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WGU OS for Programmers > Chapter 5 - CPU Scheduling > Flashcards

Chapter 5 - CPU Scheduling Flashcards

1
Q

what is the primary purpose of CPU scheduling?

A

to ensure that the computer is always working on a process (no idle time). it selects a process to run from the ready queue when the CPU goes idel

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2
Q

what is the CPU/IO burst cycle?

A

the CPU alternates between CPU and IO bound tasks, often times it alternates between the two or has several CPU bound bursts in a row

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3
Q

what is in the ready queue for the CPU scheduler?

A

PCB’s, process control blocks

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4
Q

under what four circumstances to CPU scheduling decisions get made?

A
  1. when a process switches from running to waiting
  2. when a process switches from running to ready
  3. when a process switches from waiting state to ready state
  4. when a process terminates
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5
Q

what is non-preemptive scheduling?

A

once the CPU has scheduled a process, it keeps the CPU until it releases it either by terminating or waiting

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6
Q

what is preemptive scheduling?

A

the scheduler decides when the process should be moved back into a ready state

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7
Q

what is the main drawback of preemptive scheduling?

A

it can cause race conditions if two processes share data and one is preempted for the other and the other updates the data that the first was going to update

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8
Q

what is the dispatcher?

A

the module that gives control of the CPU’s core to the process selected by the scheduler

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9
Q

what does the process of dispatching look like (handing off control to the process)

A
  1. switching context from one process to another
  2. switching to user mode
  3. jumping to the proper location in the user program that it was last running at
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10
Q

what is dispatch latency?

A

the time it takes for the dispastcher to stop one process and start the next

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11
Q

what are the 5 criterium to think about when considering CPU scheduling algorithms?

A

CPU Utilization, Throughput, Turnaround Time, Waiting Time, Response Time

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12
Q

What generally are the two things that get optimized for in scheduling algorithms?

A

CPU Utilization, Throughput

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13
Q

what is generally considered the “simplest” scheduling algorithm?

A

First come first served! the process that requests the CPU first is allocated the CPU first….FIFO baby

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14
Q

what is the drawback of FCFS?

A

making short processes wait behind long ones, it’s also non-preemptive

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15
Q

why typically has longer bursts? CPU Bound or IO Bound processes?

A

CPU Bound

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16
Q

What is the Convoy effect?

A

When CPU bound processes cause IO bound ones to wait

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17
Q

What is Shortest Job First Scheduling? SJF?

A

it prioritizes the process with the shortest CPU burst, uses FCFS in event of a tie breaker

18
Q

how does Shortest Job First estimate the length of the CPU burst of a process?

A

the scheduler users historical data from the same process and it’s essentially a weighted average to predict the next cpu burst for that process

19
Q

what is Round Robin Scheduling?

A

same as FCFS but with the ability to preempt a process to allow the system to switch between processes more easily

20
Q

how does Round Robin Scheduling work?

A

it effectively makes the CPU bursts more uniform leading to a more consistent distribution of CPU burst time

21
Q

what is Priority CPU Scheduling?

A

a more general version of the SJF. it does this by taking the inverse of the next predicted CPU burst. The higher the CPU burst, the lower the priority

22
Q

what factors may play into priority scheduling?

A

cpu average burst time, memory requirements, open files

23
Q

what interruption scheme is priority scheduling?

A

can be both preemptive and non-preemptive

24
Q

what is a major problem with Priority Scheduling?

A

a long process could be completely starved of compute time

25
Q

what is a solution to idefinite blocking or process starvation?

A

including process age in the priority algorithm

26
Q

what is multilevel queue scheduling?

A

each priority level is placed into it’s own queue

27
Q

what is asymmetric multiprocessing?

A

when one core handles all the scheduling and the other cores just execute processes

28
Q

what is symmetric multiprocessing?

A

every processor is self scheduling

29
Q

how does symmetric multiprocessing solve race conditions?

A

by having each processor have it’s own thread/process queue

30
Q

What is chip multithreading (CMT) and why is it important?

A

CMT assigns multiple hardware threads to each processing core allowing the core to switch between threads if one stalls

31
Q

what are the two main types of multithreading for processing cores?

A

course-grained (switches on long latency events) and fine-graned (switches frequently at low cost)

32
Q

How many levels of scheduling are required for a multithreaded, multicore processor and what are they?

A

two levels:
1. OS Scheduling of software threads
2. Core level scheduling of hardware threads

33
Q

how does load balancing work in a symmetric multiprocessing environment?

A

each thread queue is evenly distributed so that each core doesn’t get too many processes

34
Q

what is processor affinity

A

when a process doesn’t want to change thread queues because there is some cached data it is relying upon

35
Q

what are the two types of high level scheduling concepts for an OS?

A

soft real time and hard real time…hard real time means a process has to run critically when it’s told to run

36
Q

what is rate-monotonic scheduling?

A

uses a priority policy with preemption. lower priority processes get preempted

37
Q

what is earliest deadline firs (EDF)

A

assigns priorities according to the deadline of when the process needs to be executed by

38
Q

what is proportional share scheduling?

A

allocations proportional CPU time to processes

39
Q

what scheduler does Linux kernel use?

A

the Completely Fair Scheduler. it uses priority classes

40
Q
A

WGU OS for Programmers (14 decks)

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