Introduction
Deadlock avoidance: dynamic technique for preventing deadlocks before they occur. Method: system evaluates resource requests, grants only if resulting state is safe. Objective: ensure system never enters deadlock state without restricting resource utilization excessively.
"Deadlock avoidance requires foresight and careful resource allocation to maintain system stability." -- Abraham Silberschatz
Deadlock Fundamentals
Definition
Deadlock: condition in concurrent systems where processes wait indefinitely for resources held by each other. Result: system halt or severe performance degradation.
Necessary Conditions
Mutual exclusion: resources non-shareable. Hold and wait: processes hold resources while waiting for others. No preemption: resources cannot be forcibly taken. Circular wait: circular chain of resource requests.
Resource Types
Preemptable: can be taken from process. Non-preemptable: must be released voluntarily. Examples: CPU cycles (preemptable), printers (non-preemptable).
Deadlock Avoidance Concept
Dynamic Allocation
System evaluates each resource request at runtime. Decision: grant or delay based on potential deadlock risk.
Safe State
State where system can allocate resources to all processes in some order without deadlock.
Unsafe State
State that may lead to deadlock if further resource requests occur. System avoids entering unsafe states.
Resource Allocation Graph
Graph Structure
Vertices: processes and resources. Edges: assignment (resource to process) and request (process to resource).
Cycle Detection
Cycle presence indicates potential deadlock in single instance resource models. In multiple instances, cycle alone insufficient.
Extension for Multiple Instances
Use wait-for graph or algorithms like Banker's to analyze state safety.
Safe and Unsafe States
Safe State Definition
Existence of safe sequence: order of process execution without deadlock.
Unsafe State Definition
No known safe sequence; risk of deadlock if resources allocated.
State Transition
System transitions between states upon resource allocation or release. Avoid transitions into unsafe states.
Banker's Algorithm
Overview
Algorithm by Dijkstra for deadlock avoidance. Uses process maximum claims, allocated resources, and available resources to check safety before allocation.
Input Data
Max matrix: maximum resource demands per process. Allocation matrix: current resource allocation. Available vector: currently available resources.
Purpose
Grant resource requests only if system remains in safe state after allocation.
Banker's Algorithm Details
Safety Algorithm
Determines safe state by simulating resource allocation and process completion.
Resource-Request Algorithm
Checks requested resources against need and availability, then applies safety algorithm.
Example
Processes P0-P4, resource types A,B,C with varying maximum claims and allocations.
Available = (3,3,2)Max = P0: (7,5,3) P1: (3,2,2) P2: (9,0,2) P3: (2,2,2) P4: (4,3,3)Allocation = P0: (0,1,0) P1: (2,0,0) P2: (3,0,2) P3: (2,1,1) P4: (0,0,2)Need = Max - AllocationCheck if request can be granted without unsafe state.| Process | Allocation | Max | Need |
|---|---|---|---|
| P0 | (0,1,0) | (7,5,3) | (7,4,3) |
| P1 | (2,0,0) | (3,2,2) | (1,2,2) |
| P2 | (3,0,2) | (9,0,2) | (6,0,0) |
| P3 | (2,1,1) | (2,2,2) | (0,1,1) |
| P4 | (0,0,2) | (4,3,3) | (4,3,1) |
Limitations and Overheads
Computational Overhead
Safety checks require time-consuming computations, especially with many processes/resources.
Resource Knowledge
Processes must declare maximum resource needs in advance; impractical for dynamic demands.
Utilization Constraints
May lead to reduced resource utilization due to conservative allocations.
Comparison with Other Techniques
Deadlock Prevention
Prevents deadlocks by negating necessary conditions, often restrictive and resource-wasteful.
Deadlock Detection and Recovery
Allows deadlocks to occur, then detects and recovers, incurs runtime penalties and complexity.
Deadlock Avoidance
Balances resource utilization and safety by dynamic state analysis, more flexible but computationally intensive.
Practical Implementation
Operating Systems
Rarely implemented fully in general-purpose OS due to overhead and resource estimation difficulty.
Real-Time Systems
More likely used where predictability critical; ensures deadlock-free resource allocation.
Embedded Systems
Simplified avoidance algorithms or static analysis preferred for constrained environments.
Deadlock Avoidance in Multithreading
Resource Requests
Threads request locks or resources dynamically; avoidance mechanisms analyze requests before granting.
Lock Ordering
Static lock hierarchy used to avoid circular wait, complementing avoidance algorithms.
Timeouts and Rollbacks
Used with avoidance to prevent indefinite waiting, detect unsafe states.
Summary
Deadlock avoidance: proactive resource allocation strategy. Ensures system remains in safe states by analyzing requests and system resource state. Banker's algorithm: canonical method. Limitations: overhead, need for advance knowledge. Application: critical in real-time and embedded domains.
References
- E. Silberschatz, P. B. Galvin, G. Gagne, Operating System Concepts, 9th ed., Wiley, 2013, pp. 260-290.