Deadlock Management

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Deadlock Management
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Topics

• Introduction to Deadlocks
• Deadlock Management Approaches
• Deadlock Acceptance (Ostrich Algorithm)
• Deadlock Prevention
• Deadlock Detection and Recovery
• Deadlock Avoidance (Banker Algorithm)
• Other issues

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Introduction to Deadlocks

• Deadlock definition
• Contributing factors to deadlocks
• Concurrent processing
• Needed for high performance and utilization
• Competition over limited resources
• Examples of applications that have deadlock
• Operating systems, Database systems, Distributed
  Systems,

Deadlock is a situation where a group of
processes are permanently blocked waiting for the
resources held by each other in the group.
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System Model

• Concurrent processes P1, P2, ,Pn
• Resource types R1, R2, . . ., Rm
• CPU cycles, memory space, I/O devices, printers
• Each resource type Ri has Wi instances
• Each process utilizes a resource as follows
• Request
• Use
• Release

Represent the relationships between processes
and resources as a graph
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Resource-Allocation Graph
A set of vertices V and a set of edges E.

• V is partitioned into two types
• Processes P1, P2, , Pn
• Resource types R1, R2, , Rm
• E is partitioned into two types
• Request edge directed edge Pi Rj
• Assignment edge directed edge Rj Pi

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Resource-Allocation Graph (Contd)
Pi

• Process
• Resource Type with 4 instances
• Pi requests instance of Rj
• Pi is holding an instance of Rj

Rj
Rj
Pi
Rj
Pi
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Graph Construction
C
A
B
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Deadlock Characterization
Four necessarily and sufficient conditions

• Mutual exclusion Only one process at a time can
  use a resource
• Hold and wait A process holding at least one
  resource is waiting to acquire additional
  resources held by other processes
• No preemption A resource can be released only
  voluntarily by the process holding it, after that
  process has completed its task
• Circular wait Every process Pi is waiting for
  a resource that is held by P(i1 mod n), 1 i lt n

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Examples
Cycle, No Deadlock
Deadlock ?Cycle

• Cycles do NOT necessarily imply deadlock unless
  there is ONLY one instance of each resource type

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Topics

• Introduction to Deadlocks
• Deadlock Management Approaches
• Deadlock Acceptance (Ostrich Algorithm)
• Deadlock Prevention
• Deadlock Detection and Recovery
• Deadlock Avoidance (Banker Algorithm)
• Other issues

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Deadlock Management Approaches

• Ignore the problem altogether (Ostrich Algorithm)
• Prevent deadlocks from happening
• Negate one of the necessary conditions
• Allow deadlocks to happen
• Detect and recover if happened
• Allow deadlocks to happen (Bankers Algorithm)
• Dynamic avoidance with careful resource
  assignment

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1- The Ostrich Algorithm

• 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

Not for real-time or critical systems
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2- Deadlock Prevention
Take out any of the four necessarily conditions

• Mutual exclusion Only one process at a time can
  use a resource
• Hold and wait A process holding at least one
  resource is waiting to acquire additional
  resources held by other processes
• No preemption A resource can be released only
  voluntarily by the process holding it, after that
  process has completed its task
• Circular wait Every process Pi is waiting for
  a resource that is held by P(i1 mod n), 1 i lt n

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2- Deadlock Prevention (Breaking Mutual Exclusion)

• Not easy to break because it is a resource
  characteristic
• Not required for sharable resources (E.g. File
  opened for read)
• Must hold for non-sharable resources (E.g., tape
  drivers, printers, )
• Some devices can be spooled (requests added to
  buffer)
• Deadlock is eliminated for spooled resources,
  e.g. printers
• Problems
• Not all devices can be spooled

Buffer

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2- Deadlock Prevention (Breaking Hold and Wait)

• Must guarantee that whenever a process requests a
  resource, it does not hold any other resources
• Request and be allocated all its resources before
  it begins execution
• Allow process to request resources only when the
  process has none
• Problems
• May not know the required resources at the
  beginning
• Low resource utilization
• Indefinite postponement (Starvation)

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2- Deadlock Prevention (Breaking No Preemption)

• If a process that is holding some resources
  requests another resource that cannot be
  immediately allocated
• All resources currently being held are released
• Preempted resources are added to the list of
  resources for which the process is waiting
• Process will be restarted only when it can regain
  its old resources, as well as the new ones
• Problems
• May not be a viable solution (process is killed)
• Try to Stop/Resume the process (very expensive,
  if feasible)

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2- Deadlock Prevention (Breaking Circular Wait)

• Impose a total ordering of all resource types
• Require each process to request resources in
  order (increasing or descending)
• Problems
• May not know in advance all the required
  resources
• Low resource utilization

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Resource Ordering Example
C
A
B
1- R 2- S 3- T
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Summary of Deadlock Prevention
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Topics

• Introduction to Deadlocks
• Deadlock Management Approaches
• Deadlock Acceptance (Ostrich Algorithm)
• Deadlock Prevention
• Deadlock Detection and Recovery
• Deadlock Avoidance (Banker Algorithm)
• Other issues

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3- Deadlock Detection and Recovery

• Allow the system to enter a deadlock state
• Periodically run a deadlock detection algorithm
• Recover if deadlock is detected

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3- Deadlock Detection and Recovery
Detection with One Instance of Each Resource
Type

• Check the Resource-Allocation Graph
• If a cycle exists ? A deadlock exists

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3- Deadlock Detection and Recovery

• Recovery through preemption
• Take a resource from one process
• Recovery through rollback
• Checkpoint processes periodically
• Restart one process from its last check point
• Recovery through killing processes
• Simplest way to break a deadlock
• Free its resources for other processes to complete

-- Assign priorities -- How long a process has
computed? How much longer remains? --
Resources already allocated to it? Resources
still needed? -- Random selection
-- Good scheduling (E.g., Ageing)
Common Problems 1- Selecting a victim
2- Starvation
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4- Deadlock Avoidance
Requires that the system has some additional a
priori information available in advance

• Requires that each process declare the maximum
  number of resources of each type that it may need
• The algorithm dynamically examines the
  resource-allocation state
• Prevents the circular-wait condition
• Resource-allocation state is defined by
• The number of available resources
• The number allocated resources
• The maximum demands of the processes

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Safe State

• When a process requests a resource, system
  decides if immediate allocation leaves the system
  in a safe state
• System is in safe state if there exists a safe
  sequence of all processes
• Sequence ltP1, P2, , Pngt is safe if
• For each Pi, the resources that Pi can still
  request can be satisfied by currently available
  resources resources held by all the Pj, with
  jlti
• If Pi resource-needs are not immediately
  available, then Pi waits until all Pj have
  finished, with jlti
• When Pj is finished, Pi can obtain needed
  resources, execute, return allocated resources,
  and terminate
• When Pi terminates, Pi1 can obtain its needed
  resources, and so on

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Safe, Unsafe , Deadlock State
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Resource-Allocation Graph

• Claim edge Pi ? Rj indicate that process Pi may
  request resource Rj
• Claim edge converts to request edge when a
  process actually requests a resource
• Resources must be claimed a priori in the system

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Bankers Algorithm (Key Features)

• Multiple instances of a resource type
• Each process must a priori claim maximum use
  (claim edges)
• When a process requests a resource it may have to
  wait
• When a process gets all its resources it must
  return them in a finite amount of time
• Moves the system from one safe state to another
  safe state

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Bankers Algorithm Data Structures
Let n number of processes, and m number of
resources types.

• Available Vector of length m. If available j
  k, there are k instances of resource type Rj
  available
• Max n x m matrix. If Max i,j k, then
  process Pi may request at most k instances of
  resource type Rj
• Allocation n x m matrix. If Allocationi,j
  k then Pi is currently allocated k instances of
  Rj.
• Need n x m matrix. If Needi,j k, then Pi
  may need k more instances of Rj to complete its
  task.
• Need i,j Maxi,j Allocation i,j

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Safety Algorithm (Tested with each request)

• 1. Let Work and Finish be vectors of length m and
  n, respectively. Initialize
• Work Available
• Finish i false, for i1,3, , n
• 2. Find process i such that both
• (a) Finish i false
• (b) Needi ? Work
• If no such i exists, go to step 4.
• 3. Work Work AllocationiFinishi truego
  to step 2.
• 4. If Finish i true for all i, then the
  system is in a Safe state, otherwise, Unsafe
  state.

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Resource-Request Algorithm for Process Pi

• -Request request vector for process Pi
• -If Requesti j k then process Pi wants k
  instances of Rj
• 1. If Requesti ? Needi
• Go to step 2.
• Else
• Raise error since Pi has exceeded its
  maximum claim.
• 2. If Requesti ? Available
• Go to step 3.
• Else
• Pi must wait, since resources are not
  available.
• 3. Pretend to allocate requested resources to Pi
• Available Available - Requesti
• Allocationi Allocationi Requesti
• Needi Needi Requesti
• If safe ? the resources are allocated to Pi.
• If unsafe ? Pi must wait, and the old
  resource-allocation state is restored

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Safety Algorithm Example

• 5 processes P0 through P4
• 3 resource types A, B, C
• A(10 instances), B (5 instances), and C (7
  instances)
• Snapshot at time T0
• Allocation Max Available
• A B C A B C A B C
• P0 0 1 0 7 5 3 3 3 2
• P1 2 0 0 3 2 2
• P2 3 0 2 9 0 2
• P3 2 1 1 2 2 2
• P4 0 0 2 4 3 3

System is in a safe state because lt P1, P3, P4,
P2, P0gt is a safe sequence
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Request Algorithm Example P1 Request (1,0,2)

• Check that Request ? Available (that is, (1,0,2)
  ? (3,3,2) ? true)
• Allocation Need Available
• A B C A B C A B C
• P0 0 1 0 7 4 3 2 3 0
• P1 3 0 2 0 2 0
• P2 3 0 1 6 0 0
• P3 2 1 1 0 1 1
• P4 0 0 2 4 3 1
• Executing safety algorithm shows that sequence
  ltP1, P3, P4, P0, P2gt satisfies safety
  requirement.
• Exercise
• Can request for (3,3,0) by P4 be granted?
• Can request for (0,2,0) by P0 be granted?

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Topics

• Introduction to Deadlocks
• Deadlock Management Approaches
• Deadlock Acceptance (Ostrich Algorithm)
• Deadlock Prevention
• Deadlock Detection and Recovery
• Deadlock Avoidance (Banker Algorithm)
• Other issues

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Starvation

• Starvation definition
• Main cause is bad scheduling
• Example
• Driving license branch takes the shortest request
  first
• Solution is good scheduling
• First-come, first-serve policy
• Priorities Aging

A process may wait indefinitely to complete
without being part of a deadlock
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Two-Phase Locking in DBMS

• Two-phase locking is a concept of database
  systems
• Concurrency control
• Phase 1(Expanding phase) collect all locks a
  transaction needs
• Phase 2 (Shrinking phase) start releasing the
  locks

Notices the similarity with Requesting all
resources at once Not the same
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Topics

• Introduction to Deadlocks
• Deadlock Management Approaches
• Deadlock Acceptance (Ostrich Algorithm)
• Deadlock Prevention
• Deadlock Detection and Recovery
• Deadlock Avoidance (Banker Algorithm)
• Other issues

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Summary

• Deadlock definition
• Deadlock management
• Ignore the problem altogether (Ostrich Algorithm)
• Prevent deadlocks from happening
• Negate one of the necessary conditions
• Allow deadlocks to happen
• Detect and recover if happened
• Allow deadlocks to happen
• Dynamic avoidance with careful resource
  assignment

Deadlock is a situation where a group of
processes are permanently blocked waiting for the
resources held by each other in the group.