This document summarizes key concepts related to deadlocks in operating systems. It defines the four necessary conditions for deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. It describes methods for handling deadlocks, including deadlock prevention, avoidance, detection, and recovery. Deadlock prevention techniques aim to ensure that at least one of the necessary conditions does not hold, such as imposing an ordering on how resources can be requested. Deadlock avoidance uses additional information to determine if a request could lead to a deadlocked state. Detection and recovery methods allow deadlocks to occur but provide algorithms for identifying and resolving deadlocked processes.
This document summarizes a chapter on deadlocks from an operating systems textbook. It defines deadlock as when a set of blocked processes wait for resources held by each other. Four conditions must be met for deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. Methods to handle deadlocks include prevention, avoidance, detection, and recovery. Prevention ensures deadlocks cannot occur by restricting resource usage. Avoidance dynamically checks the system state remains safe to prevent deadlocks. Detection allows deadlocks but recovers the system. Recovery options are terminating processes or preempting resources.
This document discusses deadlocks in operating systems. It defines deadlock as when multiple processes are waiting for resources held by each other in a cyclic manner, resulting in none of the processes making progress. It provides examples and describes the four necessary conditions for deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. It also discusses methods for handling deadlocks, including prevention, avoidance, and recovery techniques like terminating processes or preempting resources.
Deadlocks-An Unconditional Waiting Situation in Operating System. We must make sure of This concept well before understanding deep in to Operating System. This PPT will understands you to get how the deadlocks Occur and how can we Detect, avoid and Prevent the deadlocks in Operating Systems.
This document defines and explains deadlocks in an operating system. A deadlock occurs when a set of processes are blocked waiting for resources held by other processes in the set, resulting in a circular wait. Four necessary conditions for deadlock are mutual exclusion, hold and wait, no preemption, and circular wait. Resource allocation graphs can be used to visualize resource allocation relations between processes and determine if a deadlock has occurred. Methods for recovering from deadlock include violating mutual exclusion, preempting resources from processes, and aborting processes. The operating system may also ignore deadlocks altogether.
The document discusses deadlocks in operating systems. It defines deadlocks and explains the four necessary conditions for a deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. It then describes different strategies for handling deadlocks, including prevention, avoidance, detection, and recovery.
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 other processes in the set, resulting in none of the processes making any progress. Four conditions must be met for deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. The document presents examples to illustrate deadlock and discusses different strategies for dealing with it, including deadlock prevention, avoidance, and detection and recovery. It specifically describes the Banker's Algorithm for deadlock avoidance.
The document discusses deadlocks in operating systems. It defines the four conditions for deadlock - mutual exclusion, hold and wait, no preemption, and circular wait. It explains resource allocation graphs and how they can be used to detect deadlocks. The document also covers deadlock prevention methods like mutual exclusion, holding and waiting, preemption, and imposing a total ordering of resources. It describes Banker's algorithm for deadlock prevention and detection with multiple instances of resources. Finally, it discusses different approaches for deadlock recovery like process termination and resource preemption.
This document discusses deadlocks and techniques for handling them. It begins by defining the four necessary conditions for a deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. It then describes three approaches to handling deadlocks: prevention, avoidance, and detection and recovery. Prevention aims to ensure one of the four conditions never holds. Avoidance uses more information to determine if a request could lead to a deadlock. Detection and recovery allows deadlocks but detects and recovers from them after the fact. The document provides examples of different prevention techniques like limiting resource types that can be held, ordering resource types, and preemption. It also explains the banker's algorithm for deadlock avoidance.
This document summarizes a chapter on deadlocks from an operating systems textbook. It defines deadlock as when a set of blocked processes wait for resources held by each other. Four conditions must be met for deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. Methods to handle deadlocks include prevention, avoidance, detection, and recovery. Prevention ensures deadlocks cannot occur by restricting resource usage. Avoidance dynamically checks the system state remains safe to prevent deadlocks. Detection allows deadlocks but recovers the system. Recovery options are terminating processes or preempting resources.
This document discusses deadlocks in operating systems. It defines deadlock as when multiple processes are waiting for resources held by each other in a cyclic manner, resulting in none of the processes making progress. It provides examples and describes the four necessary conditions for deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. It also discusses methods for handling deadlocks, including prevention, avoidance, and recovery techniques like terminating processes or preempting resources.
Deadlocks-An Unconditional Waiting Situation in Operating System. We must make sure of This concept well before understanding deep in to Operating System. This PPT will understands you to get how the deadlocks Occur and how can we Detect, avoid and Prevent the deadlocks in Operating Systems.
This document defines and explains deadlocks in an operating system. A deadlock occurs when a set of processes are blocked waiting for resources held by other processes in the set, resulting in a circular wait. Four necessary conditions for deadlock are mutual exclusion, hold and wait, no preemption, and circular wait. Resource allocation graphs can be used to visualize resource allocation relations between processes and determine if a deadlock has occurred. Methods for recovering from deadlock include violating mutual exclusion, preempting resources from processes, and aborting processes. The operating system may also ignore deadlocks altogether.
The document discusses deadlocks in operating systems. It defines deadlocks and explains the four necessary conditions for a deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. It then describes different strategies for handling deadlocks, including prevention, avoidance, detection, and recovery.
There are three main approaches to handling deadlocks: prevention, avoidance, and detection with recovery. Prevention methods constrain how processes request resources to ensure at least one necessary condition for deadlock cannot occur. Avoidance requires advance knowledge of processes' resource needs to decide if requests can be immediately satisfied. Detection identifies when a deadlocked state occurs and recovers by undoing the allocation that caused it.
The document discusses deadlocks in computer systems. It defines deadlocks as a situation where two or more competing processes are waiting for resources held by each other, leading to indefinite blocking. The key characteristics of deadlocks are mutual exclusion of resources, hold and wait conditions, no preemption of allocated resources, and circular wait dependencies between processes. The document outlines several methods to handle deadlocks, including prevention, avoidance through algorithms like the banker's algorithm, and detection and recovery.
Salman Ahmed presents on deadlocks in computer systems. A deadlock occurs when two programs are preventing each other from accessing shared resources, causing both programs to cease functioning. Deadlocks happen when processes are holding resources and waiting to acquire more resources, but cannot release the initial resources. There are four main approaches to handling deadlocks: prevention, avoidance, detection, and recovery. Prevention eliminates one of the conditions required for a deadlock, while avoidance allows the system to refuse resource requests to avoid entering a deadlocked state.
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
Deadlock occurs when each transaction in a set of two or more transactions is waiting for a resource locked by another transaction in the set, resulting in a circular wait. Several approaches to handling deadlocks are discussed, including prevention protocols that impose ordering on transactions or require transactions to lock all resources in advance. Detection methods involve constructing a wait-for graph to identify cycles indicating deadlock. If detected, victim selection chooses which transactions to abort to resolve the deadlock. Timeouts provide a simple alternative where transactions waiting longer than a threshold are aborted.
This document discusses deadlock in computer systems. It defines deadlock as when a set of processes are blocked waiting for resources held by other processes in the set. Four necessary conditions for deadlock to occur are outlined: mutual exclusion, hold and wait, no preemption, and circular wait. Strategies to handle deadlock such as prevention, detection, avoidance, and an integrated approach combining multiple strategies are also discussed. Examples of different resource types that can be involved in deadlock are provided.
This document discusses deadlocks in operating systems. It defines a deadlock as a set of blocked processes that are each holding a resource and waiting for a resource held by another process. Four conditions must be met for a deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. Deadlocks can be modeled using directed resource allocation graphs. Methods for handling deadlocks include prevention, avoidance, detection, and recovery.
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...
In this presentation i explain about the most important thing in operating system i.e Deadlock. Here i briefly explained what is deadlock, why deadlock occurs, deadlock in real life, methods of handling deadlock. Banker's algorithm and a numerical.
I hope its worth sharing!
Deadlock occurs when multiple processes are blocked waiting for resources held by other processes in the set, resulting in no forward progress. There are four conditions required for deadlock: mutual exclusion, hold and wait, no preemption, and circular wait. Deadlock can be handled through prevention, avoidance, detection, and recovery. Prevention ensures one of the four conditions is never satisfied. Avoidance allows resource allocation if it does not lead to an unsafe state. Detection identifies when deadlock occurs. Recovery regains resources by terminating processes or preempting resources.
This document discusses deadlocks in operating systems. It defines deadlock as when a set of blocked processes each hold a resource and wait for a resource held by another process. It then covers methods for handling deadlocks such as prevention, avoidance, detection, and recovery. Prevention ensures deadlock conditions cannot occur. Avoidance allows the system to deny requests that could lead to deadlock. Detection identifies when a deadlock has occurred. Recovery breaks deadlocks by terminating or preempting processes.
Ninth class biology mcqs ( classification of living organisms)Dr. Sajid Ali Talpur
This document contains a poem about how one's perception of the passage of time changes based on their mental state, followed by a chapter of biology multiple choice questions. The questions cover topics like scientific naming conventions, taxonomy, kingdoms of life, and basic cell structures. C. Linnaeus is identified as introducing binomial nomenclature. The document provides a practice test on foundational concepts in biology.
Biology Notes for Class 9th (by: Seetal Daas)Seetal Daas
This biology notes document provides an overview of various topics in biology including the definition and branches of biology, important Muslim and non-Muslim scientists who contributed to the field of biology, and the significance and impact of biology on human life. It also discusses Islamic concepts regarding the origin of life according to the Quran including that all living things were created from water and humans were created from clay.
1. The document contains questions and answers related to short stories "Button Button" by Richard Matheson, "Clearing in the Sky" by Jesse Stuart, and "Dark They Were and Golden Eyed" by Ray Bradbury.
2. It provides character summaries, themes and plot details for each story. For example, it analyzes Norma's character in "Button Button" and how her greed led to tragic consequences despite getting money.
3. The document acts as a study guide for students by addressing questions about each story that assess comprehension of key details, themes and characters. It aims to concisely summarize essential information from the stories in a question-answer format.
The document discusses the key elements (bioelements) needed for living organisms, including the major elements of carbon, hydrogen, oxygen, nitrogen, phosphorus, and calcium which make up 99% of living matter. It also mentions other minor elements needed in small quantities like sulfur, sodium, potassium, magnesium, manganese, iron, copper, zinc, iodine and chlorine. These bioelements are obtained by plants through photosynthesis and from the soil, and then consumed by animals who distribute them throughout the ecosystem.
This document contains 50 multiple choice questions about biology and cell structure. The questions cover topics like the discovery of cells, components of plant and animal cells, cell organelles and their functions, cell membrane structure and function, mitosis and meiosis. An answer key is provided at the end of the document with the correct response for each question numbered 1-50.
The document is a collection of biology practice questions and answers from the website MCQBiology.com. It includes multiple choice questions on topics such as animal physiology, biochemistry, bioinformatics, biotechnology, biostatistics, biophysics, cytology, ecology, evolution, genetics, immunology, microbiology, plant science, plant physiology, research methodology, and zoology. Each section provides the questions, answers, and a link to find more questions on the topic on the MCQBiology website. The overarching purpose is to provide biology students practice questions and tests on various topics through the MCQBiology online resource.
The document contains 50 multiple choice questions from a 1st year chemistry notes chapter on basic concepts. The questions cover topics like atoms, molecules, ions, isotopes, relative atomic mass, and stoichiometry calculations.
The document also includes the answers to all 50 questions at the end.
The document contains questions and answers related to two short stories: "Button Button" by Richard Matheson, and "Clearing in the Sky" by Jesse Stuart.
In "Button Button", Norma persuades her husband Arthur to accept $50,000 offered by Mr. Steward if they push a button that will kill an unknown person. After Norma pushes the button, she receives news that Arthur was killed in a subway accident. The story examines themes of greed, morality, and the consequences of actions.
"Clearing in the Sky" is about an old man who ignores doctor's orders and regains his health through physical labor on his farm. At age 70, he feels a sense of
1. The document provides a set of 50 multiple choice questions related to various topics in dentistry. The questions cover subjects like dental anatomy, operative dentistry, endodontics, oral pathology, oral surgery and more.
2. The questions are based on people's memories of past exams and cannot be relied on 100% but provide an idea of topics that may be covered.
3. Answers are not provided for the questions.
This document outlines different methods for handling deadlocks in operating systems, including deadlock prevention, avoidance, detection, and recovery. It discusses the four necessary conditions for deadlock and introduces the concept of a resource-allocation graph. For deadlock prevention, it describes ways to ensure that the conditions of mutual exclusion, hold and wait, no preemption, and circular wait do not all occur simultaneously, such as requiring processes to request all resources upfront or imposing a total ordering of resource types. Deadlock avoidance uses additional information about future resource needs to decide if a request can be granted. Detection and recovery methods are employed if deadlocks are allowed to occur.
This document discusses deadlocks in computer systems. It defines the four necessary conditions for deadlock and introduces resource allocation graphs to model deadlock states. It describes three main approaches to handling deadlocks: prevention, detection and recovery, and ignoring the problem. The rest of the document focuses on techniques for deadlock prevention, including holding and waiting, no preemption, and circular wait conditions. It also covers deadlock avoidance using safe state models, resource allocation graphs, and Banker's algorithm to determine if a resource request would result in an unsafe state.
The document discusses deadlocks in computer systems and various approaches to handling them. It defines deadlock as when a set of processes are blocked waiting for resources held by each other in a cyclic manner. There are four conditions required for deadlock: mutual exclusion, hold and wait, no preemption, and circular wait. Methods to handle deadlocks include ignoring the problem, preventing deadlocks through design restrictions, avoiding deadlocks by dynamically checking for safety, and detecting and recovering from deadlocks after the fact. The banker's algorithm is presented as an avoidance technique using a resource allocation graph and safety checks.
Deadlock and memory management -- Operating SystemEktaVaswani2
This document discusses operating system resources, deadlocks, and memory management. It defines system resources as any usable parts of a computer that can be controlled and assigned by the operating system. It describes the four conditions required for a deadlock and methods for detecting, avoiding, and recovering from deadlocks such as terminating processes. It also discusses memory management techniques like paging, swapping, and virtual memory that allow more programs to reside in memory than the physical memory size.
The document describes operating system concepts related to resource allocation and deadlocks. It defines a system model where processes compete for shared resources. A deadlock occurs when a set of processes are blocked waiting for resources held by other processes in the set, forming a circular wait. The document outlines four conditions for deadlock and describes methods to prevent, avoid, detect, and recover from deadlocks using techniques like safe state algorithms, resource ordering, and process termination.
A deadlock occurs when a set of processes are blocked because each process is holding a resource and waiting for another resource held by another process in the set, creating a circular wait. Four necessary conditions must be met for a deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. Deadlocks can be handled through deadlock avoidance, detection and recovery, or prevention. Avoidance methods like Banker's Algorithm use safe state models to prevent deadlocks, while detection algorithms use resource allocation graphs to detect deadlocks. Once detected, deadlocks are recovered through process termination or resource preemption.
This document discusses deadlocks in operating systems. It describes deadlocks as situations where processes are waiting for resources held by other waiting processes, resulting in none advancing. It outlines three main methods to handle deadlocks: prevention, avoidance, and detection+recovery. Prevention uses protocols to ensure deadlock conditions cannot occur. Avoidance requires knowledge of future resource needs to delay requests as needed. Detection identifies deadlocks and recovers resources to allow processes to continue.
This document discusses deadlocks in operating systems. It defines a deadlock as when processes are waiting for resources held by other waiting processes, resulting in none progressing. It presents three methods to handle deadlocks: prevention, avoidance, and detection+recovery. Deadlock prevention uses protocols to ensure deadlock conditions cannot occur. Deadlock avoidance requires knowledge of future resource needs to delay requests if needed. Detection and recovery detects when a deadlock occurs and recovers resources.
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.
Distributed deadlock occurs when processes are blocked while waiting for resources held by other processes in a distributed system without a central coordinator. There are four conditions for deadlock: mutual exclusion, hold and wait, non-preemption, and circular wait. Deadlock can be addressed by ignoring it, detecting and resolving occurrences, preventing conditions through constraints, or avoiding it through careful resource allocation. Detection methods include centralized coordination of resource graphs or distributed probe messages to identify resource waiting cycles. Prevention strategies impose timestamp or age-based priority to resource requests to eliminate cycles.
This 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 circular chain. The four necessary conditions for deadlock are discussed: mutual exclusion, hold and wait, no preemption, and circular wait. Methods for handling deadlocks include prevention, detection, recovery, and avoidance. Prevention removes one of the necessary conditions. Detection identifies when a deadlock occurs. Recovery breaks existing deadlocks through preemption, rollback, or killing processes. Avoidance anticipates potential deadlocks through algorithms like the Banker's algorithm.
A deadlock in OS is a situation in which more than one process is blocked because it is holding a resource and also requires some resource that is acquired by some other process.
This document discusses operating system topics related to deadlocks, including definitions, properties, prevention, detection, and recovery from deadlocks. It defines deadlock as when a process requests resources that are held by another waiting process, creating a circular wait. Four conditions must be met for deadlock: mutual exclusion, hold and wait, no preemption, and circular wait. Techniques to prevent deadlocks include deadlock avoidance using safe states and resource allocation graphs, and deadlock prevention by ensuring one of the four conditions is never satisfied. Deadlock detection uses wait-for graphs or detection algorithms, and recovery options are terminating processes or preempting resources.
Shivangi submitted a document on deadlocks to Tapas Sangiri. The 3-sentence summary is:
The document discusses different aspects of deadlocks including definitions, characteristics, methods for handling them such as prevention, avoidance and recovery. Prevention methods aim to ensure a deadlock never occurs by restricting resource allocation in different ways. Avoidance algorithms analyze the resource allocation graph to determine if a system is in a safe state to avoid deadlocks.
1. The document discusses deadlocks in computing systems. It defines deadlock as a situation where a process requests resources that are held by another waiting process, resulting in both processes waiting indefinitely.
2. Four conditions must be satisfied for a deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. The document outlines strategies to prevent deadlocks by ensuring that at least one of these conditions is never satisfied.
3. Deadlock detection methods are described for single resource systems using wait-for graphs and for multiple resource systems using detection algorithms. Recovery from detected deadlocks can involve terminating processes or preempting resources from processes.
Deadlocks occur when processes are waiting for resources held by other processes, resulting in a circular wait. Four conditions must be met: mutual exclusion, hold and wait, no preemption, and circular wait. Deadlocks can be handled through avoidance, prevention, or detection and recovery. Avoidance algorithms allocate resources only if it ensures the system remains in a safe state where deadlocks cannot occur. Prevention methods make deadlocks impossible by ensuring at least one condition is never satisfied, such as through collective or ordered resource requests. Detection finds existing deadlocks by analyzing resource allocation graphs or wait-for graphs to detect cycles.
The document discusses various issues related to deadlocks including:
1) The four conditions required for a deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait.
2) Approaches to handling deadlocks such as detection and recovery, avoidance through careful allocation, and prevention by eliminating one of the four conditions.
3) Specific algorithms for deadlock detection, avoidance, and prevention including the banker's algorithm and various graph-based approaches.
A document about deadlocks in operating systems is summarized as follows:
1. A deadlock occurs when a set of processes form a circular chain where each process is waiting for a resource held by the next process in the chain. The four conditions for deadlock are mutual exclusion, hold and wait, no preemption, and circular wait.
2. Deadlocks can be modeled using a resource allocation graph where processes and resources are vertices and edges represent resource requests. A cycle in the graph indicates a potential deadlock.
3. Methods for handling deadlocks include prevention, avoidance, and detection/recovery. Prevention ensures deadlock conditions cannot occur while avoidance allows the system to dynamically verify new allocations will not
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.
1) A deadlock occurs when a set of processes are blocked waiting for resources held by other processes in the set, forming a circular wait.
2) For a deadlock to occur, four necessary conditions must be met: mutual exclusion, hold and wait, no preemption, and circular wait.
3) Deadlocks can be prevented by ensuring that at least one of the four conditions is not met, such as allowing resource preemption or requiring processes request all resources before starting. Deadlock avoidance uses algorithms like Banker's Algorithm to prevent unsafe states that could lead to deadlock.
The document discusses features on the Insert, Home, and Page Layout tabs in Microsoft Word that allow formatting and styling text and other elements. Galleries on the Insert tab include items that coordinate with the overall document style and can be used to insert common elements like tables and headers. Text formatting can be applied using the options on the Home tab or by selecting styles from the Quick Styles gallery. The overall document style can be changed using themes on the Page Layout tab.
The document discusses different approaches to writing data to caches, including write-through caches that update both the cache and main memory on writes, and write-back caches that only update main memory when replacing cache blocks. It also describes modern CPU cache designs, such as split instruction/data caches, write buffers, multiple cache levels, and techniques to reduce memory stalls like non-blocking caches and cache banking.
This document provides an overview and outline of a chapter that describes implementing a processor to execute instructions from the MIPS instruction set architecture. It discusses building a basic datapath with functional units like a register file, ALU, program counter, and data memory. Multiplexors are used to select data sources and a control unit decodes instructions to set control lines that direct data flow and functional unit operations. The chapter then describes refining this design to implement pipelining for improved performance.
This document provides an overview and outline of key topics in implementing a processor, including:
- It describes building a datapath and control unit to implement a simple MIPS processor, including functional units like registers, ALU, and memory.
- It explains the flow of data and control through the processor, from fetching instructions from memory to executing operations and writing results.
- It discusses adding multiplexors and control lines to direct data between functional units based on the instruction, as well as pipelining the design for improved performance.
This document provides an overview of topics in computer arithmetic covered in Chapter 3, including addition, subtraction, multiplication, division, floating point representation, parallelism, and implications for instruction sets. Key points covered include how addition and subtraction are performed at the bit level; how overflow is detected in two's complement systems; how unsigned integers handle overflow; and how parallel arithmetic units can perform simultaneous operations on vector data for applications like graphics and audio.
This document outlines topics related to sequential circuits including storage elements like latches and flip-flops, the analysis of clocked sequential circuits using state equations and diagrams, state reduction and assignment techniques, and the design procedure for synthesizing sequential circuits using different types of flip-flops. It also lists further reading materials and notes areas that need more explanation.
This document outlines the key topics in combinational circuits including analysis, design, common circuits like adders and decoders, and provides procedures for analyzing and designing combinational circuits from Boolean functions or truth tables. It describes combinational circuits as those whose outputs depend only on the present combination of inputs and discusses standard components like adders, subtractors, comparators, encoders and multiplexers. The analysis and design procedures involve determining the Boolean functions or truth table that define a circuit's behavior and using those to derive the corresponding logic diagram.
This document provides an overview of topics in computer arithmetic, including addition, subtraction, multiplication, division, and floating point representation. It discusses how operations are performed at the hardware level, including how overflows are detected in two's complement systems. It also covers parallelism techniques used in multimedia applications, such as SIMD instructions that can perform the same operation on multiple data elements simultaneously.
This document outlines topics related to sequential circuits including storage elements like latches and flip-flops, the analysis of clocked sequential circuits using state equations and diagrams, state reduction and assignment techniques, and the design procedure for synthesizing sequential circuits using different types of flip-flops. It also lists further reading materials and notes areas that need more explanation.
This document outlines the key topics covered in combinational circuits, including:
1) It introduces combinational circuits and their analysis and design procedures.
2) Some common combinational circuits that are used extensively in digital systems design are discussed, such as adders, subtractors, comparators, decoders, encoders, and multiplexers.
3) The analysis of a combinational circuit involves determining the Boolean functions or truth table that represent the circuit's functionality. The design of a combinational circuit starts with specifying the required inputs/outputs and deriving the truth table and Boolean functions that define the required logic.
The document discusses Karnaugh mapping techniques for simplifying Boolean logic expressions. It begins by explaining that Karnaugh maps are faster and easier than Boolean algebra for problems with 3 or more variables. It then covers the basics of Karnaugh maps, including how to generate Gray codes and place 1s and 0s on the maps corresponding to minterms and maxterms. Several examples are provided of using Karnaugh maps to identify groups of cells and arrive at simplified Sum of Products or Product of Sums expressions. The document emphasizes that Karnaugh mapping scales well to problems with many logic gates and variables, unlike Boolean algebra.
The document discusses Boolean algebra formulae and techniques for converting gate circuits to Boolean expressions. It explains that gates can be converted to equivalent expressions using DeMorgan's Theorems. Sum-of-Products and Product-of-Sums Boolean expressions can be generated from truth tables and lend themselves to implementations using AND/OR gates.
Virtual memory allows processes to have a virtual address space larger than physical memory. When a process accesses a page not in memory, a page fault occurs which is handled by reading the requested page from disk into a free frame. Page replacement algorithms select a victim page to replace when there are no free frames, such as FIFO which replaces the oldest page. The optimal page replacement algorithm OPT replaces the page that will not be used for the longest time in the future but cannot be implemented as it requires knowing the future.
This document discusses input/output (I/O) systems in 3 parts:
1) I/O hardware including devices, buses, controllers, polling, and interrupts. Interrupts allow hardware to notify the CPU when a device is ready rather than requiring repeated polling.
2) The application I/O interface including block/character devices, networks, clocks/timers, and blocking/non-blocking I/O.
3) The kernel I/O subsystem which transforms requests to hardware operations, handles scheduling, buffering, caching, error handling and more. Device drivers plug into the OS to support specific devices.
The document discusses key aspects of file systems and file interfaces. It covers file concepts like attributes, operations, and types. It also covers access methods like sequential, direct, and indexed access. It discusses directory structures from single-level to tree-structured directories. Other topics covered include file system mounting, file sharing through multiple users and remote file systems, and protection through access controls and permissions.
The document discusses the structure and implementation of file systems. It describes the layered design of file systems, with the physical devices at the lowest level and the logical file system at the highest level. It also outlines several important on-disk data structures used by file systems, including the boot control block, volume control block, and directory structure. These structures store information needed to boot an operating system, manage disk space and blocks, and organize the directory structure and file metadata.
This document discusses mass storage structures including magnetic disks, solid state disks, disk structure, disk attachment methods like host-attached storage, network-attached storage, and storage area networks. It also covers disk scheduling algorithms, disk management topics, swap space management, RAID structures, and stable storage implementation. Magnetic disks are organized into platters, tracks, cylinders, and sectors. Solid state disks use flash memory or DRAM instead of magnetic platters. Disks can be attached directly to hosts or accessed over a network. Disk scheduling algorithms aim to minimize seek times and rotational latency when servicing multiple requests. RAID and swap space management improve reliability, performance and memory management respectively.
The document discusses Karnaugh mapping, which is a method for simplifying Boolean logic expressions. It provides guidelines for when different simplification methods like Boolean algebra or Karnaugh maps are best. Karnaugh maps are well-suited for problems with up to 6 variables and allow forming groups of cells to minimize logic. Examples show mapping logic terms to Karnaugh maps and identifying groupings to arrive at simplified expressions.
Virtual memory allows processes to have a virtual address space larger than physical memory. When a process accesses a page not in memory, a page fault occurs which is handled by reading the requested page from disk into a free frame. Page replacement algorithms select a victim page to replace when there are no free frames, such as FIFO which replaces the oldest page. The optimal page replacement algorithm OPT replaces the page that will not be used for the longest time in the future but cannot be implemented as it requires knowing the future.
This document discusses main memory management techniques used in operating systems. It covers topics like address binding, logical versus physical address spaces, dynamic loading, swapping, contiguous memory allocation, segmentation, paging, and page tables. The key points are:
1) Main memory management aims to efficiently allocate memory for processes while allowing processes to be swapped in and out of memory. It maps logical addresses to physical addresses through techniques like segmentation and paging.
2) Early techniques allocated processes to contiguous blocks of memory, but this led to fragmentation issues. Segmentation and paging allow non-contiguous allocation to avoid fragmentation.
3) Segmentation divides a process's memory into variable-sized segments that can be placed anywhere
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Preventing Dementia - The Basics Millions of results on Google…bilalpakweb
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Sleep Optimization
Manage Vascular Risk Factors
And MUCH more (see Deter Dementia for details)
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Deadlocks final
1. 1
Deadlocks (Galvin Notes 9th Ed.)
Chapter 7: Deadlocks
SYSTEM MODEL
DEADLOCK CHARACTERIZATION
o Necessary Conditions (Mutual Exclusion, Hold and Wait, No Preemption, Circular Wait)
o Resource-Allocation Graph
METHODS FOR HANDLING DEADLOCKS
DEADLOCK PREVENTION
o Mutual Exclusion
o Hold and Wait
o No Pre-emption
o Circular Wait
DEADLOCK AVOIDANCE
o Safe State
o Resource-Allocation Graph Algorithm
o Banker's Algorithm (Safety Algorithm, Resource-Request Algorithm/The Banker's Algorithm, An Illustrative Example)
DEADLOCK DETECTION
o Single Instance of Each Resource Type
o Several Instances of a Resource Type
o Detection-Algorithm Usage
RECOVERY FROMDEADLOCK
o Process Termination
o Resource Pre-emption (Selecting a victim, Rollback, Starvation)
Content
In a multiprogrammingenvironment, several processesmaycompete for a finite number of resources. A processrequests resources;if the resources are
not available at that time, the process enters a waiting state. Sometimes, a waitingprocess is never againable to change state, because the resources it
has requestedare heldbyother waitingprocesses. Thissituationis called a deadlock.
Althoughsome applications canidentifyprograms that maydeadlock, operating systems typicallydonot provide deadlock-prevention facilities, and
it remains the responsibilityof programmers to ensure that theydesign deadlock-free programs.
SYSTEM MODEL
A system consists of a finite number of resources to be distributed among a number of competing processes. The resources may be
partitioned into severaltypes (or classes), eachconsistingof some number of identical instances. CPU cycles, files, and I/O devices (such as
printers and DVD drives) are examples of resource types. If a systemhas two CPUs, then the resource type CPU has two instances. Similarly,
the resource type printer mayhave five instances. If a process requests an instance of a resource type, the allocationof any instance of the
type shouldsatisfythe request. For example, two printers maybe definedto be inthe same resource class if no one cares wh ich printer
prints which output.
Chapter 5 discussedvarious synchronizationtools, suchas mutex locks andsemaphores. These toolsare alsoconsidered system resources,
and theyare a commonsource of deadlock. However, a lock is typicallyassociatedwith protecting a specific data structure—that is, one lock
maybe used to protect accessto a queue, another to protect access to a linked list, and so forth. For that reason, each lock is typically
assigned its own resource class, and definition is not a problem.
A process must request a resource before using it andmust release the resource after using it. Under the normal mode of operation, a
process may utilize a resource in only the following sequence: 1) Request 2) Use 3) Release
The request andrelease of resources maybe system calls, as explained in Chapter 2. Examples are the request() andrelease()device, open()
and close()file, andallocate() and free()memorysystemcalls. Similarly, as we saw inChapter 5, the request andrelease of semaphores can
be accomplishedthrough the wait() andsignal() operations on semaphores or through acquire() and release() of a mutex lock. For each use
of a kernel-managedresource bya process or thread, the operating system checks to make sure that the processhas requestedandhas been
allocated the resource. A system table records whether eachresource is free or allocated. For eachresource that is allocated, the table also
records the process to which it is allocated. If a process requests a resource that is currentlyallocated to another process, it can be added to
a queue of processes waiting for this resource.
A set of processes is in a deadlockedstate wheneveryprocess in the set is waiting for anevent that canbe causedonlyby another process in
the set. A set of processes is ina deadlocked state when everyprocessinthe set is waitingfor anevent that canbe caused only by another
process inthe set. The resourcesmaybe either physical resources (for example, printers, tape drives, memory space, and CPU cycles) or
2. 2
Deadlocks (Galvin Notes 9th Ed.)
logical resources (for example, semaphores, mutex locks, andfiles). However, other types of events mayresult in deadlocks (for example, the
IPC facilities discussed in Chapter 3).
To illustrate a deadlocked state, consider a system withthree CD RW drives. Suppose each of three process es holds one of these CDRW
drives. If each process now requests another drive, the three processes will be in a deadlockedstate. Each is waiting for th e event “CD RW is
released,” whichcanbe causedonlybyone of the other waitingprocesses. This example illustrates a deadlockinvolving the same resource
type. Deadlocks mayalsoinvolve different resource types. For example, consider a system withone printer andone DVD drive. Suppose that
process Pi is holdingthe DVD and process Pj is holding the printer. If Pi requests the printer andPj requests the DVDdrive, a deadlock occurs.
Developers of multithreaded applications must remainaware ofthe possibilityof deadlocks. The locking tools presented in Ch apter 5 are
designedto avoidrace conditions. However, inusing these tools, developers must pay careful attention to how locks are acquired and
released. Otherwise, deadlock can occur, as illustrated in the dining-philosophers problem in Section 5.7.3.
DEADLOCK CHARACTERIZATION
In a deadlock, processes never finish executing, and system resources are tiedup, preventingother jobs fromstarting. Before we discuss th e various
methods for dealing with the deadlock problem, we look more closely at features that characterize deadlocks.
Necessary Conditions
A deadlock situation can arise if the following four conditions hold simultaneously in a system:
o Mutual exclusion – At least one resource must be heldin 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 delayeduntil the resource has been released.
o Hold and wait – A process must be holding at least one resource andwaiting to acquire additional resources that are currentlybeing
held by other processes.
o No pre-emption – Resources cannot be preempted;that is, a resource canbe released onlyvoluntarily by the process holding it,
after that process has completed its task.
o Circular wait – A set {P0, P1, ..., Pn} of waiting processes must exist suchthat P0 is waiting for a resource heldbyP1, 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.
We emphasize that allfour conditions must holdfor a deadlock to occur. The circular-wait conditionimplies the hold-and-wait condition, so
the four conditions are not completelyindependent. We shall see in Section 7.4, however, that it is useful to consider each condition
separately.
Resource-Allocation Graph
3. 3
Deadlocks (Galvin Notes 9th Ed.)
Given the definitionof a resource-allocationgraph, it canbe shownthat, ifthe graphcontains nocycles, then noprocess in the system is
deadlocked. If the graph does containa cycle, then a deadlock MAY exist.
If each resource type has exactlyone instance, thena cycle implies that a deadlockhas occurred. If the cycle involves only a set of resource
types, eachof which has onlya single instance, thena deadlockhas occurred. Eachprocessinvolvedinthe cycl e is deadlocked. Inthis case, a
cycle in the graphis botha necessaryanda sufficient condition for the existence of deadlock.
If each resource type has severalinstances, thena cycle doesnot necessarilyimplythat a deadlock has
occurred. In this case, a cycle inthe graphis a necessarybut not a sufficient conditionfor the existence
of deadlock.
To illustrate this concept, we returnto the resource-allocationgraphdepicted in Figure 7.1. Suppose
that process P3 requests an instance of resource type R2. Since noresource instance is currently
available, we add a request edge P3→ R2 to the graph(Figure 7.2). At this point, two minimal cycles exist
in the system:
P1 → R1 → P2 → R3 → P3 → R2 → P1
P2 → R3 → P3 → R2 → P2
Processes P1, P2, and P3 are deadlocked. ProcessP2 is waiting for the resource R3,
which is heldbyprocess P3. Process P3 is waitingfor either process P1 or processP2 to
release resource R2. Inaddition, processP1 is waiting for process P2 to release resource
R1.
Now consider the resource-allocationgraphinFigure 7.3. In this example, we also
have a cycle: P1 → R1 → P3 → R2 → P1
However, there is no deadlock. Observe that processP4 mayrelease its instance of
resource type R2. That resource canthen be allocatedto P3, breakingthe cycle.
In summary, if a resource-allocationgraphdoes not have a cycle, thenthe systemis
not in a deadlockedstate. If there is a cycle, then the system mayor maynot be ina
deadlockedstate. This observation is important when we dealwith the deadlock
problem.
METHODS FOR HANDLING DEADLOCKS
We can deal with the deadlock problem in one of three ways:
We can use a protocol to prevent or avoid deadlocks, ensuring that the system will never enter a deadlocked state.
We can allow the system to enter a deadlocked state, detect it, and recover.
We can ignore the problem altogether and pretend that deadlocks never occur in the system (and restart manually).
The third solution is the one used by most operating systems, in cluding Linux and Windows.
Next, we elaborate brieflyon eachof the three methods for handling deadlocks. Then, inSections 7.4 through 7.7, we present detailed algorithms.
To ensure that deadlocks never occur, the system can useeither a deadlock preventionor a deadlock-avoidance scheme. Deadlock prevention provides
a set of methods to ensure that at least one ofthe four necessaryconditions cannot hold. These methods prevent deadlocks by constraining how
requests for resources can be made.
Deadlock avoidance requires that the operatingsystem be givenadditional information in advance concerning which resources a process will
request and use duringits lifetime. Withthis additional knowledge, the operating system can decide for eachrequest whether or not the process should
wait. To decide whether the current request canbe satisfiedor must be delayed, the system must consider the resources curre ntly available, the
resources currently allocated to each process, and the future requests and rel eases of each process.
If a systemdoesnot employeither a deadlock-preventionor a deadlockavoidance algorithm, thena deadlocksituationmayarise. In this environment,
the systemcanprovide analgorithmthat examines the state of the system to determine whether a deadlock hasoccurred and an algorithm to recover
from the deadlock (if a deadlock has indeed occurred). We discuss these issues in Section 7.6 and Section 7.7.
4. 4
Deadlocks (Galvin Notes 9th Ed.)
DEADLOCK PREVENTION
For a deadlock to occur, each ofthe four necessary conditions must hold. Byensuring that at least one of these conditions cannot hold, we can
prevent the occurrence of a deadlock.
Mutual Exclusion: In general, we cannot prevent deadlocks by denying the mutual -exclusion condition, because some resources are
intrinsicallynonsharable. For example, a mutex lock cannot be simultaneouslysharedbyseveralprocesses. Sharable resources, incontrast, do
not require mutuallyexclusive access and thus cannot be involvedina deadlock. Read-onlyfilesare a goodexample of a sharable resource. If
several processesattempt to open a read-onlyfile at the same time, theycanbe granted simultaneous access to the file. A process never
needs to wait for a sharable resource.
Hold and Wait: To ensure that the hold-and-wait conditionnever occurs in the system, we must guarantee that, whenever a process requests
a resource, it does not holdanyother resources. One protocol that we canuse requires each process to request and be alloca ted all its
resources before it begins execution. We canimplement this provision byrequiring that system calls requesting resources for a process
precede all other system calls.
An alternative protocol allows a process to request resources onlywhenit hasnone. A process mayrequest some resources and use them.
Before it can request any additional resources, it must release all the resources that it is currently allocated.
To illustrate the difference between these two protocols, we consider a process that copies data from a DVDdrive to a file ondisk, sorts the
file, andthen prints the results to a printer. Ifallresourcesmust be requestedat the beginningof the process, thenthe process must initially
request the DVD drive, diskfile, andprinter. It will holdthe printer for its entire execution, even thoughit needs the printer only at the end.
The second method allows the processto request initiallyonlythe DVDdrive anddisk file. It copiesfrom the DVD drive to the disk and then
releases both the DVDdrive andthe diskfile. 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.
Both these protocols have twomain disadvantages. First, resource utilizationmaybe low, since resources maybe allocated but unused for
a long period. Second, starvationis possible. A process that needs several popular resources mayhave to wait indefinitely, because at least one
of the resources that it needs is always allocated to some other process.
No Pre-emption: The thirdnecessaryconditionfor deadlocks is that there be no preemptionof resources that have alreadybeenallocated. To
ensure that this condition does not hold, we can use the following protocol. If a process is holding so me resources and requests another
resource that cannot be immediatelyallocatedto it (that is, the processmust wait), thenallresources the process is curre ntly holding are
preempted. In otherwords, these resources are implicitlyreleased. The preempted resources are addedto the list of resources for which the
process is waiting. The process willbe restartedonlywhen it canregain its old resources, as well as the new ones that it is requesting.
Alternatively, if a process requests some resources, we first checkwhether theyare available. If theyare, we allocate them. If they are
not, we check whether theyare allocatedto some other process that is waiting for additional resources. If so, we preempt th e desired
resources from the waitingprocessand allocate themto the requestingprocess. If the resources are neither available nor held by a waiting
process, the requestingprocess must wait. While it is waiting, some of its resources maybe preempted, but onlyif another p rocess requests
them. A process canbe restartedonlywhen it is allocated the newresourcesit is requesting andrecovers anyresources that were preempted
while it was waiting.
This protocol is oftenapplied to resources whose state can be easilysavedand restored later, suchas CPU registers and memory space. It
cannot generally be applied to such resources as mutex locks and semaphores.
Circular Wait: The fourth and final conditionfor deadlocks is the circular-wait condition. One way to ensure that this
conditionnever holds is to imposea total ordering of all resource types and to require that each process requests
resources inanincreasing order of enumeration. To illustrate, we let R = {R1, R2, ..., Rm} be the set of resource types.
We assignto each resource type a unique integer number, which allows us to compare two resources and to
determine whether one precedes another inour ordering. Formally, we define a one -to-one
function F:R→N, where N is the set of natural numbers. For example, if the set of re source
types R includes tape drives, diskdrives, andprinters, thenthe functionF might be defined as
follows:
We can now consider the following protocol to prevent deadlocks: Each process can
request resources onlyinan increasing order of enumeration. That is, a process can initially
request anynumber of instances of a resource type —say, Ri . After that, the process can
request instances ofresource type Rj if and onlyifF(Rj ) > F(Ri ). For example, using the function
definedpreviously, a process that wants to use the tape drive and printer at the same time
must first request the tape drive andthenrequest the printer. Alternatively, we canrequire that
a process requesting aninstance of resource type Rj must have released anyresources Ri such
that F(Ri ) ≥ F(Rj ). Note alsothat if severalinstances ofthe same resource type are needed, a
single request for allof them must be issued. If these twoprotocols are used, thenthe circular-
wait conditioncannot hold. We candemonstrate this fact byassuming that a circular wait exists
(proof by contradiction---SKIPPED).
We can accomplishthis scheme in an applicationprogram bydeveloping anorderingamong
all synchronizationobjects inthe system. All requests for synchronizationobjects must be made
in increasingorder. For example, if the lockordering in the Pthreadprogram shown in Figure 7.4
was F(first mutex) = 1 and F(secondmutex) = 5 then thread twocouldnot request the locks out
of order. Keepin mindthat developing an ordering, or hierarchy, does not in itself prevent
deadlock. It is upto applicationdevelopers to write programs that follow the ordering. Also
note that the functionF should be defined according to the normalorder of usage of the resources in a system. For example, because the tape
drive is usually needed before the printer, it would be reasonable to define F(tape drive)<F(printer).
Althoughensuring that resources are acquiredinthe proper order is the responsibilityof applicationdevelopers, certain so ftware can be
usedto verifythat locks are acquiredinthe proper order andto give appropriate warnings whenlocks are acquiredout of order anddeadlock
is possible. One lock-order verifier, whichworks on BSD versions of UNIXsuchas FreeBSD, is knownas witness. Witness uses mutual-exclusion
locks to protect critical sections, as describedinChapter 5. It works bydynamicallymaintaining the relationship oflock orders ina system. Let’s
5. 5
Deadlocks (Galvin Notes 9th Ed.)
use the program shown inFigure 7.4 as anexample. Assume that thread one is the first to acquire the locks anddoes so in th e order (1) first
_mutex, (2) second_mutex. Witness records the relationshipthat first mutex must be acquired before second mutex. If thread two later
acquires the locks out of order, witness generates a warning message onthe systemconsole. It is also important to note that imposing a lock
ordering does not guarantee deadlockprevention if locks can be acquired dynamically (Rest of the content relevant, but SKIPPED at present,
including other code segment, and a short “Do it yourself” exercise).
DEADLOCK AVOIDANCE
As we saw, deadlock-prevention algorithms prevent deadlocks bylimitinghowrequests canbe made. The limits ensure that at least one of the necessary
conditions for deadlockcannot occur. Possible side effects of preventing deadlocks bythis method, however, are low device u tilization and reduced
system throughput.
An alternative methodfor avoidingdeadlocks is to require additional information about how resources are to be requested. Fo r example, in a system
with one tape drive andone printer, the systemmight needto know that process Pwill request first the tape drive and thenthe printer before releasing
both resources, whereasprocessQ will request first the printer andthen the tape drive. Withthis knowledge of the complete sequence ofrequests and
releases for each process, the system can decide for each request whether or not the processshould wait inorder to avoida possible future deadlock.
Each request requires that inmaking this decision the system consider the resources currently available, the resources curre ntly allocated to each
process, and the future requests and releases of each process.
The various algorithms that use this approach differ in the amount andtype of information required. The simplest andmost useful modelrequires that
each process declare the maximumnumber ofresources ofeachtype that it mayneed. Giventhis a priori information, it is possible to construct an
algorithmthat ensures that the system will never enter a deadlocked state. A deadlock-avoidance algorithm dynamically examines the resource -
allocationstate to ensure that a circular-wait conditioncannever exist. The resource allocation state is definedbythe number ofavailable andallocated
resources and the maximum demands of the processes. In the following sections, we explore two dead lock-avoidance algorithms.
Safe State
7. 7
Deadlocks (Galvin Notes 9th Ed.)
DEADLOCK DETECTION
If a systemdoesnot employeither a deadlock-preventionor a deadlockavoidance algorithm, thena deadlocksituationmayoccur. Inthis environment,
the system may provide:
An algorithm that examines the state of the system to determine whether a deadlock has occurred
An algorithm to recover from the deadlock
In the followingdiscussion, we elaborate onthese two requirements as theypertainto systems withonlya single instance of eachresource type, as well
as to systems with several instancesof eachresource type. At this point, however,we note that a detection -and-recoveryscheme requiresoverheadthat
includes not onlythe run-time costs of maintaining the necessaryinformationandexecuting the detection algorithm but also the potential losses
inherent in recovering from a deadlock.
Single Instance of Each Resource Type
If all resources have onlya single instance, then we candefine a deadlock detectionalgorithmthat usesa variant of the resource-allocation graph, called
a wait-for graph. We obtainthisgraphfrom the resource-allocation graph byremoving the resource nodes andcollapsing the appropriate edges. More
precisely, anedge from Pi to Pj in a wait-for graph implies that process Pi is waitingfor processPj to release a resource that Pi needs. An edge Pi → Pj
exists ina wait-for graphifandonlyif the correspondingresource allocationgraphcontains two edges Pi → Rq and Rq → Pj for some resource Rq . In
Figure 7.9, we present a resource-allocation graph and the corresponding wait-for graph.
As before, a deadlock exists in the system if andonlyif the wait-for graphcontains a cycle. To detect deadlocks, the system needs to maintain the
wait for graphandperiodicallyinvoke analgorithm that searches for a cycle inthe graph. An algorithmto detect a cycle in a graphrequires an order of
n2 operations, where n is the number of vertices in the graph.
Several Instances of a Resource Type
The wait-for graph scheme is not applicable to a resource-allocation system withmultiple instances ofeachresource type. We turn now to a deadlock
detection algorithm that is applicable to sucha system. The algorithm employs several time-varyingdata structures that are similar to those used inthe
banker’s algorithm:
• Available. A vector of length m indicates the number of available resources ofeachtype.
• Allocation. An n × m matrix defines the number of resources of each type currentlyallocated to each process.
• Request. An n × m matrix indicates the current request of each process. If Request[i][j] equals k, thenprocess Pi is requesting k more instances of
resource type Rj .
The ≤ relation betweentwo vectors is defined as in Section7.5.3. To simplifynotation, we again treat the rows in the matricesAllocationand Request as
vectors;we refer to them as Allocationi and Requesti . The detectionalgorithm describedhere simply investigateseverypossible allocationsequence for
the processes that remainto be completed. Compare this algorithm withthe banker’s algorithm of Section7.5.3.
8. 8
Deadlocks (Galvin Notes 9th Ed.)
Detection-Algorithm Usage
When should we invoke the detectionalgorithm? The answer depends ontwo factors:
How oftenis a deadlock likelyto occur?
How manyprocesses willbe affected bydeadlock when it happens?
If deadlocks occur frequently, then the detectionalgorithmshould be invokedfrequently. Resources allocated to deadlocked p rocesseswill be idle until
the deadlock can be broken. In addition, the number of processes involved in the deadlock cycle may gro w.
Deadlocks occur onlywhensome processmakes a request that cannot be grantedimmediately. Thisrequest maybe the final requ est that completes
a chain ofwaiting processes. In the extreme, then, we can invoke the deadlock detectionalgorithm everytime a request for allocation cannot be granted
immediately. In thiscase, we can identifynot onlythe deadlocked set of processesbut alsothe specific process that “caused” the deadlock. (In reality,
each of the deadlocked processesis a linkinthe cycle in the resource graph, so all ofthem, jointly, caused the deadlock.) If there are many different
resource types, one request maycreate manycycles inthe resource graph, eachcycle completedbythe most recent request and “caused” by the one
identifiable process.
Of course, invokingthe deadlock-detection algorithm for everyresource request will incur considerable overhead in computation time. A less
expensive alternative is simplyto invoke the algorithm at definedintervals—for example, once per hour or whenever CPU utilization drops below 40
percent. (A deadlock eventuallycripples systemthroughput and causes CPU utilizationto drop.) If the detection algorithm is invokedat arbitrary points
in time, the resource graphmaycontainmanycycles. Inthiscase, we generallycannot tell which of the many deadlocked processes “caused” the
deadlock.
RECOVERY FROM DEADLOCK
When a detectionalgorithmdetermines that a deadlock exists, severalalternatives are available. One possibility is to inform the operator that a
deadlock hasoccurred and to let the operator deal withthe deadlock manually. Another possibility is to let the system recover from the deadlock
automatically. There are two options for breaking a deadlock. One is simplyto abort one or more processes to break the circular wait. The other is to
preempt some resources from one or more of the deadlocked processes.
Process Termination
To eliminate deadlocks byaborting a process, we use one of two methods. In both methods, the system reclaims all resources allocated to the
terminated processes.
Abort all deadlocked processes. This methodclearlywill break the deadlockcycle, but at great expense. The deadlocked proce sses may have
computedfor a long time, andthe results ofthese partial computations must be discarded andprobably will have to b e recomputed later.
Abort one process at a time until the deadlock cycle is eliminated. This methodincurs considerable overhead, since after each process is
aborted, a deadlock-detection algorithm must be invoked to determine whether any processes are still deadlocked.
Aborting a process maynot be easy. If the process was inthe midst of updatinga file, terminating it will leave that file in anincorrect state. Similarly, if
the processwas in the midst of printing data on a printer, the system must reset the printer to a correct state before printingthe next job. If the partial
terminationmethodis used, then we must determine which deadlockedprocess (or processes) shouldbe terminated. This determi nation is a policy
decision, similar to CPU-schedulingdecisions. The questionis basicallyaneconomic one; we should abort those processes whose termination will incur
the minimum cost. Unfortunately, the term minimum cost is not a precise one. Many factors may affect which process is chosen, including:
1. What the priority of the process is
2. How long the process has computed and how much longer the process will compute before completing its designated task
3. How many and what types of resources the process has used(for example, whether the resources are simple to preempt)
4. How many more resources the process needs in order to complete
5. How many processes will need to be terminated 6. Whether the process is interactive or batch
Resource Preemption
To eliminate deadlocks using resource preemption, we successivelypreempt some resources from processes andgive these resources to other processes
until the deadlock cycle is broken. If preemption is required to deal with deadlocks, then three issues need to be addressed:
Selecting a victim. Which resources andwhich processes are to be preempted?As inprocesstermination, we must determine the order of
preemption to minimize cost. Cost factors mayinclude suchparameters as the number of resources a deadlockedprocessis hold ing and the
amount of time the process has thus far consumed.
Rollback. If we preempt a resource from a process, what should be done with that process? Clearly, it cannot continue with its normal
execution;it is missing some neededresource. We must roll backthe process to some safe state and restart it from that state. Since, in
general, it is difficult to determine what a safe state is, the simplest solutionis a total rollback:abort the process and thenrestart it. Althoughit
is more effective to roll back the process onlyas far as necessary to break the deadlock, this method requires the system to keep more
information about the state of all running processes.
Starvation. How do we ensure that starvation will not occur? That is, howcanwe guarantee that resources will not always be preemptedfrom
the same process?
In a system where victim selectionis basedprimarilyon cost factors, it mayhappenthat the same process is always pickedas a victim. As a result, this
process never completesits designatedtask, a starvationsituation anypractical systemmust address. Clearly, we must ensure that a process can be
pickedas a victim only a (small) finite number of times. The most common solution is to include the number of rollbacks in t he cost factor.
9. 9
Deadlocks (Galvin Notes 9th Ed.)
SUMMARY
A deadlockedstate occurs whentwo or more processes are waiting indefinitelyfor anevent that canbe causedonlybyone of th e waiting processes.
There are three principal methods for dealing withdeadlocks:
Use some protocol to prevent or avoiddeadlocks, ensuringthat the systemwill never enter a deadlockedstate.
Allowthe system to enter a deadlockedstate, detect it, andthenrecover.
Ignore the problem altogether andpretendthat deadlocks never occur inthe system.
The thirdsolution is the one usedbymost operating systems, including Linux andWindows.
A deadlock canoccur onlyif four necessaryconditions holdsimultaneouslyinthe system:mutual exclusion, holdandwait, no p reemption, andcircular
wait. To prevent deadlocks, we canensure that at least one of the necessaryconditions never holds.
A method for avoiding deadlocks, rather thanpreventing them, requires that the operating system have a priori informationabout how eachprocess
will utilize system resources. The banker’s algorithm, for example, requires a priori informationabout the maximum number of eachresource class that
each process mayrequest. Using this information, we candefine a deadlock avoidance algorithm.
If a system does not employa protocol to ensure that deadlocks will never occur, then a detection-and-recoveryscheme maybe employed. A deadlock
detection algorithm must be invokedto determine whether a deadlock has occurred. If a deadlock is detected, the systemmust recover either by
terminatingsome ofthe deadlockedprocesses or bypreemptingresources from some of the deadlockedprocesses.
Where preemption is used to dealwith deadlocks, three issues must be addressed:selectinga victim, rollback, and starvation . In a systemthat selects
victims for rollback primarilyonthe basisof cost factors, starvationmayoccur, andthe selectedprocess cannever complete its designated task.
Researchers have arguedthat none of the basic approachesalone is appropriate for the entire spectrum ofresource-allocationproblems inoperating
systems. The basic approaches can be combined, however, allowing us to select an optimal approach for eachclass of resources in a system.
To be cleared
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