Deadlock Avoidance

In complex systems involving multiple processes and shared resources, the potential for deadlocks arises when processes wait for each other to release resources, causing a standstill. The resulting deadlocks can cause severe issues in computer systems, such as performance degradation and even system crashes. To prevent such problems, the technique of deadlock avoidance is employed. It entails scrutinizing the requests made by processes for resources and evaluating the available resources to determine if the grant of such requests would lead to a deadlock. In cases where granting a request would result in a deadlock, the system denies the request. Deadlock avoidance is a crucial aspect of operating system design and plays an indispensable role in upholding the dependability and steadiness of computer systems.

Safe State and Unsafe State

A safe state refers to a system state where the allocation of resources to each process ensures the avoidance of deadlock. The successful execution of all processes is achievable, and the likelihood of a deadlock is low. The system attains a safe state when a suitable sequence of resource allocation enables the successful completion of all processes.

Conversely, an unsafe state implies a system state where a deadlock may occur. The successful completion of all processes is not assured, and the risk of deadlock is high. The system is insecure when no sequence of resource allocation ensures the successful execution of all processes.

Deadlock Avoidance Algorithms

  • When resource categories have only single instances of their resources, Resource- Allocation Graph Algorithm is used. In this algorithm, a cycle is a necessary and sufficient condition for deadlock.

  • When resource categories have multiple instances of their resources, Banker’s Algorithm is used. In this algorithm, a cycle is a necessary but not a sufficient condition for deadlock.

Resource-Allocation Graph Algorithm

Resource Allocation Graph (RAG) is a popular technique used for deadlock avoidance. It is a directed graph that represents the processes in the system, the resources available, and the relationships between them. A process node in the RAG has two types of edges, request edges, and assignment edges. A request edge represents a request by a process for a resource, while an assignment edge represents the assignment of a resource to a process.

To determine whether the system is in a safe state or not, the RAG is analyzed to check for cycles. If there is a cycle in the graph, it means that the system is in an unsafe state, and granting a resource request can lead to a deadlock. In contrast, if there are no cycles in the graph, it means that the system is in a safe state, and resource allocation can proceed

without causing a deadlock.

The RAG technique is straightforward to implement and provides a clear visual representation of the processes and resources in the system. It is also an effective way to identify the cause of a deadlock if one occurs. However, one of the main limitations of the RAG technique is that it assumes that all resources in the system are allocated at the start of the analysis. This assumption can be unrealistic in practice, where resource allocation can change dynamically during system operation. Therefore, other techniques such as the Banker's Algorithm are used to overcome this limitation.

Banker’s Algorithm

The banker's algorithm is a deadlock avoidance algorithm used in operating systems. It was proposed by Edsger Dijkstra in 1965. The banker's algorithm works on the principle of ensuring that the system has enough resources to allocate to each process so that the system never enters a deadlock state. It works by keeping track of the total number of resources available in the system and the number of resources allocated to each process.

The algorithm is used to prevent deadlocks that can occur when multiple processes are competing for a finite set of resources. The resources can be of different types such as memory, CPU cycles, or I/O devices. It works by first analysing the current state of the system and determining if granting a resource request from a process will result in a safe state. A state is considered safe if there is at least one sequence of resource allocations that can satisfy all processes without causing a deadlock.

The Banker's algorithm assumes that each process declares its maximum resource requirements upfront. Based on this information, the algorithm allocates resources to each Resource-Allocation Graph process such that the total number of allocated resources never exceeds the total number of available resources. The algorithm does not grant access to resources that could potentially lead to a deadlock situation. The Banker's algorithm uses a matrix called the "allocation matrix" to keep track of the resources allocated to each process, and a "request matrix" to keep track of the resources requested by each process. It also uses a "need matrix" to represent the resources that each process still needs to complete its execution.

To determine if a request can be granted, the algorithm checks if there is enough available resources to satisfy the request, and then checks if granting the request will still result in a safe state. If the request can be granted safely, the algorithm grants the resources and updates the allocation matrix, request matrix, and need matrix accordingly. If the request cannot be granted safely, the process must wait until sufficient resources become available.

1. Initialize the system

  • Define the number of processes and resource types.

  • Define the total number of available resources for each resource type.

  • Create a matrix called the "allocation matrix" to represent the current resource allocation for each process.

  • Create a matrix called the "need matrix" to represent the remaining resource needs for each process.

2. Define a request

  • A process requests a certain number of resources of a particular type.

3. Check if the request can be granted

  • Check if the requested resources are available.

  • If the requested resources are not available, the process must wait.

  • If the requested resources are available, go to the next step.

4. Check if the system is in a safe state

  • Simulate the allocation of the requested resources to the process.

  • Check if this allocation results in a safe state, meaning there is a sequence of allocations that can satisfy all processes without leading to a deadlock.

  • If the state is safe, grant the request by updating the allocation matrix and the need matrix.

  • If the state is not safe, do not grant the request and let the process wait.

Release the Resources

  • When a process has finished its execution, releases its allocated resources by updating the allocation matrix and the need matrix.

The above steps are repeated for each resource request made by any process in the system. Overall, the Banker's algorithm is an effective way to avoid deadlocks in resource constrained systems by carefully managing resource allocations and predicting potential conflicts before they arise.


Deadlock avoidance is an important concept in operating system design that is used to prevent the occurrence of deadlocks, which can result in system crashes and reduced performance.
By using various techniques such as resource allocation graphs, and banker's algorithm, operating systems can ensure that resources are allocated in a way that prevents deadlocks from occurring. While deadlock avoidance can be an effective way to prevent deadlocks, it can also be costly in terms of system resources and may result in reduced system performance. As a result, it is important for operating system designers to carefully balance the benefits of deadlock avoidance with the costs and potential drawbacks.

Overall, deadlock avoidance is an essential aspect of modern operating system design that helps to ensure the stability and reliability of computer systems. By understanding the principles and techniques of deadlock avoidance, system administrators and developers can build more robust and resilient systems that can better withstand the challenges of modern computing environments.

Updated on: 04-Apr-2023

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