The document discusses deadlocks in operating systems. It defines deadlock as a situation where a set of processes are blocked waiting for resources held by each other in a cyclic manner. Four necessary conditions for deadlock are described: mutual exclusion, hold and wait, no preemption, and circular wait. Methods for handling deadlocks include prevention, avoidance, and detection/recovery. Prevention ensures one of the conditions cannot occur through protocols like requiring exclusive resource requests, non-preemptive allocation, or imposing a total ordering of resource types.
This document discusses deadlock prevention and recovery in computer systems. It defines deadlock as when a set of blocked processes each hold a resource and wait for a resource held by another process. The document outlines the system model involving resources and processes. It describes deadlock characterization including conditions like mutual exclusion, hold and wait, no preemption, and circular wait. Methods to handle deadlocks include prevention techniques like avoiding one of the four conditions, detection of deadlocks in a resource allocation graph, and recovery methods like process termination or resource preemption.
This document discusses deadlock prevention and recovery in computer systems. It begins by defining deadlock and describing the conditions required for deadlock to occur. It then presents a system model and characterizes deadlocks using resource allocation graphs. The document outlines four main methods for handling deadlocks: prevention, avoidance, detection, and recovery. It focuses on prevention, describing four different approaches to ensure at least one deadlock condition does not hold: eliminating mutual exclusion, hold and wait, no preemption, or circular wait. The document also briefly discusses deadlock detection and two recovery methods: process termination and resource preemption.
The objectives of Deadlocks in Operating Systems are:
- To develop a description of deadlocks, which prevent sets of concurrent processes from completing their tasks
- To present a number of different methods for preventing or avoiding deadlocks in a computer system
The document discusses the deadlock problem in operating systems. It defines deadlock as a state where a set of blocked processes are each holding resources and waiting for additional resources held by other processes in the set, resulting in none of the processes being able to progress. Four conditions must hold simultaneously for deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. Methods for handling deadlocks include prevention, avoidance, and detection/recovery. The banker's algorithm is presented as a deadlock avoidance strategy that uses a resource allocation graph and ensures the system is always in a safe state where deadlocks cannot occur.
The document discusses different methods for handling deadlocks in a system. It describes deadlock characterization including the necessary conditions for deadlock, using a resource-allocation graph to model deadlocks, and examples of such graphs. It also explains several methods for handling deadlocks including deadlock prevention, avoidance, and detection and recovery. Deadlock prevention methods aim to enforce constraints to ensure the necessary conditions for deadlock cannot occur. Deadlock avoidance uses additional information to dynamically monitor the system state and ensure it remains in a safe state where deadlocks cannot happen.
A deadlock occurs in a concurrent system when two or more processes are unable to proceed because each is waiting for another to release a resource. This situation leads to a standstill where none of the processes can complete their execution.
This document describes a course on operating systems with a focus on deadlocks and memory management. It discusses deadlocks in depth, including the necessary conditions for deadlocks, methods for handling them through prevention, avoidance, detection and recovery. For deadlock prevention, it describes how to ensure the hold-and-wait, no preemption and circular wait conditions do not occur. Deadlock avoidance requires knowledge of future resource requests to determine if a process must wait. The document also provides an overview of memory management strategies like swapping, contiguous allocation and paging.
This document outlines the key aspects of deadlocks in operating systems. It defines the necessary conditions for a deadlock to occur as mutual exclusion, hold and wait, no preemption, and circular wait. A resource-allocation graph is used to model resource usage, where a cycle indicates a potential deadlock. Deadlocks can be prevented by avoiding one of the necessary conditions, or can be detected using the graph model to identify cycles. Upon detection, processes may need to be terminated or have resources preempted to recover from the deadlock.
This document discusses deadlock prevention and recovery in computer systems. It defines deadlock as when a set of blocked processes each hold a resource and wait for a resource held by another process. The document outlines the system model involving resources and processes. It describes deadlock characterization including conditions like mutual exclusion, hold and wait, no preemption, and circular wait. Methods to handle deadlocks include prevention techniques like avoiding one of the four conditions, detection of deadlocks in a resource allocation graph, and recovery methods like process termination or resource preemption.
This document discusses deadlock prevention and recovery in computer systems. It begins by defining deadlock and describing the conditions required for deadlock to occur. It then presents a system model and characterizes deadlocks using resource allocation graphs. The document outlines four main methods for handling deadlocks: prevention, avoidance, detection, and recovery. It focuses on prevention, describing four different approaches to ensure at least one deadlock condition does not hold: eliminating mutual exclusion, hold and wait, no preemption, or circular wait. The document also briefly discusses deadlock detection and two recovery methods: process termination and resource preemption.
The objectives of Deadlocks in Operating Systems are:
- To develop a description of deadlocks, which prevent sets of concurrent processes from completing their tasks
- To present a number of different methods for preventing or avoiding deadlocks in a computer system
The document discusses the deadlock problem in operating systems. It defines deadlock as a state where a set of blocked processes are each holding resources and waiting for additional resources held by other processes in the set, resulting in none of the processes being able to progress. Four conditions must hold simultaneously for deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. Methods for handling deadlocks include prevention, avoidance, and detection/recovery. The banker's algorithm is presented as a deadlock avoidance strategy that uses a resource allocation graph and ensures the system is always in a safe state where deadlocks cannot occur.
The document discusses different methods for handling deadlocks in a system. It describes deadlock characterization including the necessary conditions for deadlock, using a resource-allocation graph to model deadlocks, and examples of such graphs. It also explains several methods for handling deadlocks including deadlock prevention, avoidance, and detection and recovery. Deadlock prevention methods aim to enforce constraints to ensure the necessary conditions for deadlock cannot occur. Deadlock avoidance uses additional information to dynamically monitor the system state and ensure it remains in a safe state where deadlocks cannot happen.
A deadlock occurs in a concurrent system when two or more processes are unable to proceed because each is waiting for another to release a resource. This situation leads to a standstill where none of the processes can complete their execution.
This document describes a course on operating systems with a focus on deadlocks and memory management. It discusses deadlocks in depth, including the necessary conditions for deadlocks, methods for handling them through prevention, avoidance, detection and recovery. For deadlock prevention, it describes how to ensure the hold-and-wait, no preemption and circular wait conditions do not occur. Deadlock avoidance requires knowledge of future resource requests to determine if a process must wait. The document also provides an overview of memory management strategies like swapping, contiguous allocation and paging.
This document outlines the key aspects of deadlocks in operating systems. It defines the necessary conditions for a deadlock to occur as mutual exclusion, hold and wait, no preemption, and circular wait. A resource-allocation graph is used to model resource usage, where a cycle indicates a potential deadlock. Deadlocks can be prevented by avoiding one of the necessary conditions, or can be detected using the graph model to identify cycles. Upon detection, processes may need to be terminated or have resources preempted to recover from the deadlock.
Deadlock occurs in a system when multiple processes are waiting indefinitely for resources held by other waiting processes, resulting in no progress. The four conditions required for deadlock are mutual exclusion, hold and wait, no preemption, and circular wait. Deadlock can be avoided by ensuring that at least one of these conditions does not occur through methods like deadlock prevention, deadlock avoidance using safe sequences, and the banker's algorithm.
This document discusses different approaches to handling deadlocks in operating systems. It begins by defining deadlocks and providing examples. It then describes four conditions required for deadlock to occur. Various methods for handling deadlocks are discussed, including prevention, avoidance, detection, and recovery. Prevention methods aim to restrict system states that could lead to deadlocks. Avoidance methods dynamically allocate resources to ensure the system cannot reach an unsafe state. Detection methods identify deadlocks after they occur, while recovery rolls back processes or terminates them. A combined approach uses elements of multiple methods.
This document discusses different methods for handling deadlocks in computer systems, including deadlock prevention, avoidance, and detection. Deadlock prevention methods aim to ensure a system will never enter a deadlocked state by enforcing rules like mutual exclusion of resources and requiring processes to request all resources before starting. Deadlock avoidance uses algorithms like the banker's algorithm to dynamically examine the system state and ensure it remains in a "safe" state where deadlocks cannot occur. Deadlock detection allows the system to enter a deadlocked state but periodically checks a wait-for graph representing resource dependencies between processes to detect any cycles that indicate a deadlock.
This document discusses deadlocks in a multiprogramming system. It defines deadlock as a situation where a set of processes are waiting indefinitely for resources held by each other in a circular chain. Four necessary conditions for deadlock are explained: mutual exclusion, hold and wait, no preemption, and circular wait. Methods for handling deadlocks include prevention, avoidance, detection and recovery. Prevention methods aim to enforce restrictions to ensure at least one condition cannot be met, such as allocating all resources for a process upfront or not allowing processes to hold resources while waiting for others.
In these slides I discussed about deadlock,causes of deadlock,effects of deadlock,conditions of deadlock,resource allocation graph,deadlock handling strategies,deadlock prevention,deadlock avoidance,deadlock avoidance and resolution....I haven't touch algorithms section in these slides.....and last thing I want to say that don't forget to follow me...
This document discusses deadlocks in computer systems. It defines a deadlock as when two or more processes are waiting for resources held by each other in a circular chain. It describes the four conditions required for a deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. Methods to handle deadlocks include deadlock prevention, avoidance, detection, and recovery. Prevention ensures conditions don't hold, avoidance uses additional process information, detection uses graphs to find cycles indicating deadlocks, and recovery aborts processes or preempts resources.
The document discusses deadlocks in operating systems. It defines deadlocks as a situation where a set of concurrent processes are prevented from completing tasks due to circular resource dependencies. The document outlines four conditions required for deadlock and presents methods for handling deadlocks, including prevention, avoidance, detection, and recovery. It provides examples using resource allocation graphs and the banker's algorithm to model processes and resources to ensure the system remains in a safe state to avoid deadlocks.
The document summarizes key topics about deadlocks in operating systems including deadlock characterization, prevention, and avoidance. It defines deadlock as a set of blocked processes where each process is waiting for a resource held by another in the set. Four conditions must be met for deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. Deadlock prevention methods ensure systems never enter a deadlock state while avoidance uses a safe state algorithm and resource ordering to dynamically prevent unsafe states.
This document discusses deadlocks in operating systems. It defines deadlock as when a set of processes are all waiting for resources held by each other in a cyclic manner, preventing any progress. Four conditions must hold for deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. Detection methods include using a resource allocation graph to model processes and resources. Prevention techniques enforce restrictions like requiring processes request resources in a predefined order to avoid circular waits. The banker's algorithm is also described, which uses matrices to model available resources and ensure the system remains in a safe state to avoid deadlocks.
Deadlock occurs in a system when processes are waiting for resources held by other waiting processes, resulting in a circular wait. Four necessary conditions for deadlock are: mutual exclusion, hold and wait, no preemption, and circular wait. A resource-allocation graph can model a system's resource usage, with vertices for processes and resources and edges showing requests and allocations. A cycle in the graph indicates possible deadlock, but multiple instances of a resource mean a cycle does not guarantee deadlock.
Useful documents for engineering students of CSE, and specially for students of aryabhatta knowledge university, Bihar (A.K.U. Bihar). It covers following topics: System model deadlock characterization, methods for handling deadlocks, deadlock prevention,
deadlock avoidance, deadlock detection, recovery from deadlock
Deadlocks operating system To develop a description of deadlocks, which prevent sets of concurrent processes from completing their tasks
To present a number of different methods for preventing, avoiding, or detecting deadlocks in a computer system
e.t.c
The document discusses deadlocks in computer systems. It defines deadlocks as when a set of blocked processes each hold a resource and wait for a resource held by another process in the set, creating a circular wait. It presents methods for handling deadlocks including prevention, avoidance, detection, and recovery. Prevention methods restrict how processes can request resources to avoid deadlock conditions. Avoidance methods use additional information to ensure the system never enters an unsafe state allowing deadlocks. Detection finds cycles in a wait-for graph to detect existing deadlocks while recovery schemes deal with detected deadlocks.
This document discusses deadlocks in operating systems. It defines deadlock and indefinite postponement. It describes the four conditions required for deadlock: mutual exclusion, hold and wait, no preemption, and circular wait. It discusses different approaches to handling deadlocks such as prevention, avoidance, detection, and recovery. It provides examples of resource allocation graphs and the banker's algorithm to illustrate deadlock avoidance techniques.
The document discusses techniques for preventing deadlocks in operating systems. It covers four main techniques - hold and wait protocol, no preemption protocol, requesting resources in increasing order of enumeration to prevent circular wait conditions, and ensuring at least one of the four necessary conditions for deadlock is not met. The protocols aim to enforce that processes request all resources before starting or release resources before requesting new ones to avoid deadlock situations.
Deadlock in Real Time operating Systempptxssuserca5764
This document discusses deadlocks in operating systems. It begins by defining a deadlock as a set of blocked processes each holding a resource and waiting for a resource held by another process. It then covers various methods for handling deadlocks including prevention, avoidance, detection, and recovery. Prevention methods restrain how processes can request resources to ensure deadlocks cannot occur. Avoidance methods allow requests but ensure the system never enters an unsafe state. Detection finds deadlocks after they occur, while recovery rolls back processes or preempts resources to break deadlock cycles.
This document discusses deadlocks in computer systems. It begins by defining a deadlock as a state where a set of blocked processes are each holding resources and waiting for resources held by others in a cyclic manner. It then presents methods for handling deadlocks, including prevention, avoidance, and detection and recovery. For avoidance, it describes using a resource allocation graph to model processes and resources, and the banker's algorithm to ensure the system is always in a safe state where deadlocks cannot occur.
E-commerce Application Development Company.pdfHornet Dynamics
Your business can reach new heights with our assistance as we design solutions that are specifically appropriate for your goals and vision. Our eCommerce application solutions can digitally coordinate all retail operations processes to meet the demands of the marketplace while maintaining business continuity.
Deadlock occurs in a system when multiple processes are waiting indefinitely for resources held by other waiting processes, resulting in no progress. The four conditions required for deadlock are mutual exclusion, hold and wait, no preemption, and circular wait. Deadlock can be avoided by ensuring that at least one of these conditions does not occur through methods like deadlock prevention, deadlock avoidance using safe sequences, and the banker's algorithm.
This document discusses different approaches to handling deadlocks in operating systems. It begins by defining deadlocks and providing examples. It then describes four conditions required for deadlock to occur. Various methods for handling deadlocks are discussed, including prevention, avoidance, detection, and recovery. Prevention methods aim to restrict system states that could lead to deadlocks. Avoidance methods dynamically allocate resources to ensure the system cannot reach an unsafe state. Detection methods identify deadlocks after they occur, while recovery rolls back processes or terminates them. A combined approach uses elements of multiple methods.
This document discusses different methods for handling deadlocks in computer systems, including deadlock prevention, avoidance, and detection. Deadlock prevention methods aim to ensure a system will never enter a deadlocked state by enforcing rules like mutual exclusion of resources and requiring processes to request all resources before starting. Deadlock avoidance uses algorithms like the banker's algorithm to dynamically examine the system state and ensure it remains in a "safe" state where deadlocks cannot occur. Deadlock detection allows the system to enter a deadlocked state but periodically checks a wait-for graph representing resource dependencies between processes to detect any cycles that indicate a deadlock.
This document discusses deadlocks in a multiprogramming system. It defines deadlock as a situation where a set of processes are waiting indefinitely for resources held by each other in a circular chain. Four necessary conditions for deadlock are explained: mutual exclusion, hold and wait, no preemption, and circular wait. Methods for handling deadlocks include prevention, avoidance, detection and recovery. Prevention methods aim to enforce restrictions to ensure at least one condition cannot be met, such as allocating all resources for a process upfront or not allowing processes to hold resources while waiting for others.
In these slides I discussed about deadlock,causes of deadlock,effects of deadlock,conditions of deadlock,resource allocation graph,deadlock handling strategies,deadlock prevention,deadlock avoidance,deadlock avoidance and resolution....I haven't touch algorithms section in these slides.....and last thing I want to say that don't forget to follow me...
This document discusses deadlocks in computer systems. It defines a deadlock as when two or more processes are waiting for resources held by each other in a circular chain. It describes the four conditions required for a deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. Methods to handle deadlocks include deadlock prevention, avoidance, detection, and recovery. Prevention ensures conditions don't hold, avoidance uses additional process information, detection uses graphs to find cycles indicating deadlocks, and recovery aborts processes or preempts resources.
The document discusses deadlocks in operating systems. It defines deadlocks as a situation where a set of concurrent processes are prevented from completing tasks due to circular resource dependencies. The document outlines four conditions required for deadlock and presents methods for handling deadlocks, including prevention, avoidance, detection, and recovery. It provides examples using resource allocation graphs and the banker's algorithm to model processes and resources to ensure the system remains in a safe state to avoid deadlocks.
The document summarizes key topics about deadlocks in operating systems including deadlock characterization, prevention, and avoidance. It defines deadlock as a set of blocked processes where each process is waiting for a resource held by another in the set. Four conditions must be met for deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. Deadlock prevention methods ensure systems never enter a deadlock state while avoidance uses a safe state algorithm and resource ordering to dynamically prevent unsafe states.
This document discusses deadlocks in operating systems. It defines deadlock as when a set of processes are all waiting for resources held by each other in a cyclic manner, preventing any progress. Four conditions must hold for deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. Detection methods include using a resource allocation graph to model processes and resources. Prevention techniques enforce restrictions like requiring processes request resources in a predefined order to avoid circular waits. The banker's algorithm is also described, which uses matrices to model available resources and ensure the system remains in a safe state to avoid deadlocks.
Deadlock occurs in a system when processes are waiting for resources held by other waiting processes, resulting in a circular wait. Four necessary conditions for deadlock are: mutual exclusion, hold and wait, no preemption, and circular wait. A resource-allocation graph can model a system's resource usage, with vertices for processes and resources and edges showing requests and allocations. A cycle in the graph indicates possible deadlock, but multiple instances of a resource mean a cycle does not guarantee deadlock.
Useful documents for engineering students of CSE, and specially for students of aryabhatta knowledge university, Bihar (A.K.U. Bihar). It covers following topics: System model deadlock characterization, methods for handling deadlocks, deadlock prevention,
deadlock avoidance, deadlock detection, recovery from deadlock
Deadlocks operating system To develop a description of deadlocks, which prevent sets of concurrent processes from completing their tasks
To present a number of different methods for preventing, avoiding, or detecting deadlocks in a computer system
e.t.c
The document discusses deadlocks in computer systems. It defines deadlocks as when a set of blocked processes each hold a resource and wait for a resource held by another process in the set, creating a circular wait. It presents methods for handling deadlocks including prevention, avoidance, detection, and recovery. Prevention methods restrict how processes can request resources to avoid deadlock conditions. Avoidance methods use additional information to ensure the system never enters an unsafe state allowing deadlocks. Detection finds cycles in a wait-for graph to detect existing deadlocks while recovery schemes deal with detected deadlocks.
This document discusses deadlocks in operating systems. It defines deadlock and indefinite postponement. It describes the four conditions required for deadlock: mutual exclusion, hold and wait, no preemption, and circular wait. It discusses different approaches to handling deadlocks such as prevention, avoidance, detection, and recovery. It provides examples of resource allocation graphs and the banker's algorithm to illustrate deadlock avoidance techniques.
The document discusses techniques for preventing deadlocks in operating systems. It covers four main techniques - hold and wait protocol, no preemption protocol, requesting resources in increasing order of enumeration to prevent circular wait conditions, and ensuring at least one of the four necessary conditions for deadlock is not met. The protocols aim to enforce that processes request all resources before starting or release resources before requesting new ones to avoid deadlock situations.
Deadlock in Real Time operating Systempptxssuserca5764
This document discusses deadlocks in operating systems. It begins by defining a deadlock as a set of blocked processes each holding a resource and waiting for a resource held by another process. It then covers various methods for handling deadlocks including prevention, avoidance, detection, and recovery. Prevention methods restrain how processes can request resources to ensure deadlocks cannot occur. Avoidance methods allow requests but ensure the system never enters an unsafe state. Detection finds deadlocks after they occur, while recovery rolls back processes or preempts resources to break deadlock cycles.
This document discusses deadlocks in computer systems. It begins by defining a deadlock as a state where a set of blocked processes are each holding resources and waiting for resources held by others in a cyclic manner. It then presents methods for handling deadlocks, including prevention, avoidance, and detection and recovery. For avoidance, it describes using a resource allocation graph to model processes and resources, and the banker's algorithm to ensure the system is always in a safe state where deadlocks cannot occur.
E-commerce Application Development Company.pdfHornet Dynamics
Your business can reach new heights with our assistance as we design solutions that are specifically appropriate for your goals and vision. Our eCommerce application solutions can digitally coordinate all retail operations processes to meet the demands of the marketplace while maintaining business continuity.
Takashi Kobayashi and Hironori Washizaki, "SWEBOK Guide and Future of SE Education," First International Symposium on the Future of Software Engineering (FUSE), June 3-6, 2024, Okinawa, Japan
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Allez au-delà du battage médiatique autour de l’IA et découvrez des techniques pratiques pour utiliser l’IA de manière responsable à travers les données de votre organisation. Explorez comment utiliser les graphes de connaissances pour augmenter la précision, la transparence et la capacité d’explication dans les systèmes d’IA générative. Vous partirez avec une expérience pratique combinant les relations entre les données et les LLM pour apporter du contexte spécifique à votre domaine et améliorer votre raisonnement.
Amenez votre ordinateur portable et nous vous guiderons sur la mise en place de votre propre pile d’IA générative, en vous fournissant des exemples pratiques et codés pour démarrer en quelques minutes.
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Web:- https://undressbaby.com/
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Regarding mobile operating systems, two major players dominate our thoughts: Android and iPhone. With Android leading the market, software development companies are focused on delivering apps compatible with this OS. Ensuring an app's functionality across various Android devices, OS versions, and hardware specifications is critical, making Android app testing essential.
OpenMetadata Community Meeting - 5th June 2024OpenMetadata
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* How to run your own data quality framework
* What is the performance impact of running data quality frameworks
* How to run the test cases in your own ETL pipelines
* How the Incident Manager is integrated
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Watch the meeting recording here - https://www.youtube.com/watch?v=UbNOje0kf6E
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Sven will talk about Atlassian’s journey from a monolith to a multi-tenanted architecture and how it affected the way the engineering teams work. You will learn how we shifted to service ownership, moved to more autonomous teams (and its challenges), and established platform and enablement teams.
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Neo4j - Product Vision and Knowledge Graphs - GraphSummit ParisNeo4j
Dr. Jesús Barrasa, Head of Solutions Architecture for EMEA, Neo4j
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2. • In a multiprogramming environment, several processes may compete
for a finite number of resources.
• A process requests resources; if the resources are not available at that
time, the process enters a waiting state.
• Sometimes, a waiting process is never again able to change state,
because the resources it has requested are held by other waiting
processes.
• This situation is called a deadlock.
• What other situations you know relate to deadlock??
• In this chapter, we will describe methods that an operating system
can use to prevent or deal with deadlocks.
Lecture 5 Dead Lock, Simalike P 2
3. • A process must request a resource before using it and must release
the resource after using it.
• A process may request as many resources as it requires to carry out
its designated task.
• Obviously, the number of resources requested may not exceed the
total number of resources available in the system.
• In other words, a process cannot request three printers if the system
has only two.
Lecture 5 Dead Lock, Simalike P 3
4. • Under the normal mode of operation, a process may utilize a resource
in only the following sequence:
1. Request. The process requests the resource. If the request cannot
be granted immediately (for example, if the resource is being used
by another process), then the requesting process must wait until it
can acquire the resource.
2. Use. The process can operate on the resource (for example, if the
resource is a printer, the process can print on the printer).
3. Release. The process releases the resource.
Lecture 5 Dead Lock, Simalike P 4
5. • A system table records whether each resource is free or allocated.
For each resource that is allocated, the table also records the process
to which it is allocated.
• If a process requests a resource that is currently allocated to another
process, it can be added to a queue of processes waiting for this
resource.
• A set of processes is in a deadlocked state, when every process in the
set is waiting for an event that can be caused only by another process
in the set.
• The resources may be either physical resources (for example, printers,
tape drives, memory space, and CPU cycles) or logical resources (for
example, semaphores, mutex locks, and files)
Lecture 5 Dead Lock, Simalike P 5
6. • An example of a deadlocked state, consider a system with three CD
RW drives.
• Suppose each of three processes holds one of these CD RW drives.
• If each process now requests another drive, the three processes will
be in a deadlocked state.
• This example illustrates a deadlock involving the same resource type.
• Deadlocks may also involve different resource types.
• For example, consider a system with one printer and one DVD drive.
Suppose that process Pi is holding the DVD and process Pj is holding
the printer.
• If Pi requests the printer and Pj requests the DVD drive, a deadlock
occurs.
Lecture 5 Dead Lock, Simalike P 6
7. Necessary Conditions for a deadlock to occur
• A deadlock can arise if the following four conditions hold
simultaneously in a system:
1. Mutual exclusion. At least one resource must be held in a non
sharable mode; that is, only one process at a time can use the
resource. If another process requests that resource, the requesting
process must be delayed until the resource has been released.
2. Hold and wait. A process must be holding at least one resource and
waiting to acquire additional resources that are currently being
held by other processes. Eg, P1 is holding a resource R1 and
waiting for a resource R2
Lecture 5 Dead Lock, Simalike P 7
8. 3. No preemption. Resources cannot be preempted; that is, a resource
can be released only voluntarily by the process holding it, after that
process has completed its task.
4. Circular wait. A set {P0, P1, ..., Pn} of waiting processes must exist
such that P0 is waiting for a resource held by P1, P1 is waiting for a
resource held by P2, ..., Pn−1 is waiting for a resource held by Pn, and
Pn is waiting for a resource held by P0.
Conditions for a deadlock to occur …
Lecture 5 Dead Lock, Simalike P 8
9. Resource-Allocation Graph
• Deadlocks can be described more precisely in terms of a directed
graph called a system resource-allocation graph.
• This graph consists of a set of vertices V and a set of edges E.
• The set of vertices V is partitioned into two different types of nodes: P
= {P1, P2, ..., Pn}, the set consisting of all the active processes in the
system, and R = {R1, R2, ..., Rm}, the set consisting of all resource
types in the system.
Lecture 5 Dead Lock, Simalike P 9
10. Resource-Allocation Graph…
• A directed edge from process Pi to resource type Rj is denoted by Pi
→ Rj ; it signifies that process Pi has requested an instance of
resource type Rj and is currently waiting for that resource.
• A directed edge from resource type Rj to process Pi is denoted by Rj
→ Pi ; it signifies that an instance of resource type Rj has been
allocated to process Pi .
• A directed edge Pi → Rj is called a request edge; a directed edge Rj →
Pi is called an assignment edge.
• Pictorially, we represent each process Pi as a circle and each resource
type Rj as a rectangle
Lecture 5 Dead Lock, Simalike P 10
11. Resource-Allocation Graph…
• Since resource type Rj may have more than one instance, we
represent each such instance as a dot within the rectangle.
• Note that a request edge points to only the rectangle Rj , whereas an
assignment edge must also designate one of the dots in the rectangle.
• When process Pi requests an instance of resource type Rj , a request
edge is inserted in the resource-allocation graph.
• When this request can be fulfilled, the request edge is
instantaneously transformed to an assignment edge.
• When the process no longer needs access to the resource, it releases
the resource.
• As a result, the assignment edge is deleted.
Lecture 5 Dead Lock, Simalike P 11
13. Resource-allocation graph …
◦ P = {P1, P2, P3}
◦ R = {R1, R2, R3, R4}
◦ E = {P1 → R1, P2 → R3, R1 → P2, R2 → P2, R2 → P1, R3 → P3}
Resource instances:
◦ One instance of resource type R1
◦ Two instances of resource type R2
◦ One instance of resource type R3
◦ Three instances of resource type R4
Lecture 5 Dead Lock, Simalike P 13
14. Resource-allocation graph …
• Process states:
◦ Process P1 is holding an instance of resource type R2 and is waiting for an
instance of resource type R1.
◦ Process P2 is holding an instance of R1 and an instance of R2 and is
waiting for an instance of R3.
◦ Process P3 is holding an instance of R3.
Lecture 5 Dead Lock, Simalike P 14
15. Resource-allocation graph with a deadlock
• If the graph contains no cycles, then no process in the system is deadlocked.
• If the graph does contain a cycle, then a deadlock may exist.
• If each resource type has exactly one instance, then a cycle implies that a
deadlock has occurred.
• If the cycle involves only a set of resource types, each of which has only a
single instance, then a deadlock has occurred.
• Each process involved in the cycle is deadlocked.
• In this case, a cycle in the graph is both a necessary and a sufficient
condition for the existence of deadlock. (see figure below)
Lecture 5 Dead Lock, Simalike P 15
17. • Processes P1, P2, and P3 are deadlocked?
Resource-allocation graph with a deadlock
Lecture 5 Dead Lock, Simalike P 17
18. • Process P2 is waiting for the resource R3, which is held by process P3.
• Process P3 is waiting for either process P1 or process P2 to release resource
R2.
• In addition, process P1 is waiting for process P2 to release resource R1.
Resource-allocation graph with a deadlock
Lecture 5 Dead Lock, Simalike P 18
19. Resource-allocation graph with a cycle but no
deadlock.
• If each resource type has several instances, then a cycle does not necessarily
imply that a deadlock has occurred.
• In this case, a cycle in the graph is a necessary but not a sufficient condition
for the existence of deadlock.
Lecture 5 Dead Lock, Simalike P 19
20. • However, from the figure above, there is no deadlock.
• Observe that process P4 may release its instance of resource type R2.
• That resource can then be allocated to P3, breaking the cycle.
• In summary, if a resource-allocation graph does not have a cycle, then the
system is not in a deadlocked state.
• If there is a cycle, then the system may or may not be in a deadlocked state.
This observation is important when we deal with the deadlock problem.
Lecture 5 Dead Lock, Simalike P 20
21. Basing on Case A and Case B which case involve deadlock? Why?
Lecture 5 Dead Lock, Simalike P 21
Case A
Case B
22. Methods for Handling Deadlocks
We can deal with the deadlock problem in one of three ways:
i. We can use a protocol to prevent or avoid deadlocks, ensuring that the
system will never enter a deadlocked state.
ii. We can allow the system to enter a deadlocked state, detect it, and
recover.
iii. We can ignore the problem altogether and pretend that deadlocks never
occur in the system.
• But to ensure that deadlocks never occur, the system can use either a
deadlock prevention or a deadlock-avoidance scheme
Lecture 5 Dead Lock, Simalike P 22
23. Methods for Handling Deadlocks
• Deadlock prevention provides a set of methods to ensure that at least one
of the necessary conditions cannot hold.
• Deadlock avoidance requires that the operating system be given additional
information in advance concerning which resources a process will request
and use during its lifetime.
• With this additional knowledge, the operating system can decide for each
request whether or not the process should wait.
• To decide whether the current request can be satisfied or must be delayed,
the system must consider the resources currently available, the resources
currently allocated to each process, and the future requests and releases of
each process.
Lecture 5 Dead Lock, Simalike P 23
24. Deadlock Prevention
• As we noted before for a deadlock to occur, each of the four
necessary conditions must hold.
• By ensuring that at least one of these conditions cannot hold, we can
prevent the occurrence of a deadlock.
• A gain we discus the four conditions for a deadlock to occur as
methods that can be used to prevent the occurrence of a deadlock.
Lecture 5 Dead Lock, Simalike P 24
25. Mutual Exclusion
• The mutual exclusion condition must hold. That is, at least one resource
must be non-sharable.
• Sharable resources, in contrast, do not require mutually exclusive access
and thus cannot be involved in a deadlock.
• Read-only files are a good example of a sharable resource. If several
processes attempt to open a read-only file at the same time, they can be
granted simultaneous access to the file.
• A process never needs to wait for a sharable resource
Lecture 5 Dead Lock, Simalike P 25
26. Hold and Wait
• To ensure that the hold-and-wait condition never occurs in the system,
we must guarantee that, whenever a process requests a resource, it does
not hold any other resources.
• One protocol that we can use requires each process to request and be
allocated all its resources before it begins execution.
• An alternative protocol allows a process to request resources only when
it has none. A process may request some resources and use them.
• Before it can request any additional resources, it must release all the
resources that it is currently allocated.
Lecture 5 Dead Lock, Simalike P 26
27. Hold and Wait Continue
• To illustrate the difference between these two protocols, we consider a
process that copies data from a DVD drive to a file on disk, sorts the file,
and then prints the results to a printer.
• If all resources must be requested at the beginning of the process, then
the process must initially request the DVD drive, disk file, and printer.
• It will hold the printer for its entire execution, even though it needs the
printer only at the end.
Lecture 5 Dead Lock, Simalike P 27
28. Hold and Wait Continue
• The second method allows the process to request initially only the DVD
drive and disk file. It copies from the DVD drive to the disk and then
releases both the DVD drive and the disk file.
• The process must then request the disk file and the printer.
• After copying the disk file to the printer, it releases these two resources
and terminates.
Lecture 5 Dead Lock, Simalike P 28
29. Disadvantages of protocol methods
• First, resource utilization may be low, since resources may be allocated
but unused for a long period.
• Second, starvation is possible. A process that needs several popular
resources may have to wait indefinitely, because at least one of the
resources that it needs is always allocated to some other process.
Lecture 5 Dead Lock, Simalike P 29
30. No Preemption
• The third necessary condition for deadlocks is that there be no
preemption of resources that have already been allocated.
• To ensure that this condition does not hold, we can use the following
protocol.
• If a process is holding some resources and requests another resource
that cannot be immediately allocated to it, then all resources the
process is currently holding are preempted.
• In other words, these resources are implicitly released.
• The preempted resources are added to the list of resources for which the
process is waiting.
• The process will be restarted only when it can regain its old resources, as
w ell as the new ones that it is requesting.
Lecture 5 Dead Lock, Simalike P 30
31. No Preemption
• Alternatively, if a process requests some resources, we first check
whether they are available.
• If they are, we allocate them. If they are not, we check whether they are
allocated to some other process that is waiting for additional resources.
• If so, we preempt the desired resources from the waiting process and
allocate them to the requesting process.
• If the resources are neither available nor held by a waiting process, the
requesting process must wait.
• While it is waiting, some of its resources may be preempted, but only if
another process requests them.
Lecture 5 Dead Lock, Simalike P 31
32. Circular Wait
• The fourth and final condition for deadlocks is the circular-wait
condition.
• One way to ensure that this condition never holds is to impose a total
ordering of all resource types and to require that each process
requests resources in an increasing order of enumeration/list.
• Let R = {R1, R2, ..., Rm} be the set of resource types representing
resource types tape drives (3), disk drives(4), and printers(5).
• Initially a process can request any number of instances of a resource
type say, Ri. (i is any number)
• After that, the process can request instances of resource type Rj if
and only if Rj > Ri. Let say Ri=3 and Rj=5 (only request for resources
more than R3 will be granted.)
Lecture 5 Dead Lock, Simalike P 32
33. Circular Wait
• Alternatively, we can require that a process requesting an instance of
resource type Rj must have released any resources Ri such that Ri ≥ Rj.
• Note also that if several instances of the same resource type are needed,
a single request for all of them must be issued.
Lecture 5 Dead Lock, Simalike P 33
Editor's Notes
“When two trains approach each other at a crossing, both shall come to a full stop and neither shall start up again until the other has gone.” by Kansas Legislature
We emphasize that all four conditions must hold for a deadlock to nnnnnoccur.
The sets P, R, and E:
◦ P = {P1, P2, P3}
◦ R = {R1, R2, R3, R4}
◦ E = {P1 → R1, P2 → R3, R1 → P2, R2 → P2, R2 → P1, R3 → P3}
• Resource instances: