deadlockavoidance

Deadlock avoidance is a class of techniques in concurrent computing that prevent deadlocks by ensuring the system only enters safe states. A deadlock occurs when a set of processes are each waiting for resources held by others, creating a cycle of dependencies and no progress. Avoidance requires knowledge or estimation of future resource requests and uses policies to grant resources only when the resulting state can still proceed.

Approaches include enforcing a resource allocation policy such as a fixed ordering of resource types so that requests are made in a monotonic order, or using algorithms that simulate allocations to test safety before granting resources. In multi-resource environments, the Banker's algorithm is a classic method that grants requests only if the resulting state is safe.

The Banker's algorithm maintains maximum claims and current allocations for each process and checks for a safe sequence of future allocations. If granting a request would lead to an unsafe state, the request is delayed. Resource ordering prevents circular wait by restricting hold-and-wait on certain resource combinations, thereby reducing the chances of deadlock.

Deadlock avoidance trades potential throughput or latency for safety and may increase scheduling complexity. It complements deadlock prevention (which imposes constraints to rule out deadlock-prone conditions) and deadlock detection (which allows deadlocks to occur but provides recovery). It is commonly applied in systems with known resource usage patterns, such as database management systems and certain real-time environments.

See also: resource allocation graph, safe state, Banker's algorithm, deadlock prevention, deadlock detection, circular wait.