Deadlocks in Operating Systems
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Questions and Answers

What is deadlock?

Deadlock occurs when two or more processes are waiting indefinitely for an event that can be caused by only one of the waiting processes.

What are the four necessary conditions for deadlock to occur?

The four necessary conditions for deadlock are: mutual exclusion, hold and wait, no preemption, and circular wait.

Which of these options are not a necessary condition for deadlock? (Select all that apply)

  • Resource preemption (correct)
  • Mutual Exclusion
  • Circular Wait
  • No preemption (correct)
  • Hold and Wait

    • A ______ is a directed graph used to describe deadlocks more precisely.

    <p>system resource-allocation graph</p> Signup and view all the answers

    Which edge represents a request for a resource in a resource-allocation graph?

    <p>Request edge</p> Signup and view all the answers

    A system is in a safe state if a deadlock is possible.

    <p>False</p> Signup and view all the answers

    Which of these approaches can ensure that the system will never enter a deadlocked state?

    <p>Deadlock prevention</p> Signup and view all the answers

    What is the main difference between deadlock prevention and deadlock avoidance?

    <p>Deadlock prevention aims to prevent deadlocks by constraining resource access, while deadlock avoidance allows for resource requests but ensures that the system stays in a safe state.</p> Signup and view all the answers

    What is the Banker's Algorithm and how does it work?

    <p>The Banker's algorithm is a deadlock avoidance algorithm used when resources have multiple instances. It works by dynamically examining the resource-allocation state to ensure that there can never be a circular-wait condition, based on maximum resource needs and availability.</p> Signup and view all the answers

    What is meant by a priori information in the context of deadlock avoidance?

    <p>In deadlock avoidance, <code>a priori</code> information refers to the knowledge of the maximum resource needs of each thread, which the operating system uses to decide whether to grant or delay resource requests.</p> Signup and view all the answers

    The Detection Algorithm is a proactive approach to dealing with deadlocks.

    <p>False</p> Signup and view all the answers

    What is the purpose of a Waif-for Graph in deadlock detection?

    <p>Representing the dependencies between threads waiting for resources</p> Signup and view all the answers

    What are the two primary methods for recovering from deadlock?

    <p>The two main methods for recovery from deadlock are process termination (aborting processes) and resource preemption (taking resources away from processes).</p> Signup and view all the answers

    Which of these recovery methods is typically more expensive?

    <p>Process termination</p> Signup and view all the answers

    What is the main challenge associated with using rollback as a recovery method from deadlock?

    <p>A significant challenge with rollback is determining a safe state to which the system can rewind. Often, the simplest solution is a total rollback, potentially involving substantial loss of progress.</p> Signup and view all the answers

    Signup and view all the answers

    Study Notes

    Deadlocks

    • Deadlocks occur when two or more processes are waiting indefinitely for an event that can only be caused by one of the waiting processes
    • Four necessary conditions for deadlocks:
      • Mutual exclusion: only one thread at a time can use a resource
      • Hold and wait: a thread holding at least one resource is waiting to acquire additional resources held by other threads
      • No preemption: a resource can only be released voluntarily by the thread holding it, typically after it completes its task
      • Circular wait: there exists a set of waiting threads such that each thread is waiting for a resource held by the next thread in the set, with the final thread waiting for a resource held by the first thread

    Methods for Handling Deadlocks

    • Three ways to handle deadlocks:
      • Prevention: Use protocols to prevent the system from entering a deadlock state.
      • Avoidance: The operating system is given prior information to decide if a request can be fulfilled immediately or needs to be delayed. This information includes the currently available resources, current allocations to every thread, and future request/release patterns.
      • Detection and Recovery: The system enters a deadlock state and then the operating system detects it and recovers from the deadlock. This method needs deadlock detection and recovery algorithms

    Deadlock Prevention

    • Mutual Exclusion: Ensure that at least one resource must be sharable (convert resources to sharable if possible). Read-only resources that don't require exclusive access cannot be involved in a deadlock. Mutex locks are an example of non-sharable resource.
    • Hold and Wait: Threads must request all necessary resources before execution or release all held resources before requesting any new resource.
    • No Preemption: Resources cannot be preempted from one thread to another unless the thread voluntarily releases them.
    • Circular Wait: Impose a total ordering of all resource types and require threads requesting them to obey that ordering. Assign unique numbers to resources to impose a consistent order for resource requests.

    Deadlock Avoidance

    • System has a priori information available, to decide if a request can be immediately granted or need delay
    • The required information includes:
      • Number of available resources
      • Number of allocated resources to each thread
      • Maximum demand of each thread

    Safe State

    • A safe state is a state where the system can allocate resources to each thread (up to its maximum) in some order and still avoid a deadlock.
    • If a system is in a safe state, no deadlocks will occur.
    • If a system is in an unsafe state, a deadlock is possible.
    • Deadlock avoidance algorithms aim to ensure that the system never enters an unsafe state.

    Banker's Algorithm

    • Used to avoid deadlocks in systems with multiple instances of a resource type.
    • Each thread must declare its maximum resource needs in advance.
    • The algorithm checks if a request can be safely granted without leading to a deadlock.

    Detection Algorithm

    • Used to detect a deadlock in systems that do not employ deadlock prevention or avoidance algorithms.
    • The system must periodically invoke a cycle-detection algorithm.
    • If a cycle (deadlock) is detected, the system must recover.
    • O(m × n²) operations are needed to detect the deadlock condition
      • m is the number of resource types
      • n is the number of threads

    Recovery from Deadlock

    • Termination: The method of recovering from a deadlock by terminating deadlocked threads.

    • Preemption: The method of recovering from a deadlock by preempting resources held by deadlocked threads.

      • Selecting a victim: To minimize the cost, the order of preempting resources should be determined.

    • Rollback: Returning the system to a safe state. Restart threads from a previous safe state.

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    Chapter 8: Deadlocks PDF

    Description

    This quiz explores the concept of deadlocks in operating systems, discussing their conditions and methods for handling them. You'll learn about mutual exclusion, hold and wait, no preemption, and circular wait, as well as strategies like prevention and avoidance. Test your understanding of these critical concepts that affect system performance.

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