Operating System Concepts: Deadlock Avoidance

Deadlock Avoidance

• Unique numbers assigned to resources (e.g., mutex locks)
• Resources should be acquired in a specific order to prevent deadlocks
• Threads declare maximum resources they may need beforehand
• The algorithm checks the resource-allocation state to avoid circular-wait scenarios

Safe State Definition

• A system is in a safe state if a sequence of all threads allows each thread Ti to be satisfied by currently available resources and resources held by threads Tj (where j < i)
• If required resources for Ti are unavailable, Ti can wait until other threads finish

Basic Facts on Safety

• Safe state guarantees no deadlocks
• Unsafe state implies potential for deadlocks
• To avoid deadlocks, resources are tentatively allocated to threads, and their safety is reassessed

Example of Banker's Algorithm

• System has 5 threads (T0 to T4) and 3 resource types (A, B, C) with specified instances
• Snapshot details show resource Allocation, Max need, and Availability
• Need matrix calculates remaining resources for each thread (Max - Allocation)
• Safety criteria check shows sequence < T1, T3, T4, T2, T0> is safe

Safety Request Process

• When a thread makes a request (e.g., (1,0,2)), it must be checked against currently available resources
• If the request is legitimate, a safety algorithm assesses the new allocation sequence
• Example shows that T4's request for (3,3,0) can't be granted immediately if it leads to any unsafe situation

Avoidance Algorithms

• Single instance of resource type managed via resource-allocation graph
• Multiple resource types handled using Banker's Algorithm

Resource-Allocation Graph Scheme

• Edges represent resource claims, requests, and assignments between threads and resources
• Claim edge signifies a thread's potential to request a resource
• Request edges convert to assignment edges upon resource allocation
• Resources must be claimed in advance by threads

Handling Unsafe States in Graphs

• Resource allocation graph visually represents current allocations and requests
• Requests only granted if they do not create a cycle in the resource allocation graph
• Cycle creation would indicate the potential for deadlock

Banker’s Algorithm Summary

• Applies to multiple resource instances
• Threads must declare maximum necessary resources beforehand
• Resource requests can lead to waiting if conditions for safe allocation are not satisfied
• Threads must return resources within a specified timeframe after completion

Deadlock Avoidance

• Assign unique numbers to resources like mutex locks.
• Resources must be acquired in ascending order to prevent deadlocks.
• Threads need prior knowledge of maximum resources to declare availability.

Safe State

• A system is safe if it can allocate resources without leading to deadlocks.
• For a safe state, a sequence must exist for all threads where requests can be satisfied with available resources plus those held by previous threads.
• A thread can wait for resources until its preceding threads finish executing.

Conditions of State

• Safe state guarantees no deadlocks.
• Unsafe state indicates potential for deadlock.
• Avoidance measures prevent the system from entering an unsafe state.

Avoidance Algorithms

• Use a resource-allocation graph for a single resource type.
• Employ the Banker’s Algorithm for multiple resource types.

Resource-Allocation Graph

• Claim edge indicates a potential resource request; represented by a dashed line.
• Transform a claim edge to a request edge when a resource is requested.
• An assignment edge represents the allocation of resources.
• Releasing resources reverts assignment edges back to claim edges.

Banker’s Algorithm

• Designed for managing multiple instances of resources.
• Each thread must declare maximum resource use upfront.
• Threas may have to wait for resource allocation depending on current availability.

Data Structures for Banker’s Algorithm

• Utilizes matrices for tracking processes, available resources, and needs.
• Adjusts available, allocation, and need matrices upon resource requests.
• If the system remains safe, resources are allocated; if unsafe, the previous state is restored.

Example of Banker’s Algorithm

• Five threads (T0 to T4) with three resource types (A, B, C).
• Snapshot at time T0 includes allocations, maximum requirements, and available resources.

Need Matrix Calculation

• Need matrix is defined as Max - Allocation, representing remaining resource requests.

Safety Check Example

• The sequence must satisfy the safety requirement for a safe state.
• If a thread requests resources, the safety algorithm must confirm that the resulting state remains safe.

This quiz covers concepts from the 10th edition of Operating System Concepts by Silberschatz, Galvin, and Gagne, focusing on deadlock avoidance strategies. It discusses resource allocation and the importance of acquiring resources in a specific order to prevent deadlocks. Test your understanding of these critical operating system principles.