Operating Systems - CS604 Power Point Slides Lecture 29

Operating

Systems Lecture 29

Syed Mansoor Sarwar

January 27, 2016 © Copyright Virtual University of Pakistan

Agenda for TodayReview of previous lectureDeadlock detection: resources with

single and multiple instancesRecovery from deadlocks

Process terminationResource preemption

Recap of lecture


Review of Lecture 28Deadlock avoidanceResource with single

instancesResources with multiple

instances: Banker’s algorithm


Deadlock DetectionAllow system to enter

deadlock state

Detection algorithm

Recovery scheme


Single Instance of Each Resource Type

Maintain a wait-for graph

Nodes are processes.

Pi Pj if Pi is waiting for Pj.


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.


RAG and Wait-for Graph

Resource-Allocation Graph Corresponding wait-for graph


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.

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.

Multiple Instance of Each Resource Type


Detection Algorithm1. 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;elseFinish[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.

Detection Algorithm


3. Work = Work + Allocationi

Finish[i] = truego 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.

Detection Algorithm


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


Finish Sequence: <>

Process

P0

P1

P2

P3

P4

Work

A B C

0 0 0

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 0

1 0 0

0 0 2

Example System State


Finish Sequence: <P0>

Process

P0

P1

P2

P3

P4

Work

A B C

0 0 0

0 1 0

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 0

1 0 0

0 0 2



Finish Sequence: <P0, P2>

Process

P0

P1

P2

P3

P4

Work

A B C

0 0 0

0 1 0

3 1 2

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 0

1 0 0

0 0 2



Finish Sequence: < P0, P2, P3>

Process

P0

P1

P2

P3

P4

Work

A B C

0 0 0

0 1 0

3 1 2

5 2 3

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 0

1 0 0

0 0 2



Finish Sequence: < P0, P2, P3, P4 >

Process

P0

P1

P2

P3

P4

Work

A B C

0 0 0

0 1 0

3 1 2

5 2 3

5 2 5

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 0

1 0 0

0 0 2



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 >…



P2 requests an additional instance of C.

Do we have a

finish sequence?


Request

A B C

0 0 0

2 0 2

0 0 1

1 0 0

0 0 2

Process

P0

P1

P2

P3

P4


Finish Sequence: <>

Process

P0

P1

P2

P3

P4

Work

A B C

0 0 0

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



Finish Sequence: <P0>

Process

P0

P1

P2

P3

P4

Work

A B C

0 0 0

0 1 0

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



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.



Detection AlgorithmHow 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


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.

Detection Algorithm


Recovery from Deadlock: 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 processes?

Priority of a process.

How long the process has computed, and how much longer to completion.



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?



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.

Recovery from Deadlock: Resource Preemption


Recap of Lecture

Deadlock detection Recovery from deadlock

Process termination Resource preemption


Operating Systems

Lecture 29

Syed Mansoor Sarwar

Operating Systems - CS604 Power Point Slides Lecture 29

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