CSE120 Principles of Operating Systems. Prof Yuanyuan (YY) Zhou Midterm Review

Transcription

1 CSE120 Principes of Operating Systems Prof Yuanyuan (YY) Zhou Midterm Review

2 Overview The midterm Architectura support for OSes OS modues, interfaces, and structures Processes Threads Synchronization Scheduing 2

3 Midterm Covers materia through scheduing & deadock Based upon ecture materia, homeworks, and projects No cheat sheets Bring your ID TA may take photo of you and your exam Pease, do not cheat Do not copy from your neighbor You wi be noticed No one invoved wi be happy, particuary the teaching staff 3

4 Review Format I go over the major topics You need to ask questions 4 Reward: candy 10/21/18 CSE 120 Midterm Review

5 Arch Support for OSes Types of architecture support Manipuating privieged machine state Generating and handing events Events: Interrupts, exceptions, system cas, etc 5

6 Privieged Instructions What are privieged instructions? Who gets to execute them? How does the CPU know whether they can be executed? Difference between user and kerne mode Why do they need to be privieged? What do they manipuate? Protected contro registers Memory management I/O devices 6

7 Events Events Synchronous: faut (exceptions), system cas Asynchronous: interrupts, software interrupt What are fauts, and how are they handed? What are system cas, and how are they handed? What are interrupts, and how are they handed? How do I/O devices use interrupts? What is the difference between exceptions and interrupts? 7

8 Processes What is a process? What is the difference between a process and a program? What is contained in a process? 8

9 Process Data Structures Process Contro Bocks (PCBs) What information does it contain? How is it used in a context switch? State queues What are process states? What is the process state graph? When does a process change state? How does the OS use queues to keep track of processes? 9

10 Process Manipuation What does fork() on Unix do? What does it mean for it to return twice? What does exec() on Unix do? How is it different from fork? How are fork and exec used to impement shes? 10

11 Threads What is a thread? What is the difference between a thread and a process? How are they reated? Why are threads usefu? What is the difference between user-eve and kerne-eve threads? What are the advantages/disadvantages of one over another? 11

12 Thread Impementation How are threads managed by the run-time system? Thread contro bocks, thread queues How is this different from process management? What operations do threads support? Fork, yied, seep, etc. What does thread yied do? What is a context switch? What is the difference between non-preemptive scheduing and preemptive thread scheduing? Vountary and invountary context switches 12

13 Synchronization Why do we need synchronization? Coordinate access to shared data structures Coordinate thread/process execution What can happen to shared data structures if synchronization is not used? Race condition Corruption Bank account exampe When are resources shared? Goba variabes, static objects Heap objects 13

14 Mutua Excusion What is mutua excusion? What is a critica section? What guarantees do critica sections provide? What are the requirements of critica sections? Mutua excusion (safety) Progress (iveness) Bounded waiting (no starvation: iveness) Performance How does mutua excusion reate to critica sections? What are the mechanisms for buiding critica sections? Locks, semaphores, monitors, condition variabes 14

15 Locks 15 What does Acquire do? What does Reease do? What does it mean for Acquire/Reease to be atomic? How can ocks be impemented? Spinocks Disabe/enabe interrupts Bocking (Nachos) How does test-and-set/swap work? What kind of ock does it impement? What are the imitations of using spinocks, interrupts? Inefficient, interrupts turned off too ong

16 Semaphores What is a semaphore? What does P/Decrement do? What does V/Increment do? How does a semaphore differ from a ock? What is the difference between a binary semaphore and a counting semaphore? When do threads bock on semaphores? When are they woken up again? Using semaphores to sove synchronization probems Readers/Writers probem Bounded Buffers probem 16

17 Monitors What is a monitor? Shared data Procedures Synchronization In what way does a monitor provide mutua excusion? To what extent is it provided? How does a monitor differ from a semaphore? How does a monitor differ from a ock? What kind of support do monitors require? Language, run-time support 17

18 Condition Variabes What is a condition variabe used for? Coordinating the execution of threads Not mutua excusion Operations What are the semantics of Wait? What are the semantics of Signa? What are the semantics of Broadcast? How are condition variabes different from semaphores? 18

19 Impementing Monitors What does the impementation of a monitor ook ike? Shared data Procedures A ock for mutua excusion to procedures (w/ a queue) Queues for the condition variabes What is the difference between Hoare and Mesa monitors? Semantics of signa (whether the woken up waiter gets to run immediatey or not) What are their tradeoffs? What does Java provide? 19

20 Locks and Condition Vars In Nachos, we don t have monitors But we want to be abe to use condition variabes So we isoate condition variabes and make them independent (not associated with a monitor) Instead, we have to associate them with a ock (mutex) Now, to use a condition variabe Threads must first acquire the ock (mutex) CV::Wait reeases the ock before bocking, acquires it after waking up 20

21 Scheduing Components Scheduer (dispatcher) When does scheduing happen? Job changes state (e.g., waiting to running) Interrupt, exception Job creation, termination 21

22 Scheduing Goas Goas Maximize CPU utiization Maximize job throughput Minimize turnaround time Minimize waiting time Minimize response time What is the goa of a batch system? What is the goa of an interactive system? 22

23 Starvation Starvation Indefinite denia of a resource (CPU, ock) Causes Side effect of scheduing Side effect of synchronization Operating systems try to prevent starvation 23

24 Scheduing Agorithms 24 What are the properties, advantages and disadvantages of the foowing scheduing agorithms? First Come First Serve (FCFS)/First In First Out (FIFO) Shortest Job First (SJF) /shortest remaining time first Priority Round Robin Mutieve feedback queues What scheduing agorithm does Unix use? Why?

25 Some quick carification Round robin schedue If the time sice is 10ms, it means that a thread can AT MOST use the CPU for 10ms a time If a thread gives up the CPU because it has to wait for I/O, condition variabe, etc, the scheduer wi switch to another thread in the ready queue 25

26 Deadocks Deadock conditions Deadock detection Deadock prevention How do today s systems hande deadocks? Why? 26

27 Good Luck 27