation systems. 5 Several Instances of a Resource Type Available: A vector of length m indicates the number 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 [ i j ] = k , then process P i is requesting k more instances of resource type. R j . 6 Detection Algorithm 1. Let Work and Finish be vectors of length m and n , respectively Initialize: (a) Work = Available (b) For i = 0,2, …, n-1 , if Allocation i " 0, then Finish [i] = false;otherwise, Finish [i] = true . 2. Find an index i such that both: (a) Finish [ i ] == false (b) Request i # Work If no such i exists, go to step 4.

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7 Detection Algorithm (Cont.) 3. Work = Work + Allocation i Finish [ i ] = true go to step 2. 4. If Finish [ i ] == false, for some i , 0 # i # n-1 , then the system is in deadlock state. Moreover, if Finish [ i ] == false , then P i is deadlocked. Algorithm requires an order of O( m x n 2) operations to detect whether the system is in deadlocked state. 8 Example of Detection Algorithm Five processes P 0 through P 4 ; three resource types A (7 instances), B (2 instances), and C (6 instances).
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12_Deadlocks_III - CSC 4103 Operating Systems Fall 2009...

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