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TM129 > OS Topic 9 - Processes and scheduling > Flashcards

OS Topic 9 - Processes and scheduling Flashcards

1
Q

The ability of an operating system to run more than one program at a time is known as m________________

A

multitasking

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2
Q

Processes are concurrent if their lifetimes o______, even if that happens because a single processor rapidly switches execution between different processes so they are never r_______ at the same instant.

A

overlap
running

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3
Q

Of course, with multiple cores, processes can be running at the same instant, and we could then use the technical term p________

A

parallel

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4
Q

The issues for the OS for concurrent and true parallel processes are the s______, so OSs must be designed for the more g______ case of concurrent processes.

A

same
general

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5
Q

The issues for the OS for concurrent and true parallel processes are the same. What are some of the issues?

A

In both cases, there could be contention for the same resources: shared files, I/O devices and memory.

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6
Q

Most operating systems provide an alternative to the process. This alternative is called a t________.

A

thread

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7
Q

A thread represents a separate execution of a program, just as a process does, but threads share the m_______ and other r___________ allocated to a single process. This means that the program can be structured as two or more tasks that are being executed apparently at the same time and that share the same data.

A

memory
resources

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8
Q

An advantage of threads over processes is that, because they share memory and other resources, the operating system can create them more q_________ and switch between them more q__________.

A

quickly
quickly

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9
Q

True or False?

Each process is an independent entity.

A

True

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10
Q

Task Manager

The number of active tasks will vary according to what software you are running.

Each one has a unique PID.
What does PID stand for?

A

process identifier

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11
Q

Task Manager

RSS: the amount of physical memory currently used. What does RSS stand for?

A

(resident set size)

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12
Q

Task Manager

What does VM-Size stand for?

A

(virtual memory size) the total amount of memory used by the process

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13
Q

Task Manager

What does User mean?

A

name of process user

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14
Q

Task Manager

What is ‘State’

A

process state, shown as a variety of letters

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15
Q

Task Manager

What is ‘Prio’

A

process priority

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16
Q

Task Manager

What does PPID stand for?

A

parent process identifier.

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17
Q

Scheduling

One of the key tasks for an operating system is to manage d_____ on the p______, allocating appropriate processor time to different applications and to the operating system itself.

A

demand on the processor

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18
Q

Scheduling

The central problem is choosing which process to r_____ n______ and for h___ l____; this is known as scheduling.

A

run next
how long

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19
Q

Scheduling

Switches between processes need to happen many times a second to give the user the illusion of m_______________.

A

multitasking

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20
Q

Scheduling

Each process is allowed to run only for a short time-slice, often referred to as a q_______, for example 10 ms.

A

a quantum

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21
Q

Scheduling

A process that is replaced by another when its quantum expires is said to have been p___-e_______.

A

pre-empted

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22
Q

Scheduling

Frequent switching is particularly needed for an OS described as a real-time operating system.

What is meant by a real-time operating system?

A

This is a claim that it can respond to events ‘in real time’, something particularly important when software controls equipment.

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23
Q

Scheduling

Why is frequent switching important in a real-time operating system that we might see in equipment such as smart-devices in cars?

A

It would be disastrous for example if the computer system of an autonomous car stopped controlling the steering while the processor was busy calculating the best route.

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24
Q

Scheduling

Real-time must be specified in terms of time take to note an event, and time taken to deal with an event.

What is the maximum amount of time a real-time OS can take to identify an event?

What is the maximum amount of time that a real-time OS can take to handle the event?

A

10ms
50ms

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25
Q

Scheduling

Why don’t mainstream OSs have to offer such strong guarantees like real-time OS’s?

A

Because most processes are not time-critical and the consequences of failure are less severe.

26
Q

What does FIFO stand for?

A

First in First Out

27
Q

If there are four processes lined in order A, B, C, D list the order of execution in FIFO.

A

A, B, C, D

28
Q

How would Round Robin scheduling work on 4 tasks called A, B, C, D?

A

A for a quantum, B for a quantum, C for a quantum, D for a quantum. Repeat until all processes have finished.

29
Q

How does FIFO do on fairness?

A

Not well, some processes may take a lot longer than others.

30
Q

How does Round Robin do on fairness?

A

Well, all processes get an equal time.

31
Q

What does it mean when we say a process is ‘starved’?

A

The process is ready to run but never gets a chance.

32
Q

Priority

An alternative strategy is to give each process a priority level, usually expressed as a small number, for example 0 to _____.

A

100

33
Q

Priority

The queue of ready processes is kept as a priority queue: the head of the queue always has the __________ priority

A

highest

34
Q

Priority

When a new high-priority process joins a queue, it ‘q_______ j________’ over lower-priority processes so that the queue is always kept in order of priority.

A

queue jumps

35
Q

Priority

Is Priority Scheduling fair?

A

It can be unfair: a low-priority process may never get a turn.

36
Q

Priority Classes

A variant is to classify processes into a number of classes of priority and keep a q_______ for each class.

A

queue

37
Q

Priority Classes

Let’s say we had two classes, high priority and low priority.
There may be several process in each class,
e.g.

High - DD
CCC

Low - B
AAAA

What letters are in the queue for the high priority class?

A

DD
CCC

38
Q

Priority Classes

Let’s say we had two classes, high priority and low priority.
There may be several process in each class,
e.g.

High - DD
CCC

Low - B
AAAA

What letters are in the queue for the low priority class?

A

B
AAAA

39
Q

Priority class

For the following classes of priority, what scheduling system can we use for each queue?

High - DD
CCC

Low - B
AAAA

What order would the letters be in?

A

Round Robin

CDCDC ABAAA

40
Q

Dynamic scheduling

Another approach is to change p__________ dynamically.

A

priorities

41
Q

Dynamic scheduling

Dynamic scheduling can avoid ‘s___________’: a low-priority process that is making no progress – because there are always higher-priority processes waiting – can have its p____________ increased temporarily.

A

starvation
priority

42
Q

What does ‘pre-emption’ refer to?

A

“Pre-emption” refers to the ability of an operating system or a scheduler to interrupt the execution of a currently running process or task in order to give control to another process.

43
Q

The Linux scheduling system is based around priority and allows the programmer and user some control. A process priority is given as a number: ___________ numbers indicate higher priority.

A

lower

44
Q

Normal programs start with a priority of ___ (number)

A

20

45
Q

Operating system processes can be given __________ numbers (higher priority).

A

lower

46
Q

There is a mechanism called ‘n_________’ which allows a process to change its own priority or that of another.

A

niceness

47
Q

The final process priority is the initial priority and niceness added together.

If the initial process were 0, and the niceness was 5, what would the final process priority be?

A

5

48
Q

Is a process priority of 5 high priority, normal or low priority?

A

normal

49
Q

What is the highest priority (lowest number) that the root can set the niceness of a priority to?

A

-20

50
Q

What range of priority might you see given that an Initial priority for a user-space process (whether created by a normal user or root) is 20?

For a process started by a normal user?

A

20 - 39
(we can add a ‘niceness’ of 0 - 19)

51
Q

What range of priority might you see given that an Initial priority for a user-space process (whether created by a normal user or root) is 20?

For a process started by root?

A

The root user can change niceness by values from −20 to 19, so final priority will be in the range 0–39.

52
Q

Kernel threads can be given negative priorities so they can be guaranteed to take priority over any user-space processes; these are referred to as r_____-t_______ processes. The highest priority is −_____;

A

real-time processes
-100

53
Q

True or False

Real-time processes do not take priority over other processes.

A

False - they do take priority and are used for kernel operations.

54
Q

Scheduling real-time processes

A real-time process can be scheduled using one of three different schedulers:

FIFO
Round Robin
D____________

A

Deadline

55
Q

Scheduling real-time processes

Deadline. A scheduler that allows the process to b_____ an amount of CPU time needed before a deadline.

A

book

56
Q

Scheduling real-time processes

Deadlines are suited to handling a______ and v_______ in a buffer where processing must be completed in time for the next block of data to arrive.

A

audio and video

57
Q

Scheduling normal processes

The current (2023?) Linux ‘normal’ scheduler is called the C_______ F______ S___________.

A

The Completely Fair Scheduler

58
Q

The Completely Fair Scheduler honours the initial priority and niceness values but a_______ p_______ d_______ so that all processes get a ‘fair’ slice of CPU time.

A

adjusts priority dynamically

59
Q

What is ‘idle’ process which is found in most OSs and created when the system boots?

A

The sole purpose of the system idle process is to run when the processor has nothing else to do. Having such a process simplifies the design of the operating system since it can always find at least one process to run.

60
Q
A

TM129 (19 decks)

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