CS 153 Design of Operating Systems Spring 18

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1 CS 153 Design of Operating Systems Spring 18 Midterm Review

2 Midterm in class on Friday (May 4) Covers material from arch spport to deadlock Based pon lectre material and modles of the book indicated on the class schedle Semaphore book: chapter 1-4 Closed book. No additional sheets of notes CS 153 Midterm Review 2

3 Overview Architectral Spport Processes Schedling Threads Synchronization CS 153 Midterm Review 3

4 Roles an OS Abstraction: defines a set of logical resorces (objects) and well-defined operations on them (interfaces) Why? Easier app programming Hmans are good at abstraction instead of details Virtalization: Isolates and mltiplexes physical resorces via spatial and temporal sharing Why? Better hardware tilization Access Control: who, when, how Why? Fairness, performance, secrity, privacy, etc. CS 153 Midterm Review 4

5 Arch Spport Prpose of architectre spport Why we need them? Easier OS implementation Types of architectre spport Maniplating privileged machine state Generating and handling events CS 153 Midterm Review 5

6 Privileged Instrctions What are privileged instrctions? Who gets to execte them? How does the CPU know whether they can be exected? Difference between ser and kernel mode Why do they need to be privileged? What do they maniplate? Protected control registers Memory management I/O devices CS 153 Midterm Review 6

7 Events Events Synchronos: falts (exceptions), system calls Asynchronos: interrpts What are falts, and how are they handled? What are system calls, and how are they handled? What are interrpts, and how are they handled? How do I/O devices se interrpts? What is the difference between exceptions and interrpts? CS 153 Midterm Review 7

8 OS Abstractions Applications Process File system Virtal memory Operating System CPU Hardware Disk RAM CS 153 Midterm Review 8

9 Processes What is a process? What resorce does it virtalize? What is the difference between a process and a program? What is contained in a process? CS 153 Midterm Review 9

10 Process Data Strctres Process Control Blocks (PCBs) What information does it contain? How is it sed in a context switch? State qees What are process states? What is the process state graph? When does a process change state? How does the OS se qees to keep track of processes? CS 153 Midterm Review 10

11 Process Interface What operations can be performed on processes? What does CreateProcess on Windows do? What does fork() on Unix do?» What does it mean for it to retrn twice? What does exec() on Unix do?» How is it different from fork? How are fork and exec sed to implement shells? CS 153 Midterm Review 11

12 Schedling Why we need schedling? Long-term schedling / Goals Short-term schedling / Goals Components Schedler (dispatcher) When does schedling happen? Job changes state (e.g., waiting to rnning) Interrpt, exception Job creation, termination CS 153 Midterm Review 12

13 Schedling Goals Goals Maximize CPU tilization Maximize job throghpt Minimize trnarond time Minimize waiting time Minimize response time What is the goal of a batch system? What is the goal of an interactive system? CS 153 Midterm Review 13

14 Starvation Starvation Cases Indefinite denial of a resorce (CPU, lock) Side effect of schedling Side effect of synchronization Operating systems try to prevent starvation CS 153 Midterm Review 14

15 Schedling Algorithms What are the properties, advantages and disadvantages of the following schedling algorithms? First Come First Serve (FCFS)/First In First Ot (FIFO) Shortest Job First (SJF) Priority Rond Robin Mltilevel feedback qees What schedling algorithm does Unix se? Why? CS 153 Midterm Review 15

16 Threads What is a thread? What is the difference between a thread and a process? How are they related? Why are threads sefl? Concrrency à tilizing more CPU powers» The thermal wall of single thread performance Lightweight, fast commnication What is the difference between ser-level and kernellevel threads? What are the advantages/disadvantages of one over another? CS 153 Midterm Review 16

17 Thread Implementation How are threads managed by the rn-time system? Thread control blocks, thread qees How is this different from process management? What operations do threads spport? Fork, yield, sleep, etc. What does thread yield do? What is a context switch? What is the difference between non-preemptive schedling and preemptive thread schedling? Volntary and involntary context switches CS 153 Midterm Review 17

18 Synchronization Why do we need synchronization? Coordinate access to shared data strctres Coordinate thread/process exection (active schedling) What can happen to shared data strctres if synchronization is not sed? Race condition Corrption Bank accont example When are resorces shared? Global variables, static objects Heap objects CS 153 Midterm Review 18

19 Mtal Exclsion What is mtal exclsion? What is a critical section? What garantees do critical sections provide? What are the reqirements of critical sections?» Mtal exclsion (safety)» Progress (liveness)» Bonded waiting (no starvation: liveness)» Performance How does mtal exclsion relate to critical sections? What are the mechanisms for bilding critical sections? Locks, semaphores, monitors, condition variables CS 153 Midterm Review 19

20 Locks What does Acqire do? What does Release do? What does it mean for Acqire/Release to be atomic? How can locks be implemented? Spinlocks Disable/enable interrpts How does test-and-set work? What kind of lock does it implement? What are the limitations of sing spinlocks, interrpts? Inefficient, interrpts trned off too long CS 153 Midterm Review 20

21 Semaphores What is a semaphore? What does Wait/P/Decrement do? What does Signal/V/Increment do? How does a semaphore differ from a lock? What is the difference between a binary semaphore and a conting semaphore? When do threads block on semaphores? When are they woken p again? Using semaphores to solve synchronization problems Readers/Writers problem Bonded Bffers problem CS 153 Midterm Review 21

22 Monitors What is a monitor? Shared data Procedres Synchronization In what way does a monitor provide mtal exclsion? To what extent is it provided? How does a monitor differ from a semaphore? How does a monitor differ from a lock? What kind of spport do monitors reqire? Langage, rn-time spport CS 153 Midterm Review 22

23 Condition Variables What is a condition variable sed for? Coordinating the exection of threads Not mtal exclsion Operations What are the semantics of Wait? What are the semantics of Signal? What are the semantics of Broadcast? How are condition variables different from semaphores? CS 153 Midterm Review 23

24 Implementing Monitors What does the implementation of a monitor look like? Shared data Procedres A lock for mtal exclsion to procedres (w/ a qee) Qees for the condition variables What is the difference between Hoare and Mesa monitors? Semantics of signal (whether the woken p waiter gets to rn immediately or not) What are their tradeoffs? What does Java provide? CS 153 Midterm Review 24

25 Deadlock Deadlock happens when processes are waiting on each other and cannot make progress What are the conditions for deadlock? Mtal exclsion Hold and wait No preemption Circlar wait How to visalize, represent abstractly? Resorce allocation graph (RAG) Waits for graph (WFG) CS 153 Midterm Review 25

26 Deadlock Approaches Dealing with deadlock Ignore it Prevent it (prevent one of the for conditions) Avoid it (have tight control over resorce allocation) Detect and recover from it What is the Banker s algorithm? Which of the for approaches above does it implement? CS 153 Midterm Review 26

27 Smmary Yo have encontered many concepts Process, thread, locks, semaphore, deadlock, etc. They are invented to solve problems/challenges Recall what are the drivers of OS change» Usability, performance, fnctionality, secrity, etc. Think abot the evalation of OS This may help yo better nderstand them Remember this qestion: what is an OS? OS is a far general concept than jst kernel Yo re likely to design/implement yor own OS in the ftre!» API, SDK, platform, etc. CS 153 Midterm Review 27

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