Deadlock in Operating Systems

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Questions and Answers

Explain how preventing the 'hold and wait' condition can lead to low resource utilization. Provide a specific scenario as an example.

Preventing 'hold and wait' often requires processes to request all needed resources upfront and hold them throughout execution. If a process holds resources for a long time but uses them intermittently, those resources remain unavailable to other processes, leading to underutilization. For example, a process might allocate a printer early, even if it only needs to print at the very end, blocking others from using it.

Describe a scenario where enforcing a resource ordering to prevent circular wait might be difficult to implement in a real-world system.

Consider a system where processes frequently need to access a database and a logging service. If the enforced ordering requires always requesting the database before the logging service, but a process needs to log an error before accessing the database to report a setup problem, the enforced ordering becomes a hindrance.

Explain why the mutual exclusion condition is necessary for certain types of resources and provide an example.

Mutual exclusion is necessary for resources that cannot be safely shared, such as a printer or a writeable file. Allowing multiple processes to write to the same file simultaneously, without any locks, would lead to data corruption and inconsistent state.

Under what circumstances is it appropriate to use resource preemption as a deadlock prevention strategy, and what are its limitations?

<p>Resource preemption is appropriate when the state of a resource can be easily saved and restored, like CPU registers or memory. It's less suitable for resources like printers or database locks, where preemption would lead to inconsistent or unrecoverable states. A limitation is the overhead of saving and restoring resource states.</p> Signup and view all the answers

Describe a potential drawback of requiring a process to release all currently held resources when it requests a resource that cannot be immediately allocated.

<p>A process might repeatedly release and request the same set of resources, leading to a situation where it can never make progress because other processes keep acquiring the needed resources in between its release and request phases. This can lead to starvation.</p> Signup and view all the answers

Explain how using only sharable resources can prevent deadlocks. Provide an example of a sharable resource.

<p>Sharable resources, by definition, can be used concurrently by multiple processes without any restrictions. This eliminates the need for mutual exclusion, one of the four necessary conditions for deadlock. A read-only file is a good example of a sharable resource.</p> Signup and view all the answers

What is the main problem with the deadlock prevention technique that requires a process to request resources only when it holds none?

<p>The main problem with this technique is it is often impractical for processes that need a varying number of resources throughout their execution. They would have to release all resources, even those still needed, before requesting new ones, leading to inefficient resource management and potential delays.</p> Signup and view all the answers

Why is ignoring the possibility of deadlock generally not a practical solution?

<p>While seemingly simple, ignoring deadlock is generally not practical because the consequences of deadlock can be severe (system halts, data corruption, etc.). End users will find the system performs unreliably, which can be more expensive in terms of wasted productivity than implementing complex deadlock prevention schemes.</p> Signup and view all the answers

In the context of deadlock prevention, why is it often impossible to prevent the 'mutual exclusion' condition?

<p>Certain resources are inherently non-sharable, meaning they can only be used by one process at a time to maintain data consistency or integrity. For example, writing to a specific sector on a hard drive must be done exclusively to prevent data corruption.</p> Signup and view all the answers

Discuss the trade-offs between deadlock prevention strategies and system efficiency.

<p>Deadlock prevention strategies typically involve restricting resource allocation to break one of the necessary conditions for deadlock. These restrictions, such as requiring processes to request all resources upfront or enforcing a resource ordering, can lead to lower resource utilization and reduced concurrency, thereby decreasing overall system efficiency. The trade-off is between guaranteed deadlock avoidance and potential performance degradation.</p> Signup and view all the answers

Flashcards

Deadlock

A situation where two or more programs are holding resources, waiting indefinitely for resources held by others.

Mutual Exclusion

At least one resource must be held in a non-sharable mode; only one process at a time can use the resource.

Hold and Wait

A process holds at least one resource and waits to acquire others held by other processes.

No Preemption

Resources can be released only voluntarily by the process holding it, after it completes its task.

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Circular Wait

A set of processes is waiting for each other in a circular chain.

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Deadlock Prevention

Constraining resource requests to prevent at least one necessary deadlock condition.

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Preventing Hold and Wait

Ensure a process doesn't hold resources while requesting more; request all resources upfront or none at all.

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Preventing No Preemption

If a process requests an unavailable resource, release all held resources to allow another process to use them.

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Preventing Circular Wait

Impose an order on resource requests; processes must request resources in increasing order.

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Study Notes

  • Deadlock is a situation in which two or more computer programs are holding resources and waiting indefinitely for resources that are held by other programs

Necessary Conditions for Deadlock

  • Deadlock arises if and only if all four of the following conditions hold simultaneously
    • Mutual exclusion: At least one resource is held in a non-sharable mode; only one process at a time can use the resource. If another process requests that resource, the requesting process is delayed until the resource has been released
    • Hold and wait: A process holds at least one resource and waits to acquire additional resources currently held by other processes
    • No preemption: Resources cannot be preempted; a resource can be released only voluntarily by the process holding it, after that process has completed its task
    • Circular wait: A set {P0, P1, ..., Pn} of waiting processes must exist such that P0 waits for a resource held by P1, P1 waits for a resource held by P2, ..., Pn–1 waits for a resource held by Pn, and Pn waits for a resource held by P0

Deadlock Prevention

  • Deadlock prevention aims to constrain resource requests, ensuring that at least one of the necessary conditions for deadlock cannot occur
  • By denying these conditions, deadlocks can be structurally prevented

Mutual Exclusion

  • Mutual exclusion is essential for non-sharable resources
  • Mutual exclusion cannot always be prevented
  • Sharable resources like read-only files don't require mutual exclusion and won't cause deadlocks
  • Trying to avoid mutual exclusion for all resources is generally not feasible

Hold and Wait

  • To prevent the hold-and-wait condition, guarantee that whenever a process requests a resource, it does not hold any other resources
  • One protocol requires each process to request and be allocated all its resources before execution begins
  • Another protocol allows a process to request resources only when the process has none
  • A process may request some resources and, before using these resources, must release all resources that it is currently holding
  • The process can then request additional resources
  • A problem with this is that resource utilization may be low because resources may be allocated but unused for a long period
  • Starvation is possible, where a process needing several common resources may wait indefinitely because at least one of the resources is always allocated to some other process

No Preemption

  • To ensure that the no-preemption condition does not hold, use the following protocols
  • If a process holding some resources requests another that cannot be immediately allocated to it, then all currently held resources are released
  • The process will be restarted only when it can regain its old resources, as well as what it has requested
  • If a process requests a resource currently held by another process, the operating system may preempt the second process, requiring it to release its resources
  • This only works if the state of the resource can be easily saved and restored later, such as with CPU registers and memory space
  • This is applicable to resources whose state can be easily saved and restored, such as CPU registers and memory space

Circular Wait

  • To prevent the circular-wait condition, impose a total ordering of all resource types, requiring that each process requests resources in an increasing order of enumeration
  • Assign each resource type a unique integer number
  • If a process has been allocated resources Ri, it may request only resources Rj such that j > i
  • Alternatively, require that a process release all Ri such that j ≤ i before requesting Rj
  • If several instances of the same resource type are needed, a process can make repeated requests for that one resource
  • If a process needs to request resources of type i and type j, and i > j, then it must release resource i before requesting resource j
  • Determining an ordering can be difficult, and may not match the order of resource usage; also, unused resources may need to be allocated

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