Chapter06.ppt
- 2. 2 Agenda: • Two problems that affects all efforts to support concurrent processing: – Deadlock and Starvation • Principles of deadlock • Related problem of starvation • Common approaches to dealing with deadlock Prevention ,Detection, Avoidance • Classic problems used to illustrate both synchronization and deadlock issues: – The dining philosophers problem. • Note: Consideration of concurrency and deadlock on a single system.
- 3. 3 Deadlock • What??? • Permanent blocking of a set of processes that either compete for system resources or communicate with each other. • Deadlock is permanent. • What to do when …… – No efficient solution – Deadlocks involve Involve conflicting needs for resources by two or more processes
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- 6. • Different execution paths: • 1. Q acquires B and then A and then releases B and A. When P resumes execution, it will be able to acquire both resources. • 2. Q acquires B and then A. P executes and blocks on a request for A. Q releases B and A. When P resumes execution, it will be able to acquire both resources. • 3. Q acquires B and then P acquires A. Deadlock is inevitable, because as execution proceeds, Q will block on A and P will block on B. • 4. P acquires A and then Q acquires B. Deadlock is inevitable, because as execution proceeds, Q will block on A and P will block on B. • 5. P acquires A and then B. Q executes and blocks on a request for B. P releases A and B. When Q resumes execution, it will be able to acquire both resources. • 6. P acquires A and then B and then releases A and B. When Q resumes execution, it will be able to acquire both resources. 6
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- 8. 8 Reusable Resources • Used by only one process at a time and not depleted by that use • Processes obtain resources that they later release for reuse by other processes • Processors, I/O channels, main and secondary memory, devices, and data structures such as files, databases, and semaphores • Deadlock occurs if each process holds one resource and requests the other
- 10. 10 Another Example of Deadlock • Space is available for allocation of 200Kbytes, and the following sequence of events occur • Deadlock occurs if both processes progress to their second request P1 . . . . . . Request 80 Kbytes; Request 60 Kbytes; P2 . . . . . . Request 70 Kbytes; Request 80 Kbytes;
- 11. 11 Consumable Resources • Created (produced) and destroyed (consumed) • Interrupts, signals, messages, and information in I/O buffers • Deadlock may occur if a Receive message is blocking • May take a rare combination of events to cause deadlock
- 12. 12 Example of Deadlock • Deadlock occurs if receive is blocking P1 . . . . . . Receive(P2); Send(P2, M1); P2 . . . . . . Receive(P1); Send(P1, M2);
- 13. 13 Resource Allocation Graphs • Directed graph that depicts a state of the system of resources and processes
- 15. 15 Conditions for Deadlock • When???? • Mutual exclusion – Only one process may use a resource at a time • Hold-and-wait – A process may hold allocated resources while awaiting assignment of others • No preemption – No resource can be forcibly removed form a process holding it
- 16. 16 Conditions for Deadlock • Circular wait – A closed chain of processes exists, such that each process holds at least one resource needed by the next process in the chain
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- 18. 18 Possibility of Deadlock • Mutual Exclusion • No preemption • Hold and wait
- 19. 19 Existence of Deadlock • Mutual Exclusion • No preemption • Hold and wait • Circular wait
- 20. 20 Deadlock Prevention • Mutual Exclusion – Must be supported by the operating system • Hold and Wait – Require a process request all of its required resources at one time
- 21. 21 Deadlock Prevention – Prob: – Process may be held up for a long time waiting for all of its resource requests to be filled, when in fact it could have proceeded with only some of the resources. – Resources allocated to a process may remain unused for a considerable period, during which time they are denied to other processes. – A process may not know in advance all of the resources that it will require.
- 22. 22 Deadlock Prevention • No Preemption – Process must release resource and request again – Operating system may preempt a process to require it releases its resources • Circular Wait – Define a linear ordering of resource types
- 23. 23 Deadlock Avoidance • A decision is made dynamically whether the current resource allocation request will, if granted, potentially lead to a deadlock • Requires knowledge of future process request
- 24. 24 Two Approaches to Deadlock Avoidance • Do not start a process if its demands might lead to deadlock • Do not grant an incremental resource request to a process if this allocation might lead to deadlock
- 25. 25 Resource Allocation Denial • Referred to as the banker’s algorithm • State of the system is the current allocation of resources to process • Safe state is where there is at least one sequence that does not result in deadlock • Unsafe state is a state that is not safe
- 27. 27 Determination of a Safe State Initial State
- 28. • Can any of the four processes be run to completion with the resources available? 28
- 29. 29 Determination of a Safe State P2 Runs to Completion
- 30. 30 Determination of a Safe State P1 Runs to Completion
- 31. 31 Determination of a Safe State P3 Runs to Completion
- 32. following deadlock avoidance strategy, ensures that the system of processes and resources is always in a safe state. 32
- 33. 33 Determination of an Unsafe State
- 34. 34 Determination of an Unsafe State
- 35. • The deadlock avoidance strategy does not predict deadlock with certainty; • It merely anticipates the possibility of deadlock and assures that there is never such a possibility 35
- 38. 38 Deadlock Avoidance • Adv. – Less restrictive than prevention strategy. • Limitations: – Maximum resource requirement must be stated in advance – Processes under consideration must be independent; no synchronization requirements – There must be a fixed number of resources to allocate – No process may exit while holding resources
- 39. 39 Deadlock Detection • Deadlock prevention strategies are conservative • They solve the problem of deadlock by limiting access to resources and by imposing restrictions on processes. • Deadlock detection strategies do not limit resource • access or restrict process actions. • Deadlock detection, requested resources are granted to processes whenever possible. • Periodically, the OS performs an algorithm that allows it to detect the circular wait condition described earlier in condition.
- 40. 40 Deadlock Detection • Frequency of checking???? – each resource request OR – depending on how likely it is for a deadlock to occur. • Adv 1st – It leads to early detection, – The algorithm is relatively simple because it is based on incremental changes to the state of the system. • Disadv: – frequent checks consume considerable processor time.
- 41. 41 Deadlock Detection • The algorithm proceeds by marking processes that are not deadlocked. • Initially, all processes are unmarked.
- 42. Deadlock Detection Algorithm 1. Mark each process that has a row in the allocation matrix of all zeros 2. Initialize a temporary vector W to equal to available vector. 3. Find an index i such that process i is currently unmarked and the ith row of request matrix is less than or equal to W. • If no such row is found, terminate the algorithm and all unmarked processes are those involved in the deadlock. 4. If such a row is found, mark process i and add the corresponding row of the allocation matrix to W. Return to step 3
- 43. Deadlock Detection Algorithm • A deadlock exists if and only if there are unmarked processes at the end of the algorithm. • Each unmarked process is deadlocked. • Algorithm does not guarantee to prevent deadlock • All that it does is determine if deadlock currently exists.
- 44. 44 Strategies once Deadlock Detected • Abort all deadlocked processes • Back up each deadlocked process to some previously defined checkpoint, and restart all process – Original deadlock may occur • Successively abort deadlocked processes until deadlock no longer exists • Successively preempt resources until deadlock no longer exists
- 45. 45 Selection Criteria Deadlocked Processes • Least amount of processor time consumed so far • Least number of lines of output produced so far • Most estimated time remaining • Least total resources allocated so far • Lowest priority
- 46. 46 Strengths and Weaknesses of the Strategies
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- 53. 53 UNIX Concurrency Mechanisms • Pipes • Messages • Shared memory • Semaphores • Signals
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- 55. 55 Linux Kernel Concurrency Mechanisms • Includes all the mechanisms found in UNIX • Atomic operations execute without interruption and without interference
- 58. 58 Linux Kernel Concurrency Mechanisms • Spinlocks – Used for protecting a critical section
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- 61. 61 Solaris Thread Synchronization Primitives • Mutual exclusion (mutex) locks • Semaphores • Multiple readers, single writer (readers/writer) locks • Condition variables
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