Circular Wait - TarisMajor/5143-OpSystems GitHub Wiki

circular-wait

Circular wait is one of the four necessary conditions for deadlock in a system. It refers to a situation where a set of processes are waiting for resources in a circular chain, such that each process is waiting for a resource held by the next process in the chain. This condition is critical to understand as it highlights a fundamental issue in resource allocation that can lead to system deadlock.

Key Characteristics of Circular Wait

  1. Circular Chain of Processes: In a circular wait, there is a closed chain of processes where each process holds at least one resource and is waiting to acquire a resource held by the next process in the chain.
  2. Dependency Cycle: The processes form a cycle of dependencies, where the completion of each process depends on the release of resources by another process within the same cycle.

Four Necessary Conditions for Deadlock

For a deadlock to occur, all four of the following conditions must be present simultaneously:

  1. Mutual Exclusion: At least one resource must be held in a non-shareable mode.
  2. Hold and Wait: Processes holding resources are also waiting for additional resources.
  3. No Preemption: Resources cannot be forcibly taken from a process holding them.
  4. Circular Wait: There exists a set of processes {P1, P2, ..., Pn} such that P1 is waiting for a resource held by P2, P2 is waiting for a resource held by P3, and so on, with Pn waiting for a resource held by P1.

Preventing Circular Wait

To prevent circular wait and, consequently, deadlocks, the system must ensure that the circular wait condition cannot hold. Several strategies can be employed to achieve this:

  1. Impose a Resource Ordering: Assign a unique numerical identifier to each resource type and require that processes request resources in an increasing order of these numbers. By doing this, the system can ensure that a circular wait cannot occur because processes can only request resources in a specific order.

  2. Avoid Holding Multiple Resources: Design the system so that processes do not hold multiple resources while waiting for others. This can be done by requiring processes to release all held resources before requesting new ones.

  3. Prevention Algorithms: Use deadlock prevention algorithms that dynamically check for cycles and either deny resource requests that would create a circular wait or preempt resources to break potential cycles.

Advantages of Circular Wait Prevention

  1. System Stability: Preventing circular wait ensures that the system remains stable and free from deadlocks.
  2. Efficiency: By avoiding deadlocks, the system can operate more efficiently without the need for complex deadlock detection and resolution mechanisms.
  3. Predictability: Ensuring that circular waits do not occur makes resource allocation more predictable and manageable.

Disadvantages of Circular Wait Prevention

  1. Resource Utilization: Imposing strict resource ordering can lead to suboptimal resource utilization, as processes may need to wait longer for resources to become available in the specified order.
  2. Implementation Complexity: Implementing circular wait prevention strategies requires careful planning and design, which can add complexity to the system.
  3. Performance Overhead: Continuously checking for potential circular waits and managing resource allocation can introduce performance overhead.

Use Cases for Circular Wait Prevention

  1. Operating Systems: Essential for operating systems managing multiple processes and resources to ensure smooth and uninterrupted process execution.
  2. Database Management Systems: Critical for database systems where transactions frequently lock resources, leading to potential deadlocks.
  3. Real-Time Systems: Important in real-time systems where resource availability and predictable resource allocation are crucial for meeting system deadlines.

Sources for Further Reading