Home Definition Understanding Deadlocks: Causes & Resolution

Understanding Deadlocks: Causes & Resolution

by Marcin Wieclaw
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what is a deadlock

In the realm of operating systems, deadlocks are a critical issue that can bring a system to a standstill. A deadlock occurs when two or more processes are unable to proceed because each is waiting for the other to release a resource. This article explores the concept of deadlocks, their causes, prevention strategies, and resolution techniques.

Deadlocks are system deadlock situations that can disrupt the normal functioning of operating systems. They can cause delays and even system failures, leading to significant impacts on productivity and user experience. Understanding the causes and having effective resolution strategies in place is crucial for maintaining system stability.

What is a Deadlock?

In the realm of operating systems, deadlocks can be detrimental to system stability. But what exactly is a deadlock? In simple terms, a deadlock is a state in a computing system where each member of a set of processes is waiting for an event that will never occur. This often involves the allocation of resources within the operating system.

To fully understand a deadlock, it is essential to be familiar with its necessary conditions. Four conditions must exist simultaneously for a deadlock to occur:

  1. Mutual Exclusion: Processes must hold exclusive control of the resources they need, meaning only one process can use a resource at any given time.
  2. Hold and Wait: Processes already holding resources can request additional resources while still retaining the ones they currently possess.
  3. No Preemption: Resources cannot be forcibly taken away from a process; they can only be released voluntarily.
  4. Circular Wait: A circular chain of processes exists, where each process holds a resource that the next process in the chain requires.

When all of these conditions are met, a deadlock can occur, leading to system inefficiency and potential crashes.

The image above visually depicts the concept of a deadlock and its relation to resource allocation within operating systems.

Causes of Deadlocks

Deadlocks can arise due to various reasons, such as poor resource management, incorrect synchronization mechanisms, or insufficient system resources. These causes can lead to the system becoming unresponsive and unable to proceed with the execution of processes. Understanding the underlying factors that contribute to deadlocks is essential in devising effective prevention and resolution strategies.

“Limited resources, incorrect resource allocation, and inadequate synchronization are common culprits behind deadlocks.”

One of the primary causes of deadlocks is limited resources. When multiple processes compete for a finite amount of resources, such as memory or disk space, deadlock situations can occur. In such cases, processes may enter a state of waiting indefinitely, unable to proceed due to the unavailability of the required resources.

Incorrect resource allocation is another significant cause of deadlocks. When resources are not allocated properly, and processes hold onto resources while waiting for others, a deadlock can arise. This occurs due to the hold and wait condition, where a process may hold resources it has already acquired, preventing other processes from accessing them.

Inadequate synchronization mechanisms can also contribute to the occurrence of deadlocks. If processes are not synchronized correctly when accessing shared resources, conflicts can arise, leading to potential deadlocks. For example, if two processes concurrently attempt to modify the same data, a deadlock may occur as each process waits for the other to release the resource.

Identifying and understanding these causes of deadlocks is crucial in developing robust systems that can effectively manage resource allocation and synchronization. By addressing these issues, system designers and developers can reduce the likelihood of deadlocks and enhance the overall reliability and performance of operating systems.

causes of deadlocks

Cause Description
Limited resources Competition for scarce resources leads to deadlock situations.
Incorrect resource allocation Improper allocation of resources can result in processes holding onto resources while waiting for others, causing deadlocks.
Inadequate synchronization Insufficient coordination among processes when accessing shared resources can lead to conflicts and potential deadlocks.

Prevention Strategies

To prevent deadlocks, several strategies can be implemented. These strategies involve the use of resource allocation graph, banker’s algorithm, and timeouts.

Resource Allocation Graph

The resource allocation graph (RAG) is a graphical representation that helps identify and avoid deadlocks by tracking the allocation and request of resources by processes. It provides a visual representation of the resource allocation status and can highlight potential deadlock situations.

Banker’s Algorithm

The banker’s algorithm is a deadlock avoidance algorithm that ensures the system remains in a safe state. It considers the available resources and the maximum demands of processes before allocating resources. By carefully managing the allocation of resources, the banker’s algorithm can prevent deadlocks from occurring.


Implementing timeouts for resource requests can also prevent deadlocks. If a process fails to acquire the necessary resources within a specified time, the timeout mechanism releases the resources held by that process. This ensures that other processes can obtain the resources they need, reducing the likelihood of deadlocks.

By utilizing the resource allocation graph, banker’s algorithm, and timeouts, system administrators and developers can effectively prevent deadlocks and ensure the smooth operation of computer systems.

Prevention Strategies Description
Resource Allocation Graph A graphical representation that tracks the allocation and request of resources by processes.
Banker’s Algorithm An algorithm that considers available resources and maximum demands of processes for safe resource allocation.
Timeouts An implementation that releases resources held by a process if it fails to acquire necessary resources within a specified time.

Resolution Techniques

When a deadlock is detected, there are two common resolution techniques that can be employed: process termination and resource preemption. These approaches aim to break the deadlock by either releasing resources or reallocating them to different processes.

Process termination:

If a deadlock is identified, one or more processes involved in the deadlock can be terminated. By terminating these processes, the resources they were holding can be released and made available for other processes to use. It’s important to note that process termination should be used judiciously, as it can lead to data corruption or loss if not handled properly.

Resource preemption:

An alternative technique to resolve deadlocks is resource preemption. In this approach, resources are preempted or taken away from one process and allocated to another. By preempting resources from one process, the deadlock can be broken, allowing the system to continue executing. Similarly to process termination, resource preemption should be used with caution to ensure data integrity and prevent adverse effects on the system.


“Resource preemption is like taking away a toy from a child and giving it to another. By reallocating the resources, we can avoid a deadlock and ensure the system keeps running smoothly.” – Jane Henderson, System Administrator

While process termination and resource preemption can be effective in resolving deadlocks, it’s essential to carefully analyze the situation and evaluate the potential impact on the system. These techniques should be implemented in conjunction with other deadlock prevention strategies to minimize the occurrence of deadlocks and maintain system stability.

Resolution Technique Advantages Disadvantages
Process Termination
  • Quickly releases resources
  • Potential to resolve deadlock immediately
  • Possible data corruption or loss
  • May disrupt ongoing processes
  • Requires careful handling
Resource Preemption
  • Allows system to continue execution
  • Can break the deadlock and maintain resource utilization
  • Possible resource contention
  • Potential impact on the preempted process
  • Requires careful resource management

By employing these resolution techniques, system administrators and developers can effectively address deadlocks and ensure the smooth operation of their systems.

Monitoring and Detection

Utilizing the system_health session

To effectively monitor and detect deadlocks in SQL Server, one valuable tool is the system_health session. This session serves as a comprehensive system health tracking mechanism, capturing vital information about the system’s performance, including valuable deadlock data.

The system_health session enables administrators to gain insight into the occurrence of deadlocks within the SQL Server environment. By capturing deadlock graphs, it provides a visual representation of the deadlock scenario, highlighting the relationships between processes and the resources they contend for.

Implementing the system_health session allows for ongoing monitoring of deadlock events and provides administrators with the necessary information to analyze and diagnose issues that may arise. By identifying the processes involved and the specific resources in contention, administrators can take appropriate steps to resolve deadlocks swiftly and effectively.

Extended Events for Detailed Analysis

In addition to the system_health session, extended events prove to be a powerful tool for capturing and analyzing deadlock events in SQL Server. Extended events provide a more granular level of detail, offering a deeper understanding of the deadlock scenarios.

With extended events, administrators can configure event sessions tailored to their specific monitoring requirements, capturing not only deadlock graphs but also additional information such as the statement text that triggered the deadlock and the application or user responsible for it. This level of detailed analysis can greatly assist in troubleshooting and resolving deadlock issues.

By leveraging extended events alongside the system_health session, administrators can gain comprehensive insight into the occurrence and characteristics of deadlocks within their SQL Server environment. Armed with this information, they can devise appropriate strategies and implement effective resolutions to mitigate the impact of deadlocks on system performance and stability.


Deadlocks pose complex challenges that can have a significant impact on the performance and reliability of operating systems. To ensure system stability, it is crucial for operating system designers and developers to have a thorough understanding of the conditions that lead to deadlocks and to implement effective prevention strategies and resolution techniques.

By identifying the causes of deadlocks, such as limited resources, incorrect resource allocation, and inadequate synchronization, designers can devise robust solutions that minimize the occurrence of these issues. Prevention measures like the resource allocation graph and the banker’s algorithm can help prevent deadlocks by carefully managing resource allocation and ensuring the system remains in a safe state.

In the event that a deadlock does occur, effective resolution techniques such as process termination and resource preemption can be employed. However, it is important to exercise caution when employing these techniques to avoid potential data corruption or loss.

In summary, maintaining system stability requires a comprehensive approach that includes understanding, prevention, and resolution of deadlocks. Operating system designers and developers play a vital role in creating systems that proactively address deadlock situations and provide mechanisms for recovery, ensuring the smooth and reliable operation of the system.


What is a deadlock?

A deadlock is a state in a computing system where each member of a set of processes is waiting for an event that will never occur. In the context of operating systems, this often involves the allocation of resources.

What are the causes of deadlocks?

Deadlocks can arise due to various reasons, such as poor resource management, incorrect synchronization mechanisms, or insufficient system resources. Some common causes include limited resources, incorrect resource allocation, and inadequate synchronization.

How can deadlocks be prevented?

Several strategies can be implemented to prevent deadlocks. These include using the resource allocation graph (RAG) to identify and avoid deadlocks, implementing the banker’s algorithm to ensure a safe state, and implementing timeouts for resource requests.

What are the resolution techniques for deadlocks?

If a deadlock is detected, one or more processes involved in the deadlock can be terminated to release resources. Another technique is resource preemption, where resources are preempted from one process and allocated to another to break the deadlock.

How can deadlocks be monitored and detected in SQL Server?

Deadlocks in SQL Server can be monitored and detected using the system_health session, which captures deadlock information. Extended events can also be used to capture deadlock events and provide more detailed information, including the deadlock graph.


  • Marcin Wieclaw

    Marcin Wieclaw, the founder and administrator of PC Site since 2019, is a dedicated technology writer and enthusiast. With a passion for the latest developments in the tech world, Marcin has crafted PC Site into a trusted resource for technology insights. His expertise and commitment to demystifying complex technology topics have made the website a favored destination for both tech aficionados and professionals seeking to stay informed.

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