1. Lecture 29 Syed Mansoor Sarwar
Operating Systems
Lecture 29
Syed Mansoor Sarwar

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Agenda for Today
Review of previous lecture
Deadlock detection: resources with single and multiple instances
Recovery from deadlocks
Process termination
Resource preemption
Recap of lecture
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Review of Lecture 28
Deadlock avoidance
Resource with single instances
Resources with multiple instances: Banker’s algorithm
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Deadlock Detection
Allow system to enter deadlock state
Detection algorithm
Recovery scheme
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5. Single Instance of Each Resource Type
Maintain a wait-for graph
Nodes are processes.
Pi  Pj if Pi is waiting for Pj.
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6. Single Instance of Each Resource Type
Periodically invoke an algorithm that searches for a cycle in the wait-for graph.
The algorithm is expensive—it requires an order of n2 operations, where n is the number of vertices in the graph.
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7. RAG and Wait-for Graph Resource-Allocation Graph
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Resource-Allocation Graph
Corresponding wait-for graph

8. Multiple Instance of Each Resource Type
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.
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9. Multiple Instance of Each Resource Type
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. Pj.
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Detection Algorithm
Let Work and Finish be vectors of length m and n, respectively Initialize:
Work = Available;
for i = 1, 2, …, n
if Allocationi  0
then
Finish[i] = false;
else
Finish[i] = true;
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Detection Algorithm
Find an index i such that both:
(a) Finish[i] == false
(b) Requesti  Work
If no such i exists, go to step 4.
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Detection Algorithm
Work = Work + Allocationi Finish[i] = true go to step 2.
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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13. Example of Detection Algorithm
Consider the following system:
P = { P0, P1, P2, P3, P4 }
R = { A, B, C }
A: 7 instances
B: 2 instances
C: 6 instances
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Example System State
Process P0 P1 P2 P3 P4
Allocation
A B C
0 1 0
2 0 0
3 0 2
2 1 1
0 0 2
Request
A B C
0 0 0
2 0 2
1 0 0
0 0 2
Work A B C
Finish Sequence: <>
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Example System State
Process P0 P1 P2 P3 P4
Allocation
A B C
0 1 0
2 0 0
3 0 2
2 1 1
0 0 2
Request
A B C
0 0 0
2 0 2
1 0 0
0 0 2
Work A B C 1
Finish Sequence: <P0>
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Example System State
Process P0 P1 P2 P3 P4
Allocation
A B C
0 1 0
2 0 0
3 0 2
2 1 1
0 0 2
Request
A B C
0 0 0
2 0 2
1 0 0
0 0 2
Work A B C 1 3 2
Finish Sequence: <P0, P2>
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Example System State
Process P0 P1 P2 P3 P4
Allocation
A B C
0 1 0
2 0 0
3 0 2
2 1 1
0 0 2
Request
A B C
0 0 0
2 0 2
1 0 0
0 0 2
Work A B C 1 3 2 5
Finish Sequence: < P0, P2, P3>
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Example System State
Process P0 P1 P2 P3 P4
Allocation
A B C
0 1 0
2 0 0
3 0 2
2 1 1
0 0 2
Request
A B C
0 0 0
2 0 2
1 0 0
0 0 2
Work A B C 1 3 2 5
Finish Sequence: < P0, P2, P3, P4 >
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Example System State
Final finish sequence:
< P0, P2, P3, P4 , P1 >
Other possible finish sequences:
< P0, P2, P3, P1 , P4 >
< P0, P2, P4, P1 , P3 >
< P0, P2, P4, P3 , P1 > …
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Example System State
P2 requests an additional instance of C.
Do we have a
finish sequence?
Process P0 P1 P2 P3 P4
Request
A B C
0 0 0
2 0 2
0 0 1
1 0 0
0 0 2
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Example System State
Process P0 P1 P2 P3 P4
Allocation
A B C
0 1 0
2 0 0
3 0 2
2 1 1
0 0 2
Request
A B C
0 0 0
2 0 2
0 0 1
1 0 0
0 0 2
Work A B C
Finish Sequence: <>
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Example System State
Process P0 P1 P2 P3 P4
Allocation
A B C
0 1 0
2 0 0
3 0 2
2 1 1
0 0 2
Request
A B C
0 0 0
2 0 2
0 0 1
1 0 0
0 0 2
Work A B C 1
Finish Sequence: <P0>
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Example System State
P0’s request can be satisfied with currently available resources, but request for no other process can be satisfied after that.
 Deadlock exists, consisting of processes P1, P2, P3, and P4.
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Detection Algorithm
How often should the detection algorithm be invoked?
Every time a request for allocation cannot be granted immediately—expensive but process causing the deadlock is identified, along with processes involved in deadlock
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Detection Algorithm
Periodically, or based on CPU utilization
Arbitrarily—there may be many cycles in the resource graph and we would not be able to tell which of the many deadlocked processes “caused” the deadlock.
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26. Recovery from Deadlock: Process Termination
Abort all deadlocked processes.
Abort one process at a time until the deadlock cycle is eliminated.
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27. Recovery from Deadlock: Process Termination
In which order should we choose to abort processes?
Priority of a process.
How long the process has computed, and how much longer to completion.
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28. Recovery from Deadlock: Process Termination
Resources the process has used.
Resources the process needs to complete.
How many processes will need to be terminated.
Is the process interactive or batch?
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29. Recovery from Deadlock: Resource Preemption
Selecting a victim – minimize cost.
Rollback – return to some safe state, restart process from that state.
Starvation – same process may always be picked as victim; include number of rollbacks in cost factor.
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Recap of Lecture
Deadlock detection
Recovery from deadlock
Process termination
Resource preemption
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Operating Systems
Lecture 29
Syed Mansoor Sarwar
12 January 2019
© Copyright Virtual University of Pakistan