Operating System Concepts

1
Operating System Concepts

• Ku-Yaw Chang
• canseco_at_mail.dyu.edu.tw
• Assistant Professor, Department of Computer
  Science and Information Engineering
• Da-Yeh University

2
Chapter 8 Deadlocks

• In a multiprogramming environment
• Several processes may compete for a finite number
  of resources
• A process requests resources
• If no available, the process enter a wait state.
• Waiting process may never again change state
• Such a situation is called a deadlock
• A law passed by the Kansas legislature
• When two trains approach each other at a
  crossing, both shall come to a full stop and
  neither shall start up again until the other has
  gone.

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Chapter 8 Deadlocks

1. System Model
2. Deadlock Characterization
3. Methods for Handling Deadlocks
4. Deadlock Prevention

1. Deadlock Avoidance
2. Deadlock Detection
3. Recovery from Deadlock
4. Summary
5. Exercises

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8.1 System Model

• A process may utilize a resource in only the
  following sequence
• Request
• If this request cannot be granted immediately,
  the requesting process must wait.
• Use
• Release
• Examples
• open and close file system calls
• allocate and free memory system calls

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Deadlock State

• A set of processes is in a deadlock state when
  every process in the set is waiting for an event
  that can be caused only by another process in the
  set.
• Resources
• Physical resources
• printers, tape drives, memory space, CPU
• Logical resources
• files, semaphores, monitors
• Other types of events
• IPC facilities

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Deadlock Examples

• Example
• System has 2 tape drives.
• P1 and P2 each hold one tape drive and each needs
  another one.
• Example
• semaphores A and B, initialized to 1
• P0 P1
• wait (A) wait (B)
• wait (B) wait (A)

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Bridge Crossing Example

• Traffic only in one direction.
• Each section of a bridge can be viewed as a
  resource.
• If a deadlock occurs, it can be resolved if one
  car backs up (preempt resources and rollback).
• Several cars may have to be backed up if a
  deadlock occurs.
• Starvation is possible.

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Chapter 8 Deadlocks

1. System Model
2. Deadlock Characterization
3. Methods for Handling Deadlocks
4. Deadlock Prevention

1. Deadlock Avoidance
2. Deadlock Detection
3. Recovery from Deadlock
4. Summary
5. Exercises

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8.2.1 Necessary Conditions

• A deadlock situation arise if the following four
  conditions hold simultaneously
• Mutual exclusion
• At least one resource must be held in a
  nonsharable mode
• Hold and wait
• Hold at least one resource and wait to acquire
  additional resources that are currently being
  held by other processes
• No preemption
• Resources cannot be preempted
• Circular wait
• P0 is waiting for a resource that is held by P1,
  P1 is waiting for a resource that is held by P2,
  , Pn1 is waiting for a resource that is held by
  Pn, and Pn is waiting for a resource that is held
  by P0.

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8.2.2 Resource-Allocation Graph

• System resource-allocation graph
• A directed graph for describing deadlocks
• A set of vertices V
• Active processes
• P P1, P2, , Pn
• Resource types
• R R1, R2, , Rn
• A set of edges E
• A request edge
• Pi -gt Rj
• An assignment edge
• Rj -gt Pi

• 4 instances

Pi
Rj
Pi
Rj
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Resource Allocation Graph

• Sets P, R, and E
• P P1, P2, P3
• R R1, R2, R3 , R4
• E P1 -gtR1, P2 -gtR3, R1 -gtP2, R2
  -gtP2, R2 -gtP1, R3 -gtP3
• Resource instances
• R1 one instance
• R2 two instances
• R3 one instance
• R4 three instances

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Resource Allocation Graph

• If the graph contains no cycles
• No process in the system is deadlocked
• If the graph does contain a cycle
• A deadlock may exist

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Resource Allocation Graph

• P3 requests an instance of R2
• Two minimal cycles exist
• P1 -gt R1 -gt P2 -gt R3 -gt P3 -gt R2 -gt P1
• P2 -gt R3 -gt P3 -gt R2 -gt P2
• Processes P1, P2, and P3 are deadlocked

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Resource Allocation Graph

• A cycle exists
• P1 -gt R1 -gt P3 -gt R2 -gt P1
• No deadlock
• P4 may release its instance of R2
• That resource can the be allocated to P3,
  breaking the cycle

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Deadlock Characterization

• In summary, if a resource-allocation graph
• Does not have a cycle
• The system is not in a deadlock state
• Have a cycle
• The system may or may not be in a deadlock state

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Chapter 8 Deadlocks

1. System Model
2. Deadlock Characterization
3. Methods for Handling Deadlocks
4. Deadlock Prevention

1. Deadlock Avoidance
2. Deadlock Detection
3. Recovery from Deadlock
4. Summary
5. Exercises

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8.3 Methods for Handling Deadlocks

• Deal with the deadlock problem in one of three
  ways
• Use a protocol to prevent or avoid deadlocks,
  ensuring that the system will never enter a
  deadlock state
• Allow the system to enter a deadlock state,
  detect it, and recover.
• Ignore the problem altogether, and pretend that
  deadlocks never occur in the system
• Deadlocks occurs infrequently (say, once per year)

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Chapter 8 Deadlocks

1. System Model
2. Deadlock Characterization
3. Methods for Handling Deadlocks
4. Deadlock Prevention

1. Deadlock Avoidance
2. Deadlock Detection
3. Recovery from Deadlock
4. Summary
5. Exercises

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8.4 Deadlock Prevention

• Ensure at least one of the four necessary
  conditions cannot hold
• Mutual Exclusion
• Hold and Wait
• No Preemption
• Circular Wait

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Deadlock Prevention

• Mutual Exclusion
• Must hold for nonsharable resources ( ex. a
  printer)
• We cannot prevent deadlocks by denying the
  mutual-exclusion condition
• Some resources are intrinsically nonsharable
• Hold and Wait
• Whenever a process requests a resource, it does
  not hold any other resources
• Request and be allocated all resources before
  execution
• Request resources only when the process has none
• Main disadvantages
• Low resource utilization
• Starvation is possible

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Deadlock Prevention

• No Preemption
• If a process that is holding some resources
  requests another resource that cannot be
  immediately allocated to it, then 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 that
  it is requesting.
• Applied to resources whose state can be easily
  saved and restored
• Such as CPU registers and memory spaces
• No applied to printers and tape drives

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Deadlock Prevention

• Circular Wait
• Impose a total ordering of all resource types
• A one-to-one function F R -gt N
• N is the set of natural numbers
• require that each process requests resources in
  an increasing order of enumeration
• Initially request any number of instances of a
  resource type Ri
• Request instances of resource type Rj if and only
  if F(Rj)gtF(Ri)
• Example
• F(tape drive) 1
• F(disk drive) 5
• F(printer) 12

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Chapter 8 Deadlocks

1. System Model
2. Deadlock Characterization
3. Methods for Handling Deadlocks
4. Deadlock Prevention

1. Deadlock Avoidance
2. Deadlock Detection
3. Recovery from Deadlock
4. Summary
5. Exercises

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8.5 Deadlock Avoidance

• Possible side effects of preventing deadlocks
• Low device utilization
• Reduced system throughput
• Avoid deadlock
• Require additional information about how
  resources are to be requested
• Simplest and most useful model
• Each process declare the maximum number of
  resources of each type that it may need
• Possible to ensure that the system will never
  enter a deadlock state

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8.5.1 Safe State

• If the system can allocate resources to each
  process in some order and still avoid a deadlock
• A safe sequence ltP1, P2, , Pngt
• for each Pi , the resources that Pi can still
  request can be satisfied by currently available
  resources resources held by all the Pj , with j
  lt i.
• If Pi resource needs are not immediately
  available, then Pi can wait until all Pj have
  finished.
• 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.
• Unsafe no such sequence exists

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Basic Facts

• If a system is in safe state
• no deadlocks
• If a system is in unsafe state
• possibility of deadlock.
• Avoidance
• ensure that a system will never enter an unsafe
  state.

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An Example

• A system with 12 magnetic tape drives and 3
  processes P0, P1, and P2
• The sequence ltP1, P0, P2gt satisfies the safety
  condition.

Maximum Needs Current Needs
P0 10 5
P1 4 2
P2 9 2
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An Example

• Process P2 requests and is allocated 1 more tape
  drive
• The system is no longer in a safe state.
• If a process requests a resource that is
  currently available, it may still have to wait.

Maximum Needs Current Needs
P0 10 5
P1 4 2
P2 9 3
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8.5.2 Resource-Allocation Graph Algorithm

• Resource-allocation graph with only one instance
  of each resource type
• Used for deadlock avoidance
• Claim edge Pi -gt Rj (represented by a dashed
  line)
• Process Pj may request resource Rj
• Converts to request edge when a process requests
  a resource.
• When a resource is released by a process,
  assignment edge reconverts to a claim edge.
• Resources must be claimed a priori in the system.

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Resource-Allocation GraphFor Deadlock Avoidance
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Unsafe State InResource-Allocation Graph
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8.5.3 Bankers Algorithm

• Multiple instances
• resource-allocation graph allocation is not
  applicable
• A new process must declare the maximum number of
  instances of each resource type that it may need.
• Cannot exceed the total number of resources in
  the system
• Determine whether the allocation of these
  resources will leave the system in a safe state
• If yes, the resources are allocated
• If not, the process must wait until some other
  process releases enough resources

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Data Structuresfor the Bankers Algorithm

• 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 Allocation i, j
  k then Pi is currently allocated k instances of
  Rj.
• Need n x m matrix. If Need i, j k, then Pi
  may need k more instances of Rj to complete its
  task.
• Need i, j Max i, j Allocation i, j .

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Notationfor the Bankers Algorithm

• Let X and Y be vectors of length n.
• X Y if and only if X i Y i for all i
  1,2, , n
• For example
• if X (1, 7, 3, 2) and Y (0, 3, 2, 1),then Y
  X

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8.5.3.1 Safety Algorithm

• Find out whether or not the system is in a safe
  state
• Let Work and Finish be vectors of length m and n,
  respectively. Initialize
• Work Available
• Finish i false for i 1, 2, 3, , n.
• Find an i such that both
• Finish i false
• Needi ? Work.
• If no such i exists, go to step 4.
• Work Work AllocationiFinish i truego
  to step 2.
• If Finish i true for all i, then the system
  is in a safe state.

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8.5.3.2 Resource-Request Algorithm

• Requesti j k
• Process Pi wants k instances of resource type Rj.
• If Requesti Needi, go to step 2. Otherwise,
  raise an error condition, since the process has
  exceeded its maximum claim.
• If Requesti Available, go to step 3. Otherwise
  Pi must wait, since the resources are not
  available.
• Pretend to allocate requested resources to Pi by
  modifying the state as follows
• Available Available Requesti
• Allocationi Allocationi Requesti
• Needi Needi Requesti
• If safe -gt the resources are allocated to Pi .
• If unsafe -gt Pi must wait, and the old
  resource-allocation state is restored.

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8.5.3.3 An Illustrative Example

• Three resource types
• A 10 instances
• B 5 instances
• C 7 instances

• Five processes
• P0 through P4
• Snapshot at time T0
• Allocation M a x Available Need
• A B C A B C A B C A B C
• P0 0 1 0 7 5 3 3 3 2 7 4 3
• P1 2 0 0 3 2 2 1 2 2
• P2 3 0 2 9 0 2 6 0 0
• P3 2 1 1 2 2 2 0 1 1
• P4 0 0 2 4 3 3 4 3 1
• ltP1, P3, P4, P2, P0gt satisfies the safety
  criteria.

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8.5.3.3 An Illustrative Example

• P1 requests
• One instance of resource type A
• Two instances of resource type C
• Request1 (1, 0, 2) Available (3, 3, 2)
• Snapshot at time T0
• 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 2 6 0 0
• P3 2 1 1 0 1 1
• P4 0 0 2 4 3 1
• ltP1, P3, P4, P0, P2gt satisfies the safety
  criteria.
• Grant the request of process P1.

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Chapter 8 Deadlocks

1. System Model
2. Deadlock Characterization
3. Methods for Handling Deadlocks
4. Deadlock Prevention

1. Deadlock Avoidance
2. Deadlock Detection
3. Recovery from Deadlock
4. Summary
5. Exercises

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8.6 Deadlock Detection

• Allow the system to enter deadlock state
• An algorithm that examines the state of the
  system to determine whether a deadlock has
  occurred
• An algorithm to recover from the deadlock
• A detection-and-recovery scheme requires overhead
• Run-time costs
• Potential losses

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8.6.1 Single Instance ofEach Resource Type

• A wait-for graph obtained from
  resource-allocation graph
• Remove the nodes of type resource
• Collapse the appropriate edges

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8.6.1 Single Instance ofEach Resource Type

• A deadlock exists in the system if and only if
  the wait-for graph contains a cycle.
• To detect deadlocks
• Maintain the wait-for graph
• Periodically invoke an algorithm that searches
  for a cycle in the graph

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8.6.2 Several Instances ofa Resource Type

• The algorithm employs several time-varying data
  structures
• Available
• A vector of length m indicates the number of
  available resources of each type.
• Allocation
• An n x m matrix defines the number of resources
  of each type currently allocated to each process.
• Request
• An n x m matrix indicates the current request of
  each process. If Request i, j k, then process
  Pi is requesting k more instances of resource
  type Rj.

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Detection Algorithm

• 1. Let Work and Finish be vectors of length m and
  n, respectively. Initialize
• (a) Work Available
• (b) For i 1,2, , n, if Allocationi ? 0, then
  Finishi falseotherwise, Finishi true.
• 2. Find an index i such that both
• (a) Finishi false
• (b) Requesti ? Work
• If no such i exists, go to step 4.

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Detection Algorithm (Cont.)

• 3. Work Work AllocationiFinishi truego
  to step 2.
• 4. If Finishi false, for some i, 1 ? i ? n,
  then the system is in deadlock state. Moreover,
  if Finishi false, then Pi is deadlocked.

Algorithm requires an order of O(m x n2)
operations to detect whether the system is in
deadlocked state.
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Example of Detection Algorithm

• Five processes P0 through P4 three resource
  types A (7 instances), B (2 instances), and C (6
  instances).
• Snapshot at time T0
• Allocation Request Available
• A B C A B C A B C
• P0 0 1 0 0 0 0 0 0 0
• P1 2 0 0 2 0 2
• P2 3 0 3 0 0 0
• P3 2 1 1 1 0 0
• P4 0 0 2 0 0 2
• Sequence ltP0, P2, P3, P1, P4gt will result in
  Finishi true for all i.

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Example (Cont.)

• P2 requests an additional instance of type C.
• Request
• A B C
• P0 0 0 0
• P1 2 0 1
• P2 0 0 1
• P3 1 0 0
• P4 0 0 2
• State of system?
• Can reclaim resources held by process P0, but
  insufficient resources to fulfill other
  processes requests.
• Deadlock exists, consisting of processes P1, P2,
  P3, and P4.

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8.6.3 Detection-Algorithm Usage

• When, and how often, to invoke depends on
• How often a deadlock is likely to occur?
• How many processes will need to be rolled back?
• one for each disjoint cycle
• The detection algorithm can be invoked
• When a request cannot be granted immediately
• Identify the specific process that caused the
  deadlock
• At arbitrary points in time
• Not be able to tell which of the many deadlocked
  processes caused the deadlock

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Chapter 8 Deadlocks

1. System Model
2. Deadlock Characterization
3. Methods for Handling Deadlocks
4. Deadlock Prevention

1. Deadlock Avoidance
2. Deadlock Detection
3. Recovery from Deadlock
4. Summary
5. Exercises

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8.7.1 Process Termination

• Abort all deadlocked processes.
• Abort one process at a time until the deadlock
  cycle is eliminated.
• In which order should we choose to abort?
• Priority of the process.
• How long process has computed, and how much
  longer to completion.
• Resources the process has used.
• Resources process needs to complete.
• How many processes will need to be terminated.
• Is process interactive or batch?

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8.7.2 Resource Preemption

• Three issues
• Selecting a victim
• To minimize cost
• Number of resources
• Amount of time
• Rollback
• Return to some safe state, restart process for
  that state
• The simplest solution is a total rollback
• Starvation
• Same process may always be picked as victim,
  include number of rollback in cost factor
• Include the number of rollbacks in the cost factor

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Chapter 8 Deadlocks

1. System Model
2. Deadlock Characterization
3. Methods for Handling Deadlocks
4. Deadlock Prevention

1. Deadlock Avoidance
2. Deadlock Detection
3. Recovery from Deadlock
4. Summary
5. Exercises

53
Summary

• P.266

54
Chapter 8 Deadlocks

1. System Model
2. Deadlock Characterization
3. Methods for Handling Deadlocks
4. Deadlock Prevention

1. Deadlock Avoidance
2. Deadlock Detection
3. Recovery from Deadlock
4. Summary
5. Exercises

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Exercises

• 8.2
• 8.4
• 8.5
• 8.12
• 8.13

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The End