Lecture Outline

Aims and Objectives
Preventing Deadlock (by denying one of the conditions)
Deadlock Detection
Recovering from Deadlock
Deadlock Avoidance

Aims and Objectives

• Handling Deadlock
  • Prevention

  • Detection

  • Recovery

  • Avoidance

• Denying Mutual Exclusion

  Not desirable to deny - we want dedicated resources

• Denying Hold-And-Wait

  Force processes to request and gain all their resources in one go.

  Disadvantages are other processes are made to wait.

  A resource which will not be used until the end of a process still has to be requested at the beginning - poor resource utilisation

• Denying No Pre-emption

  Some non-shareable resources can be pre-empted if the hardware/software saves the context of the interrupted process - but what about printers?

  Only partial denial of no pre-emption is possible.

• Denying Circular Waiting

  Resources are grouped into `Frequency of Use' classes, C1, C2, C3, etc.

  All required resources belonging to the same class are requested at the same time in class order

  No circular wait because either all required resources one class are held, or process waits until all required resources within a class are available.

  Disadvantages are that resources must be requested in an artifical order - not necessarily the order in which they will be used.

• Do not prevent deadlock from occurring. Periodically look for deadlock. If found then recover.

• Only one of the conditions implies deadlock.

• `State Graph' used to show which processes hold resources and which processes have requested resources.

• Try to reduce the state graph by removing links for allocations and requests in turn for each process.

  

• If at least one link cannot be removed, then deadlock

• Every time there is an allocation or release? Time consuming.

• Update state graph periodically by examining state

• Problem determining which processes are deadlocked.

• Four Main Methods of Deadlock Recovery

  Abort all deadlocked processes

  Restart all deadlocked processes from a checkpoint

  Abort processes one at a time

  Pre-empt resources from processes

• Deadlock Recovery usually performed by operator

• Tries to anticipate deadlock

• Will deny resource requests if algorithm decides that granting the request could lead to deadlock.

• Because of nondeterminacy of OS, algorithm is not always correct, and denies requests that would not have led to deadlock.

• Deadlock does not occur immediately after allocating a resource, so cannot simply make projection on state graph and then run deadlock detection algorithm

Dijkstra's Banker's Algorithm

• Keep a track of all processes' current usage, future needs, and current availability of resources.

• Deadlock will be avoided if request plus current usage is less than or equal to process's claim, and if after the request is granted there is a sequence in which all processes can run to completion even if they request their full claim - safe state

Example, using 12 tape decks

                        Current                          Maximum            
                         Loan                            Need               
                                                                            
Process(1)                 1                               4                  
Process(2)                 4                               6                  
Process(3)                 5                               8                  
                                                                            
Available                                 2                                     

Case A

Process(2) requests and obtains 2 remaining decks. Process(2) can run to completion, releasing all 6 decks providing sufficient resources for the other processes to complete - safe state

Case B

Process(1) requests one deck. State unsafe because there are not enough resources to guarantee completion of all processes - request denied.

Advantages of the Banker's Algorithm

• Allows mutual-exclusion, hold-and-wait, and no pre-emption conditions

• System guarantees that processes will be allocated resources within finite time.

Disadvantages

As an assignment, hand in an essay which discusses the disadvantages of the Banker's Algorithm

Summary

• Different ways of handling deadlock

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