A deadlock in computer science refers to a situation in multithreaded or multi-process systems where two or more processes are blocked forever, waiting for each other to release resources, causing the system to come to a standstill. In a deadlock, none of the processes can proceed because they are each waiting for the other to release resources they need.
Example of Deadlock:
Imagine two processes, P1 and P2, both needing two resources, R1 and R2, to complete their tasks:
- P1 acquires R1 and waits for R2.
- P2 acquires R2 and waits for R1.
At this point, neither process can proceed because:
- P1 is waiting for R2, which is held by P2.
- P2 is waiting for R1, which is held by P1.
This creates a deadlock.
Conditions for Deadlock:
- Resources involved are non-shareable, meaning only one process can hold a resource at a time.
- A process is holding at least one resource and is waiting for additional resources that are currently held by other processes.
- Resources cannot be forcibly removed from the processes holding them; they must be released voluntarily.
- A circular chain of processes exists where each process is waiting for a resource that the next process in the chain holds.
Features of Deadlock:
- Processes compete for limited resources, which can result in a deadlock if the resources are allocated in a circular wait.
- Once a deadlock occurs, affected processes are blocked indefinitely until the situation is resolved.
- Deadlocks can degrade system performance or cause a complete halt in critical systems if not detected or resolved.
Advantages of Deadlock (from a theoretical perspective):
- In certain controlled environments (like a simulation or academic system), deadlocks can be useful for testing the system’s robustness and understanding resource allocation algorithms.
- Analyzing deadlocks may provide insights into flaws or inefficiencies in the resource management system, which can help improve system design.
How to Avoid or Prevent Deadlock:
- Prevention:
- If possible, design the system so that resources can be shared.
- Require that processes request all needed resources at once rather than holding some resources and requesting more.
- Allow preemption, meaning resources can be forcibly taken from processes that are holding them.
- Impose an ordering on resource allocation, where processes can only request resources in a predefined sequence, preventing circular wait conditions.
- Avoidance:
- This algorithm ensures that resources are allocated in such a way that the system remains in a “safe state,” meaning no deadlock is possible. It checks whether granting a resource request will leave the system in a safe or unsafe state.
- In this method, a directed graph is used to represent resource allocation. The system checks if granting a request will lead to a cycle in the graph, which would indicate a potential deadlock.
- Detection and Recovery:
- Continuously monitor the system to detect when a deadlock occurs. Once detected, the system can take action to resolve it.
- Once a deadlock is detected, recover by terminating processes involved in the deadlock or by forcibly preempting resources from some processes.
Advantages of Deadlock Handling Mechanisms:
- Prevention and Avoidance:
- Prevents deadlock from occurring in the first place, ensuring smooth operation.
- Deadlock avoidance techniques (like the Banker’s Algorithm) ensure that resource allocation will not lead to an unsafe state.
- Detection and Recovery:
- These methods allow deadlocks to be detected dynamically and can be used when it’s not feasible to completely prevent them.
- By detecting and resolving deadlocks after they occur, systems can continue to run even if a deadlock condition is temporarily present.
