31436 Systems Software

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31436 Systems Software NetworksAUTUMN 2002

• Tutorial 5
• Concurrency Deadlock

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What is Concurrency?

• Concurrency is multitasking!
• The technique of managing several processes in
  the system at the same time.
• The illusion that two or more processes are
  executing at the same time.

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What is Deadlock?

• A situation where two or more processes are
  unable to proceed because each is waiting for one
  of the others to do something.
• This set of processes is blocked permanently.
• Deadlock is immediately preceded by circular wait.

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Conditions for Deadlock

• Conditions to be present for deadlock
• Mutual exclusion
• Hold and Wait
• No pre-emption
• Conditions necessary for deadlock
• Circular wait

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QUESTION 1

• Why is deadlock an important issue for system
  operation?

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Deadlock is Important!

• Once in deadlock, the affected processes can
  never run to completion.
• All resources held by the processes at the time
  of deadlock are not released.
• This may cause other processes to go into a state
  of deadlock as resources run out.
• Worst case Deadlock in a busy-wait loop.
• Unnecessary usage of the CPU.

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QUESTION 2

• What is the difference among
• Deadlock Avoidance
• Deadlock Detection
• Deadlock Prevention

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Deadlock Avoidance

• A dynamic technique that examines each new
  resource request for deadlock.
• Allows the three conditions for deadlock.
• If a new resource request could lead to a
  deadlock, then the request is denied.
• Deadlock avoidance thus allows more concurrency
  than prevention.

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Deadlock Detection

• A technique in which requested resources are
  always granted when available.
• No restrictions set on access to resources.
• Periodically, the operating system runs the
  deadlock detection algorithm to detect the
  circular wait condition.

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Deadlock Prevention

• A technique that guarantees that a deadlock will
  not occur.
• Prevention is achieved by assuring that one of
  the necessary conditions for deadlock is not met.
• However, mutual exclusion cannot be disabled.

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QUESTION 3

• Three processes share four resources of the same
  type each requiring at most two.
• Bill says that you must introduce a deadlock
  detection algorithm in this system.
• Convince Bill that he is wrong.

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Bill Sucks!

• Scenario 1
• Each process requests one resource at a time.
• As each process has its turn executing, each
  process requests one resource.
• By the time each process has had a turn at
  executing, each process holds one resource.
• This still leaves one resource free for one
  process to use to let it execute to completion.
• This first process now releases its resources to
  be used by the other two processes.

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Bill STILL Sucks!

• Scenario 2
• Each process requests two resources at a time.
• As each process has its turn executing, each
  process requests two resources.
• Only the first two processes can execute to
  completion, as they hold all four resources.
• When either of the first two processes completes
  they will release their two resources, thus
  allowing the third process to execute.

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QUESTION 4

• If the probability of deadlock is low timeouts
  appear to be a suitable method for resolving a
  deadlock in a centralized or distributed system.
• What problems do you see for this approach?

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Timeouts for Deadlock

• How long do we set the timeouts?
• Too short System will time out even though a
  state of circular wait doesnt exist yet.
• Too long Waste of system time, power, and
  resources.
• What to do when a timeout occurs?
• If we kill all processes involved, they may reach
  deadlock in the same way again.

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Need help? Ask me! )

• Daniel COMARMOND
• E-Mail dacomarm_at_it.uts.edu.au
• ICQ 6606132
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