Deadlock Prevention in Operating Systems

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Questions and Answers

What approach is used in "mission critical" situations to prevent deadlocks?

  • Resource preemption
  • Circular wait prevention
  • Deadlock detection and recovery
  • Resource pre-allocation (correct)

Why is preemption, where allocated resources can be taken away, not always desirable?

  • It can lead to deadlocks.
  • It can be expensive to implement.
  • It can disrupt ongoing tasks, leading to user frustration. (correct)
  • It can be inefficient, as resources might not be used optimally.

How does assigning unique IDs to resources help prevent circular wait?

  • It ensures that all resources are used efficiently.
  • It allows processes to acquire resources in a predetermined order. (correct)
  • It allows for easy tracking of resource usage.
  • It prevents resources from being allocated to multiple processes simultaneously.

What is a potential issue with assigning unique IDs to resources for deadlock prevention?

<p>It can be difficult to assign IDs to all resources, especially dynamically created ones. (C)</p> Signup and view all the answers

Which of the following is NOT a common method for dealing with deadlocks?

<p>Deadlock allocation (C)</p> Signup and view all the answers

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

<p>Prevention is more efficient but less flexible, while avoidance is more flexible but less efficient. (D)</p> Signup and view all the answers

In the context of the cars-in-an-intersection problem, what could be interpreted as a resource?

<p>The intersection space (C)</p> Signup and view all the answers

Which of the following conditions is NOT required for a deadlock to occur?

<p>Preemption (B)</p> Signup and view all the answers

If a system begins in an unsafe state, what needs to happen before proceeding?

<p>More resources must be obtained. (B)</p> Signup and view all the answers

Which of the following data structures is NOT maintained by the Banker's algorithm?

<p>Process priority queue (A)</p> Signup and view all the answers

In the Banker's algorithm, a safe state refers to:

<p>A state where all processes can complete without blocking. (C)</p> Signup and view all the answers

What is the primary purpose of determining a safe state in the Banker's algorithm?

<p>To prevent deadlock by allocating resources safely. (B)</p> Signup and view all the answers

What does it mean for a process to 'declare' a resource claim in the context of the Banker's algorithm?

<p>The process has indicated the maximum amount of each resource it might need. (B)</p> Signup and view all the answers

Suppose Process P1 claims 3 units of R1, 2 units of R2, and 2 units of R3. What does this information tell us about P1?

<p>P1 may request up to 3 units of R1, 2 units of R2, and 2 units of R3. (A)</p> Signup and view all the answers

What is the significance of a resource availability vector in the Banker's algorithm?

<p>It indicates the units of each resource currently available in the system. (A)</p> Signup and view all the answers

When does the Banker's algorithm determine if a state is safe or unsafe?

<p>After every resource allocation. (B)</p> Signup and view all the answers

What is the main idea behind the deadlock detection and recovery approach?

<p>To accept the possibility of deadlocks and deal with them only when they happen. (C)</p> Signup and view all the answers

Why is the deadlock detection and recovery approach considered practical?

<p>Deadlocks are very rare, making complex prevention mechanisms less worthwhile. (A)</p> Signup and view all the answers

How are deadlocks detected in the system?

<p>By constructing and analyzing a resource allocation graph for cycles. (A)</p> Signup and view all the answers

What is a potential drawback of terminating all deadlocked processes as a recovery method?

<p>It can be too drastic and lead to data loss or inconsistencies. (D)</p> Signup and view all the answers

What is a potential concern when terminating deadlocked processes one at a time?

<p>It may require extensive analysis to determine the optimal termination order. (D)</p> Signup and view all the answers

What is a situation that could possibly lead to data inconsistency when recovering from a deadlock?

<p>A process is terminated while holding a resource that is not released. (D)</p> Signup and view all the answers

What is a key consideration regarding resource usage when recovering from a deadlock?

<p>Releasing resources held by terminated processes to avoid inconsistencies. (C)</p> Signup and view all the answers

What is the main principle underlying the decision to handle deadlocks in an ad-hoc way?

<p>Deadlocks are infrequent enough that a dedicated, complex approach is not warranted. (C)</p> Signup and view all the answers

Which of the following conditions is NOT essential for deadlock to occur?

<p>Preemption (A)</p> Signup and view all the answers

What is a potential consequence of choosing to implement a 'deadlock detection and recovery' strategy, instead of 'deadlock prevention' or 'deadlock avoidance'?

<p>All of the above (D)</p> Signup and view all the answers

Which of the following is NOT a valid strategy for dealing with deadlocks?

<p>Deadlock resolution (A)</p> Signup and view all the answers

Which of the following is an example of a resource that can lead to a deadlock?

<p>All of the above (D)</p> Signup and view all the answers

In the context of dealing with deadlocks, what does 'mutual exclusion' refer to?

<p>A situation where only one process can access a resource at a time (C)</p> Signup and view all the answers

Which of the following strategies for dealing with deadlocks is the most extreme, where the responsibility of detecting and recovering from deadlocks is placed on the user?

<p>Deadlock detection and recovery (B)</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 from occurring, while deadlock avoidance only aims to avoid deadlocks at certain times. (B)</p> Signup and view all the answers

Which of the following is NOT a common approach to recovery from a deadlock?

<p>Increasing the priority of the processes involved in the deadlock (D)</p> Signup and view all the answers

What are the four conditions of deadlock?

<p>Mutual exclusion, hold-and-wait, no preemption, circular wait (A)</p> Signup and view all the answers

How does deadlock prevention differ from deadlock avoidance?

<p>Deadlock prevention eliminates one of the four conditions, while avoidance checks for safe states. (A)</p> Signup and view all the answers

What is indicated by a 'safe state' in the Banker’s Algorithm?

<p>The system can allocate resources to processes without falling into deadlock. (B)</p> Signup and view all the answers

What does the 'hold-and-wait' condition entail in the context of deadlock?

<p>Processes can hold certain resources while waiting for others that are currently allocated. (C)</p> Signup and view all the answers

Which of the following conditions is violated to prevent deadlocks in 'no preemption'?

<p>A process's resources cannot be forcibly taken while it is executing. (B)</p> Signup and view all the answers

What is meant by 'incremental resource requests' in deadlock avoidance?

<p>Processes can request resources gradually as needed, to enhance safety. (D)</p> Signup and view all the answers

What condition is being prevented in a controlled intersection that allows at most three cars?

<p>Circular wait (D)</p> Signup and view all the answers

Which type of operating system would most likely implement deadlock detection and recovery?

<p>General-purpose operating system (B)</p> Signup and view all the answers

Why is an ordering that starts with P1 considered unsafe?

<p>P1 may require more R1 than available, leading to a potential deadlock. (C)</p> Signup and view all the answers

After P2 completes, what is the availability vector for R1, R2, and R3 respectively?

<p>6, 2, 3 (A)</p> Signup and view all the answers

Which of the following statements about the ordering P2, P1, P3, P4 is accurate?

<p>This ordering is a safe ordering, but other safe orderings may exist. (B)</p> Signup and view all the answers

What is the main purpose of the analysis described in the passage?

<p>To find a safe ordering for processes to avoid deadlocks. (A)</p> Signup and view all the answers

What information is necessary to determine if a given resource allocation is a 'safe state'?

<p>All of the above. (D)</p> Signup and view all the answers

What does the term 'unsafe state' refer to in the context of the passage?

<p>A state where no process can be allocated any resources without risking deadlock. (A)</p> Signup and view all the answers

Why is the order P2, P1 considered promising for avoiding deadlock?

<p>P2 requires only a small amount of additional resources, which are currently available. (B)</p> Signup and view all the answers

Which process(es), after P2 completes, will be able to finish without risking deadlock?

<p>Both Process P1 and Process P3 (B)</p> Signup and view all the answers

Flashcards

Mission Critical Situations

High-stakes scenarios where resource pre-allocation avoids deadlocks, like nuclear power control.

Pre-allocation of Resources

Allocating all necessary resources beforehand to ensure availability when needed, preventing deadlocks.

No Preemption

A condition where resources cannot be taken away from processes once allocated.

Allowing Preemption

Enabling removal of resources from processes; often leads to user dissatisfaction.

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Circular Wait

A condition where processes wait in a loop for resources, causing deadlock.

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Resource ID Ordering

Assigning unique IDs to resources and accessing them in increasing order to avoid circular wait.

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Acquiring Resources in Order

The approach of processes accessing resources in a predetermined sequence to prevent deadlocks.

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On-Demand Resources

Resources created as needed, complicating pre-allocation and ordering efforts.

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Banker's Algorithm

An algorithm used to determine safe state in resource allocation.

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Safe State

A system state where processes can complete without deadlocks.

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Unsafe State

A state where processes might not complete due to potential deadlock.

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Claim Matrix

A matrix showing maximum resources each process may claim.

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Allocation Matrix

A matrix showing currently allocated resources to processes.

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Availability Vector

A vector showing remaining available resources in the system.

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Process Order

A sequence in which processes are allowed to execute.

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Resource Request

A demand made by a process for additional resources.

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Resource Allocation

The process of distributing resources to various processes in an operating system.

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Process P1

A process currently using 1 unit of R1 and needing up to 2 units of R1.

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Blocking Condition

When a process cannot continue executing because it cannot obtain required resources.

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Process P2

A process that can claim up to 6 units of R1 and currently uses 6 units of R1, needing maybe more R3.

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Completion of Processes

The stage when all allocated resources are returned to the system after a process finishes executing.

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Deadlock Detection

The method of identifying when deadlocks have occurred in a system.

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Resource Allocation Graph

A graphical representation to track resource allocation and detect deadlocks.

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Cycle in Graph

A circular path in a graph that indicates a deadlock situation.

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Deadlock Recovery

Strategies for resolving a deadlock situation once detected.

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Process Termination

Ending a process to resolve a deadlock situation.

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Costly Termination

Ending a process that has high resource and time costs.

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Resource Inconsistency

When resources are left in a state that cannot be used effectively.

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Deadlock

A situation where processes are stuck waiting for each other, preventing progress.

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Mutual Exclusion

A condition where resources cannot be shared and are allocated exclusively.

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Hold-and-Wait

A situation where processes hold onto resources while waiting for others.

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Deadlock Prevention

Strategies designed to eliminate at least one of the necessary conditions for deadlock.

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Deadlock Avoidance

Temporary removal of conditions to prevent deadlock at specific times.

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Deadlock Detection and Recovery

Allowing deadlocks to occur but having a method to detect and recover from them.

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Incremental Resource Requests

Requesting resources gradually instead of all at once to avoid deadlocks.

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Study Notes

Chapter 6: Interprocess Communication

  • This chapter covers interprocess communication.

Chapter 7: Deadlock

  • Deadlock occurs when a set of processes are permanently blocked, awaiting the progress of each other
  • A crucial issue related to concurrent process management.
  • A deadlock occurs when there is a set of processes that are perma- nently blocked. The unblocking of one relies on the progress of another. But since none can make progress, none will be unblocked, and the deadlock condition will remain.

7.1 Resource Allocation Graphs

  • A resource allocation graph visually represents processes and resources, depicting which processes hold resources and which processes are waiting for resources.
  • Arrows from resources to processes indicate that the process is holding the resource.
  • Arrows from processes to resources indicate that the process is waiting for a resource.
  • A deadlock exists when a cycle exists in the graph, meaning a circular dependency.

7.2 The Four Conditions for Deadlock

  • Four conditions must be present for a deadlock to occur:
    • Mutual Exclusion: only one process can use a resource at a time.
    • Hold and Wait: a process holds at least one resource while waiting for others.
    • No Preemption: a resource cannot be forcibly taken away from a process holding it.
    • Circular Wait: a circular chain of processes exists, each holding a resource needed by the next process in the chain.

7.3 Deadlock Prevention

  • Deadlock prevention aims to stop deadlocks by ensuring that at least one of the four conditions for deadlock cannot happen.
  • This approach requires that the resource management system prevent the deadlock conditions, thereby avoiding deadlocks.

7.4 Deadlock Avoidance

  • Deadlock avoidance actively prevents deadlocks by carefully controlling the allocation of resources.
  • The system examines the resource allocation states of each request; if the request may lead to deadlock, the process is put on hold pending sufficient resource availability.
  • Banker's Algorithm: a specific algorithm for resource allocation that determines safe and unsafe states.

7.5 Deadlock Detection and Recovery

  • Deadlock detection identifies if a deadlock exists in a system.
  • Recovery involves terminating the deadlocked processes or resources to resolve the deadlock.
  • Resource allocation graphs are used to detect cycles.
  • The difficulty lies in processes accessing resources which leads to the occurrence of inconsistent states; processes left with partial resource allocation requests.
  • The recovery process is complex, as it requires identifying the deadlocked processes and resources.

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