Deadlocks - ?d????da

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Deadlocks - ?d????da

• Chapter 3

3.1. Resource 3.2. Introduction
to deadlocks 3.3. The ostrich algorithm
3.4. Deadlock detection and recovery
3.5. Deadlock avoidance 3.6.
Deadlock prevention 3.7. Other issues
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Resources - ?????

• Examples of computer resources
• printers
• tape drives
• tables
• Processes need access to resources in reasonable
  order
• Suppose a process holds resource A and requests
  resource B
• at same time another process holds B and requests
  A
• both are blocked and remain so
• Hardware and software deadlocks

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Resources

• Deadlocks occur when
• processes are granted exclusive access to devices
• we refer to these devices generally as resources
• Resources may have multiple copies
• Preemptable (p??e?????s?µ??) resources
• can be taken away from a process with no ill
  effects (for example memory)
• Nonpreemptable (µ?-p??e?????s?µ??) resources
• will cause the process to fail if taken away
  (e.g. CDR)

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Resources

• Sequence of events required to use a resource
• request the resource
• use the resource
• release the resource
• Must wait if request is denied
• requesting process may be blocked
• may fail with error code
• Nature of requesting a resource is highly system
  dependent (e.g. request system call)

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Resource Acquisition
t
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Introduction to Deadlocks

• Formal definition 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
• Only one thread, no interrupts
• Usually the event is release of a currently held
  resource
• None of the processes can
• run
• release resources
• be awakened
• Number of processes and resources is unimportant

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Four Conditions for Deadlock

• Mutual exclusion condition
• each resource assigned to 1 process or is
  available
• Hold and wait condition
• process holding resources can request additional
• No preemption condition
• previously granted resources cannot forcibly
  taken away
• Circular wait condition
• must be a circular chain of 2 or more processes
• each is waiting for resource held by next member
  of the chain
• All relate to a policy that a system can or can
  not have

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Deadlock Modeling

• Modeled with directed graphs
• resource R assigned to process A
• process B is requesting/waiting for resource S
• process C and D are in deadlock over resources T
  and U

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Deadlock Modeling
A B
C

• How deadlock occurs

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Deadlock Modeling
(o) (p)
(q)

• How deadlock can be avoided

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Deadlock Modeling

• Strategies for dealing with Deadlocks
• just ignore the problem altogether
• detection and recovery (a????e?s? ?a? epa?????s?)
• dynamic avoidance (ap?f???)
• careful resource allocation
• prevention (p??????)
• negating one of the four necessary conditions

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The Ostrich Algorithm ???. st??????aµ????

• Pretend there is no problem
• Reasonable if
• deadlocks occur very rarely
• cost of prevention is high
• UNIX and Windows takes this approach
• It is a trade off between
• convenience
• correctness

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Detection with One Resource of Each Type

• AG processes RW resources
• Note the resource ownership and requests
• Is this system deadlocked and if yes, which
  processes are involved?
• A cycle can be found within the graph, denoting
  deadlock

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Detection with One Resource of Each Type

• We need a formal algorithm for detecting
  deadlocks
• A simple one to detect cycles
• Take each node in turn.
• Do a DFS (depth first search) on it.
• If it comes to a node it has encountered in this
  run, then there exists a cycle.
• Previous graph has a cycle

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Detection with Multiple Resources of Each Type

• Data structures needed by deadlock detection
  algorithm
• At all times Si1Cij Aj Ej

n
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Detection with Multiple Resources of Each Type

• Deadlock detection is based on comparing vectors
• Algorithm
• Look for an unmarked process, Pi for which the
  i-th row of R is less or equal to A
• If such a process is found, add the i-th row of C
  to A, mark the process and go back to step 1
• If no such process exists the algorithm terminates

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Detection with Multiple Resources of Each Type

• An example for the deadlock detection algorithm
• (3/2/1)

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Detection with Multiple Resources of Each Type

• When to look for deadlocks?
• Every time a resource request is made
• Detection ASAP
• Expensive
• Every k minutes or whenever the CPU utilization
  drops below a certain threshold

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Recovery from Deadlock - ?pa?????s?

• Recovery through preemption
• take a resource from some other process (e.g.
  printer)
• depends on nature of the resource
• Recovery through rollback
• checkpoint a process periodically
• use this saved state
• restart the process if it is found deadlocked

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Recovery from Deadlock

• Recovery through killing processes
• crudest but simplest way to break a deadlock
• kill one of the processes in the deadlock cycle
• the other processes get its resources
• choose process that can be rerun from the
  beginning (perhaps not in cycle)

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Deadlock Avoidance - ?p?f???

• So far we assumed that all requests take place at
  the beginning
• The system must be able to decide whether
  granting a resource request is safe or not
• Is there an algorithm that can always avoid
  deadlocks?
• Yes, if certain information is known in advance

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Deadlock Avoidance - Resource Trajectories

• Two process resource trajectories
• /// and \\\ are impossible to get
• What scheduler should do at point t ?

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Safe and Unsafe States

• A state is said to be safe if it is not
  deadlocked and there is some scheduling order in
  which every process can run to completion even if
  all of them suddenly request their maximum number
  of resources immediately

(a) (b)
(c) (d)
(e)

• Demonstration that the state in (a) is safe 10
  instances

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Safe and Unsafe States
(a) (b)
(c)
(d)

• Demonstration that the state in b is not safe
• An unsafe state is not a deadlocked state

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The Banker's Algorithm for a Single Resource
(a)
(b)
(c)

• Check to see if granting the request leads to
  unsafe state
• Three resource allocation states
• safe
• safe
• unsafe

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Banker's Algorithm for Multiple Resources

• Example of banker's algorithm with multiple
  resources

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Banker's Algorithm for Multiple Resources

• Look for a row, R, whose unmet resource needs are
  all smaller than or equal to A. If no such row
  exists the system will eventually deadlock since
  no process can run to completion.
• Assume the process of the row chosen requests all
  the resources it needs (which is guaranteed to be
  possible) and finishes. Mark that process as
  terminated and add all its resources to the A
  vector
• Repeat step 1 and 2 until either all processes
  are marked terminated, in which case the state is
  safe, or until a deadlock occurs, in which case
  is not.
• B requests a printer (D,A or E, )
• E requests a printer (deadlock)

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Deadlock PreventionAttacking the Mutual
Exclusion Condition

• Some devices (such as printer) can be spooled
• only the printer daemon uses printer resource
• thus deadlock for printer eliminated
• Not all devices can be spooled
• Principle
• avoid assigning resource when not absolutely
  necessary
• as few processes as possible actually claim the
  resource

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Attacking the Hold and Wait Condition

• Goal Prevent processes that hold resources from
  waiting for more resources
• Require processes to request resources before
  starting
• a process never has to wait for what it needs
• Problems
• may not know required resources at start of run
• also ties up resources other processes could be
  using
• Variation
• process must give up temporarily all resources
  before requesting a new one
• then request all immediately needed

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Attacking the No Preemption Condition

• This is not a viable option
• Consider a process given the printer
• halfway through its job
• now forcibly take away printer
• !!??

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Attacking the Circular Wait Condition
(a)
(b)

• Normally ordered resources
• A resource graph

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Attacking the Circular Wait Condition

• Rule All requests of a process must be made in
  numerical order gt the resource allocation graph
  can not have cycles
• Either i lt j or i gt j gt cant have deadlocks
• Same logic with multiple resources at every
  instant one assigned resource will be the highest
• Problem impossible to find an ordering to
  satisfy everyone

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Attacking the Circular Wait Condition

• Summary of approaches to deadlock prevention

• Avoidance and prevention are not widely used in
  OS, but have special-purpose applications

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Other IssuesTwo-Phase Locking

• DB systems lock records for update
• Phase One
• process tries to lock all records it needs, one
  at a time
• if needed record found locked, start over
• (no real work done in phase one)
• If phase one succeeds, it starts second phase,
• performing updates
• releasing locks
• Note similarity to requesting all resources at
  once
• Algorithm works where programmer can arrange
  things so that the program can be stopped and
  restarted

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Non-resource Deadlocks

• Possible for two processes to deadlock
• each is waiting for the other to do some task
• Can happen with semaphores
• each process required to do a down() on two
  semaphores (mutex and another)
• if done in wrong order, deadlock results

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Starvation

• Algorithm to allocate a resource
• may be to give to shortest job first
• Works great for multiple short jobs in a system
• May cause long job to be postponed indefinitely
• even though not blocked
• Solution
• First-come, first-serve policy