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Topics
• Resource
• Introduction to deadlocks
• The ostrich algorithm
• Deadlock detection and recovery
• Deadlock avoidance
– Banker’s algorithm
• Deadlock prevention
• Other issues
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Introduction
• Parallel operation among many devices driven by
concurrent processes contribute significantly to
high performance. But concurrency also results in
contention for resources and possibility of
deadlock among the vying processes.
• Deadlock is a situation where a group of processes
are permanently blocked waiting for the
resources held by each other in the group.
• Typical application where deadlock is a serious
problem: Operating system, data base accesses,
and distributed processing.
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System Model
• Resource types R1, R2, . . ., Rm
CPU cycles, memory space, I/O devices
• Each resource type Ri has Wi instances.
• Each process utilizes a resource as
follows:
– request
– use
– release
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Deadlock Characterization
• 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: there exists a set {P0, P1, …, P0} of waiting
processes such that P0 is waiting for a resource that is
held by P1, P1 is waiting for a resource that is held by
P2, …, Pn–1 is waiting for a resource that is held by
Pn, and P0 is waiting for a resource that is held by P0.
Deadlock can arise if four conditions hold simultaneously.
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Resource-Allocation Graph
• V is partitioned into two types:
– P = {P1, P2, …, Pn}, the set consisting of all the
processes in the system.
– R = {R1, R2, …, Rm}, the set consisting of all
resource types in the system.
• request edge – directed edge P1 Rj
• assignment edge – directed edge Rj Pi
A set of vertices V and a set of edges E.
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Methods for Handling
Deadlocks
• Ensure that the system will never enter a
deadlock state. (pessimistic)
• Allow the system to enter a deadlock state
and then recover. Database systems;
• Ignore the problem and pretend that
deadlocks never occur in the system; Older
operating systems; (ostrich algorithm:
optimistic)
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Dealing with Deadlock
Strategies for dealing with Deadlocks
1. just ignore the problem altogether
2. detection and recovery
3. dynamic avoidance
• careful resource allocation
4. prevention
• negating one of the four necessary
conditions
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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
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Detection with One Resource of Each
Type (1)
• Note the resource ownership and requests
• A cycle can be found within the graph, denoting
deadlock
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Recovery from Deadlock (1)
• Recovery through preemption
– take a resource from some other process
– 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 (2)
• 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
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Deadlock Avoidance
• Simplest and most useful model requires that
each process declare the maximum number of
resources of each type that it may need.
• The deadlock-avoidance algorithm dynamically
examines the resource-allocation state to ensure
that there can never be a circular-wait condition.
• Resource-allocation state is defined by the
number of available and allocated resources, and
the maximum demands of the processes.
Requires that the system has some additional a priori information
available.
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Safe State
• When a process requests an available resource, system must
decide 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 <P1, P2, …, Pn> 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 j<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, Pi+1 can obtain its needed resources, and
so on.
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Resource-Allocation Graph
Algorithm
• Claim edge Pi Rj indicated that process Pj may
request resource Rj; represented by a dashed line.
• Claim edge 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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Banker’s Algorithm
• Multiple instances.
• Each process must a priori claim maximum
use.
• 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.
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Data Structures for the Banker’s
Algorithm
• 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].
Let n = number of processes, and m = number of resources types.
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Safety Algorithm
1. Let Work and Finish be vectors of length m and n,
respectively. Initialize:
Work = Available
Finish [i] = false for i - 1,3, …, n.
2. Find and i such that both:
(a) Finish [i] = false
(b) Needi Work
If no such i exists, go to step 4.
3. Work = Work + Allocationi
Finish[i] = true
go to step 2.
4. If Finish [i] == true for all i, then the system is in a safe
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 resource type Rj.
1. If Requesti Needi go to step 2. Otherwise, raise error
condition, since process has exceeded its maximum claim.
2. If Requesti Available, go to step 3. Otherwise Pi must wait,
since resources are not available.
3. Pretend to allocate requested resources to Pi by modifying the
state as follows:
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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Example of Banker’s
Algorithm
• 5 processes P0 through P4; 3 resource types A
(10 instances),
B (5instances, and C (7 instances).
• Snapshot at time T0:
AllocationMaxAvailable
A B C A B C A B C
P0 0 1 07 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 24 3 3
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Example (Cont.)
• The content of the matrix. Need is defined to be Max –
Allocation.
Need
A B C
P0 7 4 3
P1 1 2 2
P2 6 0 0
P3 0 1 1
P4 4 3 1
• The system is in a safe state since the sequence < P1, P3,
P4, P2, P0> satisfies safety criteria.
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Example P1 Request (1,0,2) (Cont.)
• Check that Request Available (that is, (1,0,2) (3,3,2) true.
Allocation NeedAvailable
A B C A B CA B C
P0 0 1 0 7 4 3 2 3 0
P1 3 0 20 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 <P1, P3, P4, P0,
P2> satisfies safety requirement.
• Can request for (3,3,0) by P4 be granted?
• Can request for (0,2,0) by P0 be granted?
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Deadlock Prevention
• Mutual Exclusion – not required for sharable
resources; must hold for nonsharable resources.
• Hold and Wait – must guarantee that whenever
a process requests a resource, it does not hold
any other resources.
– Require process to request and be allocated
all its resources before it begins execution, or
allow process to request resources only when
the process has none.
– Low resource utilization; starvation possible.
Restrain the ways request can be made.
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Deadlock Prevention (Cont.)
• 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.
• Circular Wait – impose a total ordering of all resource
types, and require that each process requests resources in
an increasing order of enumeration.
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Deadlock Prevention
Attacking 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
• 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 all resources
– 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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Other Issues
Two-Phase Locking
• 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
– program can be stopped, restarted
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Nonresource 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
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Data Structures for the Banker’s
Algorithm
• 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].
Let n = number of processes, and m = number of resources types.](https://image.slidesharecdn.com/deadlockmar21-250724200322-5b162034/85/DeadlockMar21-ppt-22-320.jpg)
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Safety Algorithm
1. Let Work and Finish be vectors of length m and n,
respectively. Initialize:
Work = Available
Finish [i] = false for i - 1,3, …, n.
2. Find and i such that both:
(a) Finish [i] = false
(b) Needi Work
If no such i exists, go to step 4.
3. Work = Work + Allocationi
Finish[i] = true
go to step 2.
4. If Finish [i] == true for all i, then the system is in a safe
state.](https://image.slidesharecdn.com/deadlockmar21-250724200322-5b162034/85/DeadlockMar21-ppt-23-320.jpg)
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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 resource type Rj.
1. If Requesti Needi go to step 2. Otherwise, raise error
condition, since process has exceeded its maximum claim.
2. If Requesti Available, go to step 3. Otherwise Pi must wait,
since resources are not available.
3. Pretend to allocate requested resources to Pi by modifying the
state as follows:
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](https://image.slidesharecdn.com/deadlockmar21-250724200322-5b162034/85/DeadlockMar21-ppt-24-320.jpg)
