Deadlock in Operating SystemSystem Model Deadlock Characterization

Deadlock in Operating System
By
Mrs.R.SABITHA .,M.Sc.,M.Phil.,
Assistant Professor,
Department of Computer Science(SF)
V.V.Vanniaperumal College for Women,
Virudhunagar.

Contents
 System Model
 Deadlock Characterization
 Methods for Handling Deadlocks
 Deadlock Prevention
 Deadlock Avoidance
 Deadlock Detection

Problem of Deadlock
• A set of blocked processes each holding a resource and waiting to
acquire a resource held by another process in the set.
• Example
– System has 2 disk drives.
– P1 and P2 each hold one disk drive and each needs another one.
• Example
– semaphores A and B, initialized to 1
P0 P1
wait (A); wait(B)
wait (B); wait(A)

Bridge Crossing Example
• Traffic only in one direction.
• Each section of a bridge can be viewed as a resource.
• If a deadlock occurs, it can be resolved if one car backs up (preempt
resources and rollback).
• Several cars may have to be backed up if a deadlock occurs.
• Starvation is possible.

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

Deadlock Characterization
Deadlock can arise if four conditions hold simultaneously.
• 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.

Resource-Allocation Graph
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

Resource-Allocation Graph
• Process
• Resource Type with 4 instances
• Pi requests instance of Rj
• Pi is holding an instance of Rj
Pi
Pi

Resource Allocation Graph With A Deadlock

Graph With A Cycle But No Deadlock

Basic Facts
• If graph contains no cycles  no deadlock.
• If graph contains a cycle 
– if only one instance per resource type, then
deadlock.
– if several instances per resource type, possibility of
deadlock.

Methods for Handling Deadlocks
• Deadlock Prevention
- Ensure that the system will never enter
a deadlock state.
Deadlock Avoidance
-Allow the system to enter a deadlock
state and then recover.
Deadlock Detection
-Ignore the problem and pretend that deadlocks never
occur in the system

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.

Safe State
• System is in safe state if there exists a sequence <P1, P2, …, Pn>
of ALL the processes is the systems such that 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.
• That is:
– 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.

Basic Facts
• If a system is in safe state  no deadlocks.
• If a system is in unsafe state  possibility of
deadlock.
• Avoidance  ensure that a system will never
enter an unsafe state

• 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
Banker’s Algorithm

Steps for Banker’s 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].

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 – 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

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:
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

Example
• 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.

Recovery from Deadlock: Process
Termination
• Abort all deadlocked processes.
• Abort one process at a time until the deadlock cycle is eliminated.
• Selecting a victim – minimize cost.
• Rollback – return to some safe state, restart process for that state.
• Starvation – same process may always be picked as victim,
include number of rollback in cost factor

Deadlock in Operating SystemSystem Model Deadlock Characterization

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