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Questions and Answers
What must a thread do if its request for a resource cannot be granted immediately?
What must a thread do if its request for a resource cannot be granted immediately?
Which of the following actions is NOT a part of the resource utilization process by a thread?
Which of the following actions is NOT a part of the resource utilization process by a thread?
What system calls are examples for the request and release of resources?
What system calls are examples for the request and release of resources?
What does the wait() operation on a semaphore typically involve?
What does the wait() operation on a semaphore typically involve?
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What happens if a thread requests more resources than are available in the system?
What happens if a thread requests more resources than are available in the system?
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What occurs when a thread requests resources that are not available?
What occurs when a thread requests resources that are not available?
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In a deadlock situation, what is true about the processes involved?
In a deadlock situation, what is true about the processes involved?
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Which of the following best describes the system model in the context of deadlocks?
Which of the following best describes the system model in the context of deadlocks?
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What can be classified as a resource type in a system model?
What can be classified as a resource type in a system model?
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What is one primary concern related to deadlock situations?
What is one primary concern related to deadlock situations?
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How many instances does each resource type have in a system model?
How many instances does each resource type have in a system model?
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What happens to threads in a deadlocked state?
What happens to threads in a deadlocked state?
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What is not a characteristic of deadlock?
What is not a characteristic of deadlock?
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Study Notes
Operating Systems - Deadlocks
- Deadlocks occur in multi-programming environments, where several threads compete for a finite number of resources.
- A thread requests resources; if they're unavailable, the thread enters a waiting state.
- Sometimes, a waiting thread can't change state because the requested resources are held by other waiting threads.
- A deadlock occurs when every process in a set of processes is waiting for an event that can only be caused by another process in the set.
- A set of threads is in a deadlocked state when each thread waits for an event caused by another thread in the set.
System Model
- Systems have a finite number of resources to be distributed among competing threads.
- Resource types are R1, R2, ..., Rm.
- Examples of resources include CPU cycles, memory, I/O devices.
- Synchronization tools (e.g., mutex locks, semaphores) are also resources.
- Each resource type (e.g., R₁) has a number of instances (e.g., W₁).
- Processes utilize resources in these steps:
- Request: The thread asks for a resource.
- If the request can't be granted immediately, the thread waits.
- Use: The thread operates on the resource.
- Release: The thread releases the resource after usage.
Deadlock Characterization
- Deadlocks arise when these four conditions hold simultaneously:
- Mutual Exclusion: Only one process can use a resource at a time.
- Hold and Wait: A process holding at least one resource is waiting to acquire additional resources held by other processes.
- No Preemption: Resources cannot be taken away from a process holding them until the process releases them.
- Circular Wait: There exists a cycle of waiting processes, where each process is waiting for a resource held by the next process in the cycle.
Deadlock Prevention
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Invalidate one of the four conditions to prevent deadlocks.
- Mutual Exclusion: Break mutual exclusion for those resources that can be shared.
- Hold and Wait: Require a process to request all its needed resources at once, preventing it holding some while waiting for others.
- No Preemption: Allow the system to preempt resources from a process that's waiting and give them to a process that's requesting them.
- Circular Wait: Impose a total ordering on all the resources. Requiring each process to seek resources in an increasing order prevents circular waits.
Deadlock Avoidance
- Systems must have a priori information about how resources are to be requested.
- Deadlock avoidance algorithms dynamically examine the resource allocation state to prevent circular waits.
- Resource allocation states use available and allocated resources and maximum demands of processes.
Safe State
- A system is in a safe state if there exists a safe sequence of all processes (e.g., P₁, P₂, ..., Pn).
- The resources needed by P₁ can be granted using current available resources and resources of processes with indices < i.
- If a process requests resources when they are unavailable in the safe state, the process can wait until the other processes have completed and can obtain the resources it requires in a safe order.
Unsafe State
- If a system is in an unsafe state, there is a possibility of deadlock.
- However, not all unsafe states lead to deadlocks.
Resource Allocation Graph
- Deadlocks can be analyzed using a system resource allocation graph, which consists of a set of vertices (V) and edges (E) representing threads (T) and resource instances (R).
- A request edge (T1 → R;) signifies that T1 has requested R₁ and is waiting.
- An assignment edge (R; → T1) indicates that R₁ has been allocated to T₁.
- Cycle in the graph might imply potential deadlock.
- If there are no cycles, then the system is in a safe state.
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Description
This quiz explores the concept of deadlocks in operating systems, particularly in multi-programming environments. You'll learn about how threads compete for resources and the conditions under which deadlocks occur. Test your understanding of resource allocation and synchronization tools.