Lecture 6- Deadlocks.pptx

Operating Systems: CSE 3204
ASTU
Department of CSE
January 4, 2023 1
Operating Systems
Lecture 6: Deadlocks
Chapter Two
Process Management

Deadlock
• System Model
• Deadlock Characterization
• Methods for handling Deadlock
• Deadlock Prevention
• Deadlock Avoidance
• Deadlock Detection
• Recovery from Deadlock
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Operating Systems

System Model
• A system contains a finite number of resource types (R1, R2, . . ., Rm) to be
distributed among competing processes
• The resource types are partitioned in to several types (e.g files, I/O
devices, CPU cycles, memory), each having a number of identical
instances
• A process must request a resource before using it and release it after
making use of it. Each process utilizes a resource as follows:
• Request
• A process requests for an instance of a resource type. If the
resource is free, the request will be granted. Otherwise the process
should wait until it acquires the resource
• Use
• The process uses the resource for its operations
• Release
• The process releases the resource
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Deadlock
• Deadlock can be defined as a permanent blocking of processes that
either compete for system resources or communicate with each other
• The set of blocked processes each hold a resource and wait to
acquire a resource held by another process in the set
• All deadlocks involve conflicting needs for resources by two or more
processes
• A set of processes or threads are deadlocked when each process or
thread is waiting for a resource to be freed which is controlled by
another process
• Example 1
• Suppose a system has 2 disk drives
• If P1 is holding disk 2 and P2 is holding disk 1 and if P1 requests
for disk 1 and P2 requests for disk 2, then deadlock occurs
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Operating Systems

Traffic gridlock is an everyday
example of a deadlock situation.
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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

Deadlock characterization
1. Mutual exclusion: only one process at a time can use a resource. No
process can access a resource unit that has been allocated to another
process
2. Hold and wait: a process holding at least one resource is waiting to
acquire additional resources held by other processes.
3. No preemption: a resource can be released only voluntarily by the
process holding it, after that process has completed its task.
4. Circular wait: there exists a set {P0, P1, …, Pn} 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
• Pn is waiting for a resource that is held by P0
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Deadlock can arise if four conditions hold simultaneously in
a system:

Resource Allocation Graph(RAG)
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• Deadlock can be better described by using a directed graph
called resource allocation graph
• The graph consists of a set of vertices V and a set of edges E
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
• If a Resource Allocation Graph contains a cycle, then a deadlock
may exist

Resource Allocation Graph /RAG (cont.)
• Process
• Resource Type with 4 instances
• Pi requests instance of Rj
• Pi is holding an instance of Rj
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p
Pi Rj
Pi Rj
• Diagrammatically, processes and resources in RAG are represented as follow:

Example of Resource Allocation Graph
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Graph With A Cycle But No Deadlock
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Basic Facts
• If graph contains no cycles then there’s no
deadlock
• If however a graph contains a cycle then
there are two possible situations:
• if there is only one instance per
resource type, then deadlock can occur
• if there are several instances per
resource type, there’s a possibility of no
deadlock

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• The RAG shown here tells us about the following
situation in a system:
• P= {P1, P2, P3}
• R= {R1,R2, R3, R4}
•E ={P1R1,P2R3, R1P2, R2P1,R3P3}
• The process states
• P1 is holding an instance of R2 and is waiting
for an instance of R1
• P2 is holding an instance of R1 and instance
of R2, and is waiting for an instance of R3
• P3 is holding an instance of R3

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• Resource Allocation Graph With a Deadlock
• There are two cycles in this graph
• P1R1P2R3P3R2P1
• P2R3P3R2P2
• Processes P1, P2 and P3 are deadlocked:
• P1 is waiting for P2 to release R1
• P2 is waiting for R3 held by P3 and
• P3 is waiting for either P1 or P2 to
release R2

Methods for handling Deadlocks
Deadlock problems can be handled in one of the following 3 ways:
1. Using a protocol that prevents or avoids deadlock by ensuring
that a system will never enter a deadlock state deadlock
prevention and deadlock avoidance scheme are used
2. Allow the system to enter a deadlock state and then recover
3. Ignore the problem and pretend that deadlocks never occur in
the system; used by most operating systems, including UNIX
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Operating Systems

Deadlock Prevention
• By ensuring at least one of the necessary conditions for deadlock will not
hold, deadlock can be prevented.
 This is mainly done by restraining how requests for resources can be
made
• Deadlock prevention methods fall into two classes:
• An indirect method of deadlock prevention prevents the occurrence
of one of the three necessary conditions listed previously (items 1
through 3)
• A direct method of deadlock prevention prevents the occurrence of a
circular wait (item 4)
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Deadlock Prevention (contd.)
1. Mutual Exclusion: This is not required for sharable resources; however to
prevent a system from deadlock, the mutual exclusion condition must hold for non-
sharable resources
2. Hold and Wait: in order to prevent the occurrence of this condition in a system,
we must guarantee that whenever a process requests a resource, it does not hold
any other resources. Two protocols are used to implement this:
1. Require a process to request and be allocated all its resources before it
begins execution or
2. Allow a process to request resources only when the process has none
• Both protocols have two main disadvantages:
o Since resources may be allocated but not used for a long period, resource
utilization will be low
o A process that needs several popular resources has to wait indefinitely
because one of the resources it needs is allocated to another process. Hence
starvation is possible.
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3. No Preemption
• If a process holding certain resources is denied further request, that
process must release its original resources allocated to it
• If a process requests a resource allocated to another process waiting for
some additional resources, and the requested resource is not being used,
then the resource will be preempted from the waiting process and
allocated to the requesting process
• 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
• This approach is practical to resources whose state can easily saved and
retrieved easily
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4. Circular Wait
• A linear ordering of all resource types is defined and each process
requests resources in an increasing order of enumeration
• So, if a process initially is allocated instances of resource type R,
then it can subsequently request instances of resources types
following R in the ordering
Ri->Ri+1->Ri+2->Ri+3
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Deadlock Avoidance
• Deadlock avoidance scheme requires each process to declare the
maximum number of resources of each type that it may need in
advance
• Having this full information about the sequence of requests and
release of resources, we can know whether or not the system is
entering unsafe state
• 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
• A state is safe if the system can allocate resources to each process
in some order avoiding a deadlock.
• A deadlock state is an unsafe state.
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• If a system is in a safe state,
then there are no deadlocks.
• If a system is in unsafe state,
then there is a possibility of
deadlock.
• Deadlock avoidance method
ensures that a system will
never enter an unsafe state
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Deadlock avoidance: Safe State
Basic Facts

Deadlock Avoidance Algorithms
• Based on the concept of safe state, we can define algorithms
that ensures the system will never deadlock.
• If there is a single instance of a resource type,
• Use a resource-allocation graph(RAG)
• If there are multiple instances of a resource type,
• Use the Dijkstra’s banker’s algorithm
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Operating Systems

• A new type of edge (Claim edge), in addition to the request and assignment edge
is introduced.
• Claim edge Pi  Rj indicates that process Pi may request resource Rj at some point
in the future. The edge resembles a request edge but is represented by a dashed
line in the graph
• Claim edge is converted to request edge when a process requests a resource
• Request edge is converted to an assignment edge when the resource is
allocated to the process
• When a resource is released by a process, assignment edge reconverts to a
claim edge
• If no cycle exists in the allocation, then system is in safe state otherwise the
system is in unsafe state
• Resources must be claimed a priori in the system
• i.e, before a process starts executing, all its claim edge must show up in the
allocation graph
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Deadlock Avoidance Algorithms (contd.)
Resource-Allocation Graph Scheme

Resource-Allocation Graph Algorithm
• Suppose that process Pi requests a resource Rj
• The request can be granted only if converting the request edge to
an assignment edge does not result in the formation of a cycle in
the resource allocation graph.
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• If we suppose P2 requests R2. We can not
allocate it since it will create a cycle.

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Unsafe State Resource-Allocation
Resource-Allocation Graph algorithm
• If P1 requests for R2 and P2 requests
for R1, then deadlock will occur

Banker’s Algorithm
• This algorithm is used when there are multiple instances of
resources
• When a process enters a system, it must declare the maximum
number of each instance of resource types it may need
• The number however may not exceed the total number of
resource types in the system
• 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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Operating Systems

• The following data structures are used in the 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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The algorithm is as follows:
1.Process pi makes requests for resources. Let Request(i) be the
corresponding request vector. So, if pi wants k instances of resource type rj,
then Request(i)[j] = k
2.If Request(i) !≤ Need(i), there is an error.
3.Otherwise, if Request(i) !≤ Available, then pi must wait
4.Otherwise, Modify the data structures as follows :
• Available = Available - Request(i)
• Allocation(i) = Allocation(i) + Request(i)
• Need(i) = Need(i) - Request(i)
5.Check whether the resulting state is safe. (Use the safety algorithm
presented in the next slide)
6.If the state is safe, do the allocation. Otherwise, pi must wait for
Request(i)

• It is used to identify whether or not a system is in a safe state. The
algorithm can be described as follow:
1. Let Work and Finish be vectors of length m and n, respectively.
Initialize:
Work = Available
Finish [i] = false for i = 0, 1, …, n- 1.
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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Operating Systems
Safety Algorithm

Resource-Request Algorithm
• This algorithm determines if a request can be safely granted. The algorithm is
described below
• If 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 – Request;
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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Examples of Banker’s algorithm:
• Assume a system has
• 5 processes P0 through P4;
• 3 resource types:
A (10 instances), B (5instances), 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
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Example (contd.)
• 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)
• 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 < 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 Detection
• If a system does not implement either deadlock prevention
or avoidance, deadlock may occur. Hence the system must
provide
• A deadlock detection algorithm that examines the state of
the system if there is an occurrence of deadlock
• An algorithm to recover from the deadlock
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Deadlock Detection:
1. Single Instance of Each Resource Type
• If there are single instances of each resources in a system, then an
algorithm that uses a type of resource allocation graph called wait-for
graph will be used
• The wait-for graph is obtained from the resource allocation graph by
removing the resource nodes and collapsing the corresponding edges
• If a process Pi points to Pj I in a wait-for graph, it indicates that A Pi is
waiting for Pj to release a resource Pj needs
• To detect deadlocks, the system needs to periodically invoke an algorithm
that searches for a cycle in the graph. If there is a cycle, there exists a
deadlock.
• An algorithm to detect a cycle in a graph requires an order of n2
operations, where n is the number of vertices in the graph.
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Resource-Allocation Graph Corresponding wait-for graph
Deadlock Detection:
1. Single Instance of Each Resource Type (contd.)

• When there are multiple instances of a resource type in a resource
allocation system, the wait-for graph is not applicable. Hence, a deadlock
detection algorithm is used
• The algorithm uses several data structures similar to the ones in banker’s
algorithm
• 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 [ij] = k, then process Pi is requesting k more
instances of resource type. Rj.
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Deadlock Detection:
2. Several Instances of a Resource Type

Deadlock 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
Finish[i] = false;otherwise, Finish[i] = true.
2. Find an index i such that both:
(a) Finish[i] == false
(b) Requesti  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] == false, for some i, 1  i  n, then the system is in deadlock
state. Moreover, if Finish[i] == false, then Pi is deadlocked.
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Deadlock Detection Algorithm (contd.)
Example
• 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 <P0, P2, P3, P1, P4> will result in Finish[i] = true for all i
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• 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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Deadlock Detection Algorithm (cont.)
Example

Deadlock Detection Algorithm Usage
• When, and how often, to invoke the detection algorithm depends on:
1. How often a deadlock is likely to occur?
2. How many processes will be affected (need to be rolled back)?
• If deadlock occurs frequently, then the algorithm is invoked frequently,
o Resources allocated to deadlocked processes will be idle until the deadlock can
be broken
o The number of processes in the deadlock may increase
• If the algorithm is invoked for every resource request not granted, it will
incur a computation time overhead on the system
• If detection algorithm is invoked arbitrarily, there may be many cycles in the
resource graph and so we would not be able to tell which of the many deadlocked
processes “caused” the deadlock
• A less expensive alternative is to invoke the algorithm at a less frequent time
• (e.g once per hour)
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• Once a deadlock has been detected, recovery strategy is needed. There are two
possible recovery approaches:
• Process termination
• Resource preemption
Process Termination
• Abort all deadlocked processes
• Abort one process at a time until the deadlock cycle is eliminated
• In which order should we choose a process to abort? Chose the process with
• Least amount of processor time consumed so far
• Least amount of output produced so far
• Most estimated time remaining
• Least total resources allocated so far
• Lowest priority
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Recovery from Deadlock

Recovery from Deadlock
Resource Preemption
• In this recovery strategy, we successively preempt resources and allocate them to
another process until the deadlock is broken
• While implementing this strategy, there are three issues to be considered
• Selecting a victim – which resources and process should be selected to
minimize cost just like in process termination. The cost factors may include
parameters like the number of resources a deadlocked process is holding,
number of resources it used so far
• Rollback – if a resource is preempted from a process, then it can not continue
its normal execution
• The process must be rolled back to some safe state and started
• Starvation – same process may always be picked as victim several times. As
a result, starvation may occur. The best solution to this problem is to only allow
a process to be picked as a vicitim for a limited finite number of times. This can
be done by including the number of rollback in the cost factor
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a. b.
c.
Exercises
1. For each of the resource allocation graphs below, determine whether there is a
deadlock or not. Give explanations for each of your answers.
d.

Exercises(cont…)
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2. There are four processes in a system and they are going to share nine tape
drives. Their current and maximum number of allocation numbers are as follows :
a. Is the system in a safe state? Why
or why not?
b. Is the system deadlocked? Why or
why not?
3. Explain if the system in the given
resource allocation graph here is
deadlocked. If not, give an
execution order of the processes
which successfully terminates
Process Allocation Maximum
P1 3 6
P2 1 2
P3 4 9
P4 0 2

4. Given the resource allocation graph below:
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Operating Systems
a. Apply the deadlock detection algorithm
and either indicate why the system is
deadlocked, or specify a safe allocation
sequence
b. If the process P2 also request 2
instances of resource r1, does the
system enter a deadlock? Why?
5. Consider the following snapshot of a system:
• Use the banker’s algorithm to answer the following
questions:
a. What is the content of the matrix Need?
b. Is the system in a safe state?
c. If a request from process P1 arrives for
(0,4,2,0), can the request be granted
immediately?
Exercises(cont…)

Lecture 6- Deadlocks.pptx

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Lecture 6- Deadlocks.pptx