Introduction to Computer Science

Transcription

1 Introduction to Computer Science CSCI 109 China Tianhe-2 Andrew Goodney Spring 2019 Lecture 8: Operating Systems March 18th, 2019

2 Operating Systems ì Working Together 1

3 Schedule 2

4 Agenda u Talk about operating systems u Reiew midterm (time permitting) 3

5 Operating Systems uwhat is an OS? uthe kernel, processes and resources uprotection/isolation/security ucompeting for time Reading: St. Amant Ch. 6 4

6 Before Operating Systems u One computer ó one program u Program runs start to finish (or crashes) Once done, load the next program u Thought experiment? Program waits 10ms to load a data item from tape (eery once in a while) Oer the course of the program execution loads 1,000,000 data items. What happens to the 10,000s while the program was waiting? 5

7 Obseration u Most programs perform some I/O I/O is slow (tape, disk, network, human user, etc.) u CPU literally does nothing while waiting for I/O This is inefficient u What if we could share the CPU so when one program is waiting we can run another program? u Operating systems came out of this need to time-share the CPU 6

8 The need for an OS Problem Solution: Algorithm + Data Structures Low-leel instructions Executes on Pseudocode Program Compiles to CPU, Memory, Disk, I/O 7

9 The OS as a executie manager Problems Solution: Algorithms + Data Structures Pseudocode Low-leel Low-leel instructions Low-leel instructions instructions Executions managed by Operating System Program Program Program Program Compile to CPU, Memory, Disk, I/O 8

10 What is an Operating System? uan executie manager for the computer umanages resources Space (i.e. memory) Time (i.e. CPU compute time) Peripherals (i.e. input and output) uos is a program that starts, runs, pauses, restarts, and ends other programs u (some content from the following slides is courtesy of Mark Redekopp and CS350) 9

11 Definition A piece of software that manages a computer s resources What resources need managing? CPU (threads and processes) Memory (Virtual memory, protection) I/O (Abstraction, interrupts, protection) Hardware Kernel User Mode mode DISK User App System Library Processes & Scheduling I/O Driers & Protocols Processor Mem. Management Unit User App System Library File Systems Memory Translation Network Graphics 10

12 Examples of Operating Systems u u u Microsoft Family MSDOS, Windows , WindowsNT -> Windows 10 Predominately x86 (Intel) hardware, some PowerPC, some ARM FreeDOS POSIX (UNIX/like) macos, FreeBSD, openbsd, netbsd, Solaris, AIX, and others u ios Linux u u u u Others Run on most processor architectures Little side project of uniersity student UNIX clone that won the war 20+ popular distributions Android: heaily customized Linux and Jaa on phone/tablet PlaystationOS, VxWorks 11

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15 Important Vocabulary u Resource Some part of the computer that programs use: u u Policy Memory, CPU, Input/Output deices Rules enforced by algorithms that share access to resources u OS Deelopers (humans) write policies that achiee some set of goals for the operating systems 14

16 What does an Operating System do? ua bare computer is just hardware uprograms are written to use that hardware, but exclusie use is inefficient uin simple terms, the OS: Enables more than one program at a time to use the computer hardware Present computer resources (CPU, disk, I/O) through abstract interfaces to allow sharing Enforce policies to manage/regulate the sharing of resources 15

17 Roles Referee Protection against other applications Enforce fair resource sharing Why doesn't an infinite loop require a reboot? Illusionist (Virtualization) Glue Each program thinks it is running separately Each program thinks it has full access to computer's resources (or unlimited resources) Common serices (such as copy/paste) Files can be read by any application UI routines for look & feel Separate applications from hardware so you don t need to know which keyboard, disk drie, etc 16

18 OS Design Criteria u Reliability (and aailability) u Security & Priacy u Performance u Portability 17

19 Reliability and Aailably u Reliable systems work properly Correct (or expected) outputs are generated for a set of inputs If this is not the case, the system has failed u Examples? u Aailable systems are aailable to do work u Aailable does not imply reliable System can be aailable but not reliable (system has bugs, generates wrong results) System can be reliable but not aailable u Crash eery 5 minutes, but saes results and restarts 5 minutes later 18

20 Priacy, Security, Isolation u For an OS security means the OS does not run unintended code or get into a compromised state No irus/malware u OS priacy means programs should not get access to data they should not hae Password keychains, files in other users directories u Security and Priacy require some tradeoffs with performance, which is why OS s are not 100% secure Some are better than others! 19

21 Portability u Many machine types exist: x86, x86_64, PPC, ARM, MIPS u Many different motherboards or hardware platforms exist: serer with 8 CPUs 12 PCIe slots to RaspberryPi, to AppleTV, etc. u OS with good portability abstracts these differences into a stable API so programmers don t notice u Also, can the OS itself be ported to new hardware easily? u Good portability leads to wide adoption Linux, Windows 20

22 Performance u What does performance mean? Lots of computation? Fluid GUI for game? Low latency disk for database? u OS balances these with policies Major axis is throughput s. response time Different OS s are tuned based on use case DB serer has different policies than Windows gaming rig 21

23 Examples of Policies utasks are gien priorities; higher priority tasks are handled first usome kind of tasks are neer interrupted uall tasks are equal priority; round-robin usome tasks can only use part of a disk usome tasks can use network 22

24 The kernel uthe kernel is the core of an OS ukernel coordinates other programs uwhen the computer starts up the kernel is copied from the disk to the memory ukernel runs until some other program needs to use the CPU ukernel pauses itself to run other program 23

25 How memory is used Start Kernel loaded User program runs User program done Free Memory Free Memory Free Memory Free Memory Executing Program Kernel Kernel Kernel 24

26 Multitasking u One program uses the CPU at a time u OS switches CPU usage (rapidly) u Creates an illusion that all the programs are running at the same time u Changeoer from one program to another is called a context switch u Examples of context switching? u Can context switching be good for a program? u Can context switching be good for a CPU? 25

27 Abstractions: Processes and Resources u Resources Space (memory) Time (CPU) Peripherals (printers etc.) u Process: an executing program Program counter Contents of registers Allocated memory & contents uos doesn t worry about what each program does uinstead OS cares about What resources does a process need? How long will it run? How important is it? 26

28 Protection/Isolation u Other processes hae to be preented from writing to the memory used by the kernel u Crash in one program shouldn t crash OS or other programs u OS has access to all resources: priileged mode u User programs hae restricted access: user mode u When a user program needs access to protected resources it makes a system call (e.g., managing files, accessing a printer) u Principle of least priilege (kernel has highest priilege) 27

29 Keep the CPU busy! u Keeping CPU busy is THE MOST IMPORTANT THING EVER! u Lets look at some policies that can help us do that. u We assume we hae lots of work (i.e. different programs) 28

30 Competing for time uthink of the time the CPU spends in chunks or blocks uhow can blocks of time be allocated to different processes so that work can be done efficiently? upolicy: rules to enforce process prioritization 29

31 Process Scheduling Policies uthe process queue u Round-robin u First-come, first-sered upriority-based Preset priority for each process Shortest-remaining-time uall these policies keep the CPU busy uare there other ways to judge a policy? 30

32 Keeping CPU busy 31

33 How to ealuate a policy? u u u u Utilization: how much work the CPU does Throughput: # of processes that use the CPU in a certain time Latency: aerage amount of time that processes hae to wait before running Fairness: eery process gets a chance to use the CPU CPU utilization Throughput Latency Fairness Round-robin Good Variable Potentially high First-come firstsered Shortest remaining time Fixed priority Good Variable Yes Good High Potentially high Good Yes No staration No Could hae staration 32

34 Eeryday policies uplanes taking off: first come first sered High efficiency for the runway If seeral smaller planes in line before a large one, not efficient for the aerage passenger u Traffic at an intersection w/light out: round robin First traffic in one direction, then another If a police car arries, then switch to priority-based Unlikely to eer be shortest remaining time 33

35 First-come, first sere (non-pre-emptie) 34

36 Round Robin (pre-emptie) 35

37 Weighted Round Robin 36

38 Shortest-time Remaining 37