The objectives of Deadlocks in Operating Systems are: - To develop a description of deadlocks, which prevent sets of concurrent processes from completing their tasks - To present a number of different methods for preventing or avoiding deadlocks in a computer system

1. Amrita
School
of
Engineering,
Bangalore
Ms. Harika Pudugosula
Lecturer
Department of Electronics & Communication Engineering

2. • System Model
• Deadlock Characterization
• Methods for Handling Deadlocks
• Deadlock Prevention
• Deadlock Avoidance
• Deadlock Detection
• Recovery from Deadlock
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3. Deadlock Detection
• If a system does not employ either a deadlock-prevention or a deadlock avoidance
algorithm, then a deadlock situation may occur
• In this environment, the system may provide -
• An algorithm that examines the state of the system to determine whether a
deadlock has occurred
• An algorithm to recover from the deadlock
• Two requirements as they pertain to systems with only a single instance of each
resource type, systems with several instances of each resource type
• Includes not only the run-time costs of maintaining the necessary information and
executing the detection algorithm but also the potential losses inherent in
recovering from a deadlock
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4. Single Instance of Each Resource Type
• If all resources have only a single instance, then we can define a deadlock detection
algorithm that uses a variant of the resource-allocation graph - Wait-for graph
• Graph obtained from the resource-allocation graph by removing the resource nodes
and collapsing the appropriate edge
• An edge from Pi to Pj in a wait-for graph implies that process Pi is waiting for
process Pj to release a resource that Pi needs
• An edge Pi → Pj exists in a wait-for graph if and only if the corresponding resource
allocation graph contains two edges Pi → Rq and Rq → Pj for some resource Rq
• A deadlock exists in the system if and only if the wait-for graph contains a cycle
• To detect deadlocks, the system needs to maintain the wait-for graph and periodically
invoke an algorithm that searches for a cycle in the graph
• 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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5. Single Instance of Each Resource Type
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fig a : Resource-Allocation Graph fig b: Corresponding wait-for graph

6. Several Instances of a Resource Type
• The wait-for graph scheme is not applicable to a resource-allocation system with
multiple instances of each 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 × m matrix defines the number of resources of each type
currently allocated to each process
• Request - An n × m matrix indicates the current request of each process. If
Request[i][j] equals k, then process Pi is requesting k more instances of resource
type Rj
• Algorithm requires an order of O(m x n2) operations to detect whether the system is
in deadlocked state
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7. Several Instances of a Resource Type - Deadlock Detection Algorithm
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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, 0<= i <=n, then the system is in deadlock state.
Moreover, if Finish[i] == false, then Pi is deadlocked

8. Example - Deadlock Detection Algorithm
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• Consider a system with five processes P0 through P4 and three resource types A, B,
and C. Resource type A has seven instances, resource type B has two instances, and
resource type C has six instances. Suppose that, at time T0, we have the following
resource-allocation state -
• System is not in a deadlocked state
Indeed, if we execute our algorithm,
we will find that the sequence
<P0, P2, P3, P1, P4> results in
Finish[i] == true for all i

9. • A= 7-7=0
• B=2-2=0
• C=6-6=0
• P0
Request<=work
0 0 0 <= 0 0 0 true
Work=work+allocation 0 0 0+0 1 0= 0 1 0 finish[0]=true
P0-false P1-false P2-false P3-false P4-false
0- true
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10. • P1
Request <=work
2 0 2 <= 0 1 0 false p1 has to wait
• P2
Request <=work
0 0 0 <= 0 1 0 true
work=work+allocation 0 1 0 + 3 0 3=3 1 3 finish[2]=true
• P3
Request <=work
1 0 0 <= 3 1 3 true
Work= work +allocation 3 1 3 + 2 1 1= 5 2 4 finish [3]= true
• P4
Request <=work
0 0 2 < = 5 2 4 true
Work=work+ allocation 5 2 4 + 0 0 2= 5 2 6 finish[4]=true
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11. • P1
Request <= work
2 0 2 <= 5 2 6
work= work + allocation 5 2 6+2 0 0= 7 2 6
• Safe sequence <p0,p2,p3,p4,p1>
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12. Example - Deadlock Detection Algorithm
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• Consider a system with five processes P0 through P4 and three resource types A, B,
and C. Resource type A has seven instances, resource type B has two instances, and
resource type C has six instances. Suppose that, at time T0, we have the following
resource-allocation state -
• System is in a deadlocked state
ALthough, reclaiming the resources
held by process P0, the number of
available resources is not sufficient
to fulfill the requests of the other
processes.
• Thus, a deadlock exists, consisting
of processes P1, P2, P3, and P4

13. Detection - Algorithm Usage
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• When, and how often, to invoke depends on:
• How often a deadlock is likely to occur?
• How many processes will be affected by deadlock when it happens?
• We can invoke the deadlock-detection algorithm every time a request for allocation
cannot be granted immediately
• Invoking the deadlock-detection algorithm for every resource request will incur
considerable overhead in computation time
• A less expensive alternative is simply to invoke the algorithm at defined intervals—
for example, once per hour or whenever CPU utilization drops below 40 percent. (A
deadlock eventually cripples system throughput and causes CPU utilization to drop)
• If the detection algorithm is invoked at arbitrary points in time, the resource graph
may contain many cycles. In this case, we generally cannot tell which of the many
deadlocked processes “caused” the deadlock.

14. Recovery from Deadlock
• When a detection algorithm determines that a deadlock exists, several alternatives
are available.
1. Inform the operator that a deadlock has occurred and to let the operator deal
with the deadlock manually
2. Let the system recover from the deadlock automatically
• Two options for breaking a deadlock -
• Process termination
a. abort all deadlocked process
Example - Assume 10 process in that 3 process suffer deadlock they have to be
aborted
P0=95% P1=90%, P2=88% execution completed
it has used CPU time, memory, I/O devices
it is expensive approach
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15. b. To abort one process at a time until deadlock cycle is eliminated i.e circular
wait
- p0,p1,p2 suffer from deadlock
- abort p0 and apply deadlock detection algorithm to find cycle is
eliminated
- similarly apply for p1,p2
• In which order should we choose to abort the process from many processes
1. Priority of the process
2. How long process has computed, and how much longer to completion
3. Resources the process has used
4. Resources process needs to complete
5. How many processes will need to be terminated
6. Is process interactive or batch?
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Recovery from Deadlock

16. Recovery from Deadlock
• Two options for breaking a deadlock -
• Resource preemption
• To eliminate deadlocks using resource preemption, we successively preempt
some resources from processes and give these resources to other processes
until the deadlock cycle is broken
• If preemption is required to deal with deadlocks, then three issues need to
be addressed
1. Selecting a victim – i.e., Which resources and which processes are to be
preempted?
• As in process termination, we must determine the order of preemption to
minimize cost
• Cost factors may include such parameters as the number of resources a
deadlocked process is holding and the amount of time the process has thus far
consumed
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17. Recovery from Deadlock
2. Rollback - If we pre-empt a resource from a process, what should be done with that
process?
• Clearly, it cannot continue with its normal execution; it is missing some needed
resource. We must roll back the process to some safe state and restart it from
that state
3. Starvation -
• How do we ensure that starvation will not occur? That is, how can we guarantee
that resources will not always be pre-empted from the same process?
• Ensure that a process can be picked as a victim only a (small) finite number of
times
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References
1. Silberscatnz and Galvin, “Operating System Concepts,” Ninth Edition,
John Wiley and Sons, 2012.

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