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CH 3 Deadlock

CH 3 Deadlock. When 2 (or more) processes remain blocked forever !. Process a Down x Gets x Down y Blocks. Process b Down y Gets y Down x Blocks. How can this happen?. Both are blocked forever !. Resources. Things for which we request exclusive access.

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CH 3 Deadlock

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  1. CH 3 Deadlock When 2 (or more) processes remain blocked forever!

  2. Process a Down x Gets x Down y Blocks Process b Down y Gets y Down x Blocks How can this happen? Both are blocked forever!

  3. Resources • Things for which we request exclusive access. • Ex. db, files, shared memory, printer, cd/dvd writer, tape drive, etc. • Types • Preemptable – can be taken away w/out ill effects • Non preemptable – cannot be take away w/out causing a failure

  4. Resource examples • Memory – preemptable • CPU – preemptable • CD writing – non preemptable • Printing – non preemptable • We will only consider non preemptable (the harder problem).

  5. Deadlock • A set of processes is deadlocked if each process in the set is waiting for an event that only another process in the set can cause.

  6. Conditions for deadlock • Mutual exclusion • Hold and wait • No preemption • Circular wait (Need all of these (all necessary).)

  7. Resource allocation graph resource process

  8. (Directed)Graphs • A graph G = (V,E) where V = vertex set and E = edge set • Ordered pairs of vertices • Ex. Let G = (V,E) where V = { C, D, T, U } and E = { <D,U>, <U,C>, <C,T>, <T,D> }

  9. Dealing w/ deadlock • Ignore it (ostrich algorithm). • Detect and recover. • Avoid by careful resource allocation. • Disallow one or more of the conditions necessary for deadlock.

  10. 2. Detect and recover • Detection with one resource of each type • Detection with multiple resources of each type

  11. Detection with one resource of each type • Detect cycle in digraph • For each vertex v in graph, • search the subtree rooted at v • see if we visit any vertex twice (by keeping a record of already visited vertices).

  12. Detection with multiple resources of each type • Skip.

  13. Recovery • Preemption • Rollback (using checkpoints) • Kill process

  14. Recovery: preemption • Temporarily take back needed resource • Ex. Printer • Pause at end of page k • Start printing other job • Resume printing original job starting at page k+1

  15. Recovery: rollback • Checkpoint – save entire process state (typically right before the resource was allocated) • When deadlock is detected, we kill the checkpointed process, freeing the resource, and then later restart the killed process starting at the checkpoint. • Requires apps that can be restarted in this manner.

  16. Rdb’s & commit/rollback From p. 62, http://www.postgresql.org/files/documentation/pdf/8.0/postgresql-8.0-US.pdf

  17. Recovery: kill process • Requires apps that can be restarted from the beginning.

  18. Deadlock avoidance • Safe state = not deadlocked and there exists some scheduling order in which every process can run to completion even if all of them suddenly request their max number of resources immediately • Unsafe != deadlocked

  19. Safe states

  20. Unsafe states

  21. Banker’s algorithm (for single resource type) • Grant only those requests that lead to safe states. • Requires future information.

  22. Banker’s algorithm • (b) is safe because from (b) we can give C 2 more (free=0) then C completes (free=4) then give either B or D . . .

  23. Banker’s algorithm • (c) is unsafe because no max can be satified

  24. Banker’s algorithm (multiple resources) N: E=existing (free) P=possessed (allocated/held) A=available ∑

  25. Banker’s algorithm (multiple resources) To check for a safe state: • Look for a row, R, in N <= A. • Assume the process of the chosen row requests all resources, runs to completion, and terminates (giving back all of its resources to A). • Repeat steps 1 and 2 until either all processes either terminate (indicating that the initial state was safe) or deadlock occurs.

  26. Banker’s algorithm (multiple resources) To check for a safe state: • Look for a row, R, in N <= A.

  27. Deadlock prevention • Attack: • Mutual exclusion • Hold and wait • No preemption • Circular wait

  28. Deadlock prevention: attack mutex • Ex. Use spooling instead of mutex on printer • No all problems lend themselves to spooling • Still have contention for disk space with spooling

  29. Deadlock prevention: attack hold & wait • Request (wait for) all resources prior to process execution • Problems: • We may not know a priori. • We tie up resources for entire time. • Before requesting a resource, we must first temporarily release all allocated resources and then try to acquire all of them again.

  30. Deadlock prevention: attack no preemption • Tricky at best. • Impossible at worst.

  31. Deadlock prevention: attack circular wait • Only allow one resource at a time. • Request all resources in (some globally assigned) numerical order. • No numerical ordering may exist.

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