Eksamen Flashcards

Question

What is a Data processing action?

Answer 1

The processor performs some arithmetric or logical operation on data.

Answer 2

An instruction may specify that the sequence of execution be altered. ## Footnote For example: The processor may fetch an instruction from location 149, which specifies that the next instruction will be from location 182. The processor sets the program counter to 182. Thus, on the next fetch stage, the instruction will be fetched from location 182 rather than 150.

Answer 3

A message which interrupts the current program to do something else, after which it will return to where it was interrupted.

Answer 4

There are two ways to deal with it: 1. Sequential interrupt processing (also called a disabled interrupt) * Ignores all interrupts until this one is done. 2. Nested interrupt processing. * If an interrupt is received while another interrupt is being handled, the newest interrupt will take priority.

Answer 5

It is a device for staging the movement of data between main memory and processor registers to improve performance.

Answer 6

SMP (symmetric multiprocessing) is the processing of programs by multiple processors that share a common operating system and memory.

Answer 7

When there are multiple processors inside the same chip. (They each have their individual cache storages, but also share at least one cache that is accessible by all of them.)

Answer 8

Multiprocessors doesn't share a cache and are not in the same chip but multicores do.

Answer 9

If a particular memory location is referenced at a particular time, then it is likely that nearby memory locations will be referenced in the near future. ## Footnote In this case it is common to attempt to guess the size and shape of the area around the current reference for which it is worthwhile to prepare faster access.

Answer 10

If memory has been accessed recently it is already in cache and is therefore fast to use again.

Answer 11

A listing of a sequence of instructions.

Answer 12

A batch job is a computer program or set of programs processed in batch mode. This means that a sequence of commands to be executed by the operating system is listed in a file (often called a batch file, command file, orshell script) and submitted for execution as a single unit.

Answer 13

A user at a terminal logs on to the system.

Answer 14

1. New. 2. Ready. 3. Running. 4. Blocked/Waiting. 5. Exit.

Answer 15

A process that cannot execute until some event occours, like the completion of I/O operation.

Answer 16

A process that has just been created, but has not yet been admitted to the pool of executable processes by the OS. ## Footnote Typically, a new process has not yet been loaded into the main memory, although its process control block has been created.

Answer 17

A process is in the Exit State when it has been released from the pool of executeable processes by the OS. Either because it halted or because it aborted for some reason.

Answer 18

Preemption is defined to be the claiming of a ressource from another process that is using it.

Answer 19

When none of the processes in main memory is in the Ready state, the OS swaps one of the blocked processes from the disk into a suspend queue.

Answer 20

Concurrency is when two tasks can start, run, and complete in overlapping time periods. It doesn't necessarily mean they'll ever both be running at the same instant. Eg. multitasking on a single-core machine. Can be implemented using software such as an OS or Virtualization software

Answer 21

Parallelism is when tasks literally run at the same time, eg. on a multicore processor. ## Footnote To implement parallelism hardware such as a multicore processor is needed.

Answer 22

A function or action implemented as a sequence of one or more instructions that appears to be indivisible. The sequence of instructions is guaranteed to execute as a group. Atomicity guarantees isolation from concurrent processes.

Answer 23

Code region that is vulnerable to race conditions by accessing shared resources

Answer 24

A collection of threads *P* are in a deadlock state if every thread in *P* is waiting for an event that can only be generated by another thread in *P*.

Answer 25

Deadlock-like state with no progress although with no cyclic wait.

Answer 26

The requirement that when one process is in a critical region that accesses shared ressources, no other process may be in a critical region that accesses any of those shared ressources.

Answer 27

There are three ways: 1. Disabling interrupts 1. Pro's: effective. 2. Con's: too effective, potentially very disruptive. 2. Software locks 1. Pro's: portable, less disruptive. 2. Con's: inefficient, vulnerable to compiler optimisation. 3. Hardware locks 1. Pro's: (more) efficient 2. Con's: less portable... still inefficient(!)

Answer 28

In computer science, a lock or mutex (from mutual exclusion) is a synchronization mechanism for enforcing limits on access to a resource in an environment where there are many threads of execution. A lock is designed to enforce a mutual exclusion concurrency control policy.

Answer 29

A situation in which multiple threads or processes read and write a shared data item and the final result depends on the relative timing of their execution.

Answer 30

* Situation where a thread never acquires a resource because other processes always get it first. * Typical problem for systems with *priorities*.

Answer 31

Priorities are the measurement of the relative importance of the task. * Hard real-time tasks may have an "absolute" priority, with the system failing if a deadline is missed. * If the system is to continue to run no matter what, then both hard and soft real-time tasks may be assigned relative priorities as a guide to the scheduler.

Answer 32

A semaphore is a variable or abstract data type that is used for controlling access, by multiple processes, to a common resource in a parallel programming or a multi user environment. A useful way to think of a semaphore is as a record of how many units of a particular resource are available, coupled with operations to safely (i.e., without race conditions) adjust that record * Semaphores which allow an arbitrary resource count are called counting semaphores * Semaphores which are restricted to the values 0 and 1 (or locked/unlocked, unavailable/available) are called binary semaphores.

Answer 33

A (very) simple semaphor. * Two states (binary): locked and unlocked. * lock: locks an unlocked mutex. * unlock: unlocks a locked mutex.

Answer 34

A data type that is used to block a process or thread until a particular condition is true.

Answer 35

* Language construct ensuring mutual exclusion. * Encapsulates critical regions. * Guaranteed by the compiler.

Answer 36

Mutual exclusion mechanism in which a process executes in an loop waiting for the value of a lock variable to indicate availability.

Answer 37

An event flag is a process synchronization primitive. ## Footnote It has two possible states: "set" or "cleared".

Answer 38

* Relocation * Protection * Sharing * Logical organization * Physical organization

Answer 39

Processes should be able to be relocated everywhere in memory and still work. This raises technical concerns related to adressing

Answer 40

A process cannot be allowed to access other processes data.

Answer 41

Despite protection, we can still allow processes to access shared data.

Answer 42

Main memory is organized as a linear or one-dimensional address space that consists of sequence of bytes or words. Secondary memory at its physical level is similarly organized. Most of the programs are organized into modules.

Answer 43

Computer memory is organized into two levels: 1. **Main memory** - Main memory is a volatile memory and it provides fast access at relatively high cost. 2. **Secondary memory** - Secondary memory is a non-volatile memory and it is slower and cheaper than main memory.

Answer 44

Because it's impossible to know how many ressources a process needs before runtime.

Answer 45

For example if a number of processes are executing the same program it is advantageous to allow each process to access the same copy of the program rather than have its own separate copy.

Answer 46

It creates less internal fragmentation because you get a memory block which fits better to the size you need.

Answer 47

In simple paging you need all information loaded into main memory to run a process as opposed to virtual memory paging that can run one page at a time.

Answer 48

When the processor spends most of the time handling page faults instead of "real" instructions.

Answer 49

Principle of locality er nødvendigt for at gøre det muligt at gætte hvad der skal bruges om lidt hvorved det undgås at smide pages væk der skal bruges snart, og derved undgås thrashing.

Answer 50

* Modified bit * Present bit * (hvis true, så har den også frame no.) * Other control bits * for example: bits used in algorithms by the OS.

Answer 51

Translation Lookaside Buffer

Answer 52

A TLB is an associative cache of page table entries and is used to avoid the necessity of accessing the main memory every time a virtual address is mapped. It improves virtual address translation speed.

Answer 53

An algorithm that is 'not set in stone'.

Answer 54

1. Demand paging. 2. Prepaging.

Answer 55

A page gets loaded into main memory when a process needs it.

Answer 56

Loads pages into memory before they have to be used. (It's not feasible since you rarely knows which pages is needed except at the start of a program)

Answer 57

In computing, resident set size (RSS) is the portion of memory occupied by a process that is held in main memory (RAM).

Answer 58

All the pages a process has accessed within a specific time interval.

Answer 59

It consists of a set of locations, defined by sequentially numbered addresses. ## Footnote Each location contains a bit pattern that can be interpreted as either an instruction or data.

Answer 60

**What is it?** A datastructure that contains sufficient information so that it is possible to interrupt a running process and later resume execution as if the interruption had not occurred. **Why is it needed?** It is the key tool that enables the OS to support multiple processes and to provide for multiprocessing.

Answer 61

It happens for three reasons: * It is created upon login. * It is spawned by an existing process. * It is created when a new job is started( by OS, user, ...).

Answer 62

1. Assign a unique process identifier to the new process. 2. Allocate space for the process. 3. Initialize the process control block. 4. Set the appropriate linkages. 1. For example: If the OS maintains each scheduling queue as a linked list, then the new process must be put in the Ready or Ready/Suspended list. 5. Create or expand other data structures. 1. For example: The OS may maintain an accounting file on each process to be used subsequently for billing and/or performance assessment purposes.

Answer 63

1. Save the relevant data for the current program. 2. Pick the program with the highest-priority, that is ready to run. 3. Re-instate the new program's data. 4. Execute.

Answer 64

It happens for four reasons: * Program completed. * Lack of resources: * Time (hard max limit). * Memory. * ... * Error condition * Invalid instruction. * Arithmetic error. * ... * Parent request/parent termination.

Answer 65

An operating system (OS) is software that manages computer hardware and software resources and provides common services for computer programs.

Answer 66

It is a collaction of process related data: * Process Control Block. * Program (code). * Stack. * Heap (user data).

Answer 67

Thread Control Block (TCB) is a data structure in the operating system kernel which contains thread-specific information needed to manage it. The TCB is "the manifestation of a thread in an operating system". An example of information contained within a TCB is: * Stack pointer: Points to thread's stack in the process. * Program counter. * State of the thread (running, ready, waiting, start, done). * Thread's register values. * Pointer to the Process control block (PCB) of the process thread lives on. The Thread Control Block acts as a library of information about the threads in a system. Specific information is stored in the thread control block highlighting important information about each process.

Answer 68

* Split of tasks that can be executed concurrently on shared processors. * Implemented by rapid switching between tasks. * Common in modern OS's

Answer 69

* Split of processes into several threads. * Always at least one thread pr. process (even in non-multithreaded systems)

Answer 70

**Performance** * Low-cost thread creation, switching, and termination. * Efficient inter-thread communication (no need to involve kernel) * Can utilise multi-processor/multi-core platforms. **Better architecture/design** * Easy split into foreground and background work. * Asynchronous processing. * Modularity.

Answer 71

Controls execution of the processors. The kernel manages: * Thread scheduling. * Process switching. * Exception and interrupt handling. * Mulitprocessor synchronization. Unlike the rest of the Executive and the user level, the Kernel's own code does not run in threads. It contains the most frequently used functions in the OS and, at a given time, other portions of the OS currently in use.

Answer 72

**Implement in kernel-space (aka. obvious choice)** * Modify/extend kernel to support threads. * Pro's: better handling of blocked threads. **Implement in user-space** * User-space library for thread creation, switching, termination, ... * Pro's: performance (kernel not involved), portable. * Con's: blocked threads. **Implement in both kernel- and user-space (aka. complex choice)** * Mainly in user-space. * Mapping user-space threads to (fewer) kernel-space threads. * Pro's: all of the above. * Con's: complex implementation, complex management.

Answer 73

Make sure that only one thread at a time accesses shared resources * Updating a shared resource happens atomically (Event, or sequence of events, that happen(s) uninterruptedly). * Critical regions are executed under mutual exclusion.

Answer 74

A "no-op" loop waiting for a condition to be fulfilled: a loop where the executing thread *actively* waits for access to the critical region.

Answer 75

A better alternative to Busy Wait. Let another thread work while we wait * Block the thread until access to the resource can be achieved. * Wake the thread when leaving the critical region. * Leave the hard work to the OS.

Answer 76

* Thread that does not run until it recieves a signal from another thread. * Special state: blocked. * Not allocated CPU time. * OS keeps account of blocked threads and why they are blocked.

Answer 77

If all four of the conditions are met, Deadlock can happen. * Mutual Exclusion * Resources cannot be shared * No preemption * Resources cannot be taken away from a thread * Hold-and-wait * Threads may always try to take more resources * Circular Wait * There is a circular dependency of resources

Answer 78

Break one of Coffman's conditions: * **Avoid**: Mutual exclusion * Make resources shareable. * Critical regions are *always* un-shareable resources. * **Allow**: Preemption * Allow pre-emption of resources (forcibly removing resources). * Only possible (safe) if state of resources can be re-created. * **Avoid**: Hold-and-wait * Require all resources to be allocated at once. * Waste of resources. * Potentially a long wait. * **Avoid**: Circular wait * Resources are allocated in a specific order. * All resources must be known beforehand.

Answer 79

* When a low-priority thread blocks a high(er)-priority thread. * Happens when a higher-priority thread needs a resource held by a lower-priority thread.

Answer 80

Nogle der har en god forklaring, der også kommer ind på de forskellige måder at undgå dead/live-lock?

Answer 81

A state in which there is at least one allocation sequence that allows all threads to finish without deadlock.

Answer 82

It defines a specific physical memory cell in primary memory.

Answer 83

References to memory location independently of physical location. * Indirection ("Every software problem can be solved by adding another layer of indirexction" (Steven M. Bellovin)). * Virtual addresses are mapped to physical addresses at runtime . * Requires hardware support (MMU); often (always) integrated into CPU. * Virtual addressing is transparent to the programmer.

Answer 84

A logical address defined relative to a known base address. * Base address is a known physical address. * Relative address is the offset to the base address. * The physical address is the sum of base address and offset.

Answer 85

A storage allocation scheme in which secondary memory can be addressed as though it were part of main memory. * The addresses a program may use to reference memory are distinguished from the addresses the memory system uses to identify physical storage sites, and program-generated addresses are translated automatically to the corresponding machine addresses. * The size of virtual storage is limited by the addressing scheme of the computer system and by the amount of secondary memory available and not by the actual number of main storage locations.

Answer 86

* Process memory is split into a number of pages of fixed size. * Physical memory is split into a number of pages frames (da: rammer) * Pages are placed in page frames. * Page frames have same fixed size as pages. * OS places pages in frames and maintains information about placement in a page table. * OS maintains one page table pr. process. * Requires MMU support.

Answer 87

* Memory can be shared by letting different page tables refer to the same frame. * Same physical memory in two different address spaces (one per process). * Shared frames does not necessarily imply same (local) page numbers. * Shared memory useful for: * IPC (Inter-process communication). * Sharing of program code, libraries (DLL, .so).

Answer 88

Memory is split into segments * Separate address space (programmer defined). * Defined size, access rights, etc. * Defined as start address (base address) and length. * Logical address on the form . * Segment table maps segment number to start address og segment length.

Answer 89

**Memory management:** * Static allocation, uniform/nom-uniform size. * Dynamic allocation. * Paged and/or segmented memory

Answer 90

A Memory Management Unit, is a computer hardware unit having all memory references passed through itself, * Primarily performs the translation of virtual memory addresses to physical addresses. * It is usually implemented as part of the central processing unit (CPU).

Answer 91

Allocation in blocks of pre-determined fixed size. * Main challenges: block size, process placement. * Main advantages: easy to implement, little overhead. * Main disadvantages: inflexible, inefficient.

Answer 92

Every block (partition) has same size. **Pros** * Easy to implement/manage. * Little/no OS overhead. **Cons:** * Internal fragmentation (a lot). * Inflexible (fixed no of proc.). * Big programs problematic (overlay).

Answer 93

Split memory into blocks of different size. * Requires strategy for allocation of processes to blocks. * Smallest-fit * May result in processes waiting for blocks of the "appropriate" size. * Smallest-available * May result in more internal fragmentation * Problem: memory requirement must be known beforehand

Answer 94

* Allocation in blocks dynamically sized during load-time. * OS allocates appropriate block * OS maintains start address and length. * Processes must specify memory requirements at start-up.

Answer 95

* Best-fit: find block that is closest in size. (Usually the best strategy) * First-fit: find first block big enough. * Next-fit: find first block big enough, after last placed block.

Answer 96

**Pros** * No (visible) internal fragmentation. * Adapts to current needs (few big or many small processes). **Cons:** * Memory requirements must be known beforehand. * Compaction * Relocation of processes to reduce external fragmentation. * Similar to de-fragmentation of harddisks. * Compaction without virtual memory requires re-writing of addresses in program code and data (pointers).

Answer 97

* Internal fragmentation is the wasted space within each allocated block because of rounding up from the actual requested allocation to the allocation granularity. * External fragmentation is the various free spaced holes that are generated in either your memory or disk space. External fragmented blocks are available for allocation, but may be too small to be of any use.

Answer 98

In computing, a logical address is the address at which an item (memory cell, storage element, network host) appears to reside from the perspective of an executing application program. ## Footnote A logical address may be different from the physical address due to the operation of an address translator or mapping function. Such mapping functions may be, in the case of a computer memory architecture, a memory management unit (MMU) between the CPU and the memory bus

Answer 99

The Fetch Policy determines when a page should be brought into main memory. The two most common alternatives are demand paging and prepaging. Deman paging: * A page is brought into main memory only when a reference is made to a location on that page. (The page is brought in as it is used) Prepaging: * Pages other than the one demanded by a page fault are brought into main memory. (All the pages are brought in before they are used).

Answer 100

The placement policy determines where in main memory a process piece is to reside.

Answer 101

When all of the frames in main memory are occupide and it is necessary to bring in a new page to satisfy a page fault, the replacement policy determines which page currently in memory is to be replaced.

Answer 102

When you have to allocate frames to processes there are four different ways to do so: 1. Equal allocation. 2. Proportional allocation. 3. The "Working set" model. 4. Page fault frequency.

Answer 103

The OS looks at the working set of each process and allocates appropriately.

Answer 104

* The primary(?) "interface" to the OS (for users). * Main purpose: organising data (in files, directories, ...). * Filesystem maintains list of filenames and corresponding block numbers. * Defines operations that can be done on files and volumes (renaming, creation, deletion, ...). * Solves administrative tasks: * Sharing and protection. * Reliability and fault tolerance. * Efficiency. * Space usage.

Answer 105

There are ways to organise a file system: 1. Flat filesystems. 2. Two-level file systems. 3. Hierarchical file systems. 4. Paths.

Answer 106

A Flat file system has no subdirectories.

Answer 107

A Two-level file system has one directory for each user, but no subdirectories.

Answer 108

A hierarchical file system uses a hierachical organisation of folders. Directories are organised as trees.

Answer 109

* Two directory entries for same file content. * Difficult to realise if meta-data is stored in directory. * Does not work across volumes. * A file is deleted (or unlinked) when there are no more hard links to it. * Hard links can be used to avoid unnecessary replication of files * Guarantees consistent file content. * Reduces space usage. * Hard links allow flexible name space organisation (across directory structure).

Answer 110

* Deleting the file pointed to by a symbolic link results in a dead symbolic link. * For flexible than hardlinks * Can link across volumes. * Can link to a directory. * Special attribute marks the file as a symbolic link. * Windows' shortcuts: a kind of symbolic link.

Answer 111

There are four ways to allocate disk blocks in a filesystem: 1. Contiguous allocation. 2. Chained/linked allocation. 3. Mapped allocation. 4. Indexed allocation.

Answer 112

Contiguous memory allocation is a classical memory allocation model that assigns a process consecutive memory blocks (that is, memory blocks having consecutive addresses). * Pros: * Easy to implement/administrate. * Cons: * Often results in fragmentation.

Answer 113

Each file is stored as a linked list of disk blocks. (The blocks may be spread out on any addresses on the disk) * Cons: * Can take relatively longer time to read the entire file (compared to continuous allocation).

Answer 114

In computer file systems, a block allocation map is a data structure used to track disk blocks that are considered "in use". Blocks may also be referred to as allocation units or clusters.

Answer 115

Linked allocation does not support random access of files, since each block can only be found from the previous. Indexed allocation solves this problem by bringing all the pointers together into an index block. ## Footnote This type of allocation will have a pointer which has the address of all the blocks of a file. This method solves the problem of fragmentation as the blocks can be stored in any location.

Answer 116

A processor register is a small amount of storage available as part of a digital processor, such as a central processing unit (CPU). Such registers are typically addressed by mechanisms other than main memory and can be accessed faster.

Answer 117

In computing, a system call is how a program requests a service from an operating system's kernel. ## Footnote This may include hardware-related services (for example, accessing a hard disk drive), creation and execution of new processes, and communication with integral kernel services such as process scheduling. System calls provide an essential interface between a process and the operating system. **Implementation:** A typical way to implement this is to use a software interrupt or trap. Interrupts transfer control to the operating system kernel so software simply needs to set up some register with the system call number needed, and execute the software interrupt.

Answer 118

* A module for the OS * The OS' interface to hardware * Disks (floppy, optical, ...) * Keyboards, mouse, ... * USB, FireWire, serial ports, printers, bluetooth, ... * Virtual devices * VPN * Plugins * File systems * Scheduling (policy) * Filtering (firewall) * A protocol * A program

Answer 119

**There are three types of I/O:** * Programmed I/O * Interrupt-driven I/O * Direct Memory Access (DMA)

Answer 120

The CPU issues a command then waits for I/O operations to be complete. As the CPU is faster than the I/O module, the problem with programmed I/O is that the CPU has to wait a long time for the I/O module of concern to be ready for either reception or transmission of data. The CPU, while waiting, must repeatedly check the status of the I/O module, and this process is known as Polling. As a result, the level of the performance of the entire system is severely degraded. * CPU requests I/O operation * I/O module performs operation * I/O module sets status bits * CPU checks status bits periodically * I/O module does not inform CPU directly * I/O module does not interrupt CPU * CPU may wait or come back later

Answer 121

**Interrupt-driven I/O:** * Unit generates interrupt when data is ready. * ISR responsible for moving data in or out of data-register. * Polling can be avoided. * CPU responsible for copying data.

Answer 122

Direct Memory Access (DMA) means CPU grants I/O module authority to read from or write to memory without involvement. ## Footnote DMA module controls exchange of data between main memory and the I/O device. Because of DMA device can transfer data directly to and from memory, rather than using the CPU as an intermediary, and can thus relieve congestion on the bus. CPU is only involved at the beginning and end of the transfer and interrupted only after entire block has been transferred.

Answer 123

Polling is the continuous checking of other programs or devices by one progam or device to see what state they are in, usually to see whether they are still connected or want to communicate. ## Footnote Specifically, in multipoint or multidrop communication (a controlling device with multiple devices attached that share the same line), the controlling device sends a message to each device, one at a time, asking each whether it has anything to communicate (in other words, whether it wants to use the line).

Answer 124

**FIFO** * First come first served. * Fair but poor performance. **Closest block first** * Good performance, but not fair. **SCAN** * The "elevator algorithm". * Maintains direction for read head. * Changes direction at inner-/outer-most cylinder. * Reasonably fair, reasonable performance. **C-SCAN (cyclic scan)** * Like SCAN, but does not chage direction (wraps around). * Lower latency than SCAN.

Answer 125

An (unpredictable) interruption of a thread-execution caused by an event in- or out-side the CPU. * Exceptions generated by the CPU itself. * Three categories of exceptions * Faults. * Traps. * Interrupts. * Often: no clear difference between exceptions, interrupts, and faults. * Exceptions allow user-mode to communicate with kernel-mode * Primarily control communication. * Data exchanged via shared memory. * Usually: a small number of different exceptions in kernel (known beforehand).

Answer 126

Generated by suddenly occurring errors: * Division by zero. * Use of illagal instructions. * Insufficient permissions.

Answer 127

* Software interrupts/trap instructions, e.g. INT. * Used for implementing system-calls.

Answer 128

In computing, scheduling is the method by which work specified by some means is assigned to resources that complete the work. **All scheduling systems have these three common goals outside of their individual goals:** 1. Fairness: fair share of CPU time to every process. 2. Policy enforcement: compliance with policy. 3. Balance: keep all sub-systems busy.

Answer 129

In addition to the three common goals Batch systems are concerned with: * Throughput: maximise jobs pr hour. * Turnaround time: minimise time spent on a task. * CPU utilisation: keep the CPU buys.

Answer 130

In addition to the three common goals Interactive systems are concerned with: * Response time: respond to user requests quickly. * Proportionality: predictability from user prespective.

Answer 131

In addition to the three common goals Real-Time systems are concerned with: * Meeting deadlines: critical for hard real-time. * Predicatbility: cirtical for hard real-time.

Answer 132

**First-Come First-Served (FIFO)** * Easy to understand. * Easy to implement. * Can result in (very) low CPU-utilisation. **Shortest Job First** * Optimal when all jobs are known and available at the same time. * Must be able to determine running time beforehand. **Shortest Remaining Time Next** * Preemptive version of Shortest Job First. * Must be able to determine running time beforehand.

Answer 133

**Round-robin** * Single queue of threads (run queue). * Active thread runs for a quantum; moved to the back of the queue when finished. * Length of quantum important for performance. * Can be expanded to multiple run queues for priorities. * Dynamic prioritisation possible by moving threads between run queues. **Priority Scheduling** * Like round-robin, but using a priority queue. * Runnable thread with highest priority is allowed to run. * May require dynamic (re-)prioritisation to ensure fairness. **Shortest Process Next** * Like Shortest Job Scheduling. * Estimate residual running time based on history. **Guaranteed Scheduling** * Fixed, dynamic amount of CPU time. * Track spent CPU-time. * Variation: thread with most unspent quantum chosen. **Lottery-scheduling** * Scheduler draws a lot (randomly). * Prioritisation by handing out different number of lottery tickets to different threads. * Threads can trade lottery tickets. * Fair sharing of resources between users. **Fair-Share Scheduling** * Take thread/process ownership into account.

Answer 134

**Cyclic Executive** * Static "scheduler", deterministic. **Fixed-Priority** * Highest-priority thread/task is scheduled. * Predictable (statically). **Earliest Deadline First** * Optimal. * Less predictable. * Failures may lead to domino effect. **Value-Based** * Flexible, genereic. * Hard to reason about.

Answer 135

In computing, a shell is a user interface for access to an operating system's services. In general, operating system shells use either a command-line interface (CLI) or graphical user interface (GUI), depending on a computer's role and particular operation.

Answer 136

An Interrupt can be sent from one of the following: * I/O module. * Program. * Timer. * Hardware failure.

Answer 137

Thread synchronization is defined as a mechanism which ensures that two or more concurrent processes or threads do not simultaneously execute some particular program segment known as mutual exclusion. ## Footnote When one thread starts executing the critical section (serialized segment of the program) the other thread should wait until the first thread finishes.

Answer 138

The seven state process model contains seven states for execution of processes: 1. **New**: processes which are newly coming for execution. 2. **Ready**: processes which are in main memory and available for execution. 3. **Running**: processes which are running or executing. 4. **Exit**: processes which are completely executed. 5. **Blocked**: processes which are in main memory and awaiting an event. 6. **Blocked suspended**: processes which are in secondary memory and awaiting an event. 7. **Ready** **suspended**: processes which are in secondary memory but is available for execution as soon as it is loaded into main memory.

Answer 139

Interprocess communication (IPC) is a set of programming interfaces that allow a programmer to coordinate activities among different program processes that can run concurrently in an operating system. This allows a program to handle many user requests at the same time. Since even a single user request may result in multiple processes running in the operating system on the user's behalf, the processes need to communicate with each other. The IPC interfaces make this possible.

Answer 140

Deadlock can be avoided if certain information about processes are available to the operating system before allocation of resources. For every resource request, the system sees whether granting the request will mean that the system will enter an unsafe state. The system only grants requests that will lead to safe states. In order for the system to be able to determine whether the next state will be safe or unsafe, it must know in advance at any time: 1. resources currently available. 2. resources currently allocated to each process. 3. resources that will be required and released by these processes in the future.

Answer 141

**Detection:** Under deadlock detection, deadlocks are allowed to occur. Then the state of the system is examined to detect that a deadlock has occurred and subsequently it is corrected. An algorithm is employed that tracks resource allocation and process states, it rolls back and restarts one or more of the processes in order to remove the detected deadlock. Detecting a deadlock that has already occurred is easily possible since the resources that each process has locked and/or currently requested are known to the resource scheduler of the operating system. It then performs the recovery strategy.

Answer 142

**Recovery:** After a deadlock is detected, it can be corrected by using one of the following methods: * Process Termination: * We can choose to abort all processes involved in the deadlock. This ensures that deadlock is resolved with certainty and speed. But the expense is high as partial computations will be lost. * We can choose to abort one process at a time until the deadlock is resolved. This approach has high overheads because after each abort an algorithm must determine whether the system is still in deadlock. * Several factors must be considered while choosing a candidate for termination, such as priority and age of the process. * Resource Preemption: * Resources allocated to various processes may be successively preempted and allocated to other processes until the deadlock is broken.

Answer 143

An index node is a data structure used to represent a filesystem object, which can be one of various things including a file or a directory. Each inode stores the attributes and disk block location(s) of the filesystem object's data.

Answer 144

Character special files or character devices provide unbuffered, direct access to the hardware device. They do not necessarily allow you to read or write single characters at a time; that is up to the device in question. The character device for a hard disk, for example, will normally require that all reads and writes are aligned to block boundaries and most certainly will not let you read a single byte. * A stream of characters * Keyboard, mouse, console, serial ports * /dev/null, /dev/zero, /dev/random * Interface * File-operations * seek, read, write, readdir, select, ioctl, mmap, open, flush, close, ... * Registered by the module * module register chrdev * module unregister chrdev * Example: /dev/zero * Delivers stream of 0's * dd if=/dev/zero of=/home/foo/overwrite.me count=512

Answer 145

Block special files or block devices provide buffered access to the hardware, such that “the hardware characteristics of the device are not visible.” Unlike character devices, block devices will always allow you to read or write any sized block (including single characters/bytes) and are not subject to alignment restrictions. The downside is that because block devices are buffered, you do not know how long it will take before a write is pushed to the actual device itself; additionally, if the same hardware exposes both character and block devices, there is a risk of data corruption due to the clients using the character device being unaware of changes made in the buffers of the block device. * Data transferred in a number of blocks of fixed size * Disks: hard, floppy, optical, ... * Includes buffering: no ordering necessary (random access, I/O scheduling) * Interface * Often formatted with a file system * Also le-operations(!) * seek, read, write, readdir, select, ioctl, mmap, open, flush, close, ... * Registered by the module * register blkdev * unregister blkdev * Example: RAM disk /dev/ram0

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