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Deadlocks

Deadlocks. CPE 261403 - Operating Systems http://www.e-cpe.org/moodle. Conditions of a Deadlock (7.2.1). Mutual exclusion Hold and wait No preemption Circular wait. Which of these are in a deadlock?. Methods for Handling Deadlocks. Never allow a deadlock state

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Deadlocks

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  1. Deadlocks CPE 261403 - Operating Systems http://www.e-cpe.org/moodle

  2. Conditions of a Deadlock (7.2.1) • Mutual exclusion • Hold and wait • No preemption • Circular wait

  3. Which of these are in a deadlock?

  4. Methods for Handling Deadlocks • Never allow a deadlock state • Allow deadlock and then recover • Ignore the problem

  5. Deadlock Prevention(7.4)Prevent deadlock conditions from occurring • Mutual exclusion • Hold and wait • No preemption • Circular wait

  6. Deadlock Avoidance(7.5) Figure 7.5

  7. Example Total number of resources = 12

  8. Resource Allocation Graph Safe Unsafe

  9. 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 Rjavailable. • Max: n x m matrix. If Max [i,j] = k, then process Pimay request at most k instances of resource type Rj. • Allocation: n x m matrix. If Allocation[i,j] = k then Pi is currently allocated k instances of Rj. • Need: n x m matrix. If Need[i,j] = k, then Pi may need k more instances of Rjto complete its task. Need [i,j] = Max[i,j] – Allocation [i,j].

  10. Example 3 resource types: A (10 instances), B (5instances), and C (7 instances). Time T0 MAX Allocation

  11. Safety Algorithm Available Need Finish

  12. Safety Algorithm Available Need Finish

  13. Example (Cont.) • The content of the matrix Need is defined to be Max – Allocation. Need A B C P0 7 4 3 P1 1 2 2 P2 6 0 0 P3 0 1 1 P4 4 3 1 • The system is in a safe state since the sequence < P1, P3, P4, P2, P0> satisfies safety criteria.

  14. Process P1 makes a request for: 1 instance of A 2 instance of C Request Vector = (1,0,2)

  15. Safety Algorithm after (1,0,2) request Work Need Finish

  16. Example: P1 Request (1,0,2) • Check that Request  Available (that is, (1,0,2)  (3,3,2)  true. AllocationNeedAvailable A B C A B C A B C P0 0 1 0 7 4 3 2 3 0 P1 3 0 2 0 2 0 P2 3 0 1 6 0 0 P3 2 1 1 0 1 1 P4 0 0 2 4 3 1 • Executing safety algorithm shows that sequence < P1, P3, P4, P0, P2> satisfies safety requirement. • Can request for (3,3,0) by P4 be granted? • Can request for (0,2,0) by P0 be granted?

  17. Deadlock Detection (7.6)

  18. Wait-for Graphs (single instance system only) Resource-Allocation Graph Corresponding wait-for graph

  19. Wait-for Graph does not work with multiple instance resources

  20. Banker’s Algorithm as a deadlock detection tool for multiple instance systems

  21. Deadlock Recovery (7.7)

  22. Process Termination • Abort all deadlock processes • Abort one at a time until deadlock cycle is eliminated

  23. 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.

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