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CPS110: Intro to Operating Systems Landon Cox January 10, 2008 About me (Landon Cox) Background BS Math/CS: Duke, ’99 PhD EECS: Michigan, ’05 Research interests OS, p2p, economics, security, mobility Why am I a professor? Research and teaching are a lot of fun Explaining things improves my understanding Syllabus: prerequisites CPS 100 Basic data structures Allocating memory on the stack versus from the heap CPS 104 Basic computer architecture, ISAs Registers: stack pointer, PC, general-purpose Virtual memory translation Page tables TLB, caching Syllabus: lectures and textbook Lecture notes on the web (125 pages) Exams based on content of lectures Textbooks Only suggested “Modern Operating Systems” Easy to find on-line Syllabus: discussion sections Two sections, starting next week Tu 1:15 - 2:30 M 1:15 - 2:30 Moved to Tu 5:30 – 6:20, D106? Teaching Assistant Amre Shakimov ([email protected]) Undergraduate Teaching Assistant Quinn Gaumer Syllabus: projects Where you will learn the most 4 projects 0: very simple intro to C++ 1: building a user-level thread package 2: building a virtual memory manager 3: hack into a vulnerable system Projects aren’t long, but are difficult Only 100-1,000 lines/code, but many hours Everything is in C++ Project 0 has been posted today Syllabus: homework problems Posted on web by Friday Should be done before discussion section Not graded, but count toward participation Syllabus: project groups All projects done in groups of 2 or 3 Email groups to [email protected] By Tuesday (January 15)! Group members will rate each other Procedure for firing, quitting in syllabus Syllabus: project auto-grading All projects are auto-graded Allows groups to immediate feedback Use submit110 script on cs machines One submission/group/day gets feedback Can’t use to debug your project Any group member’s submission counts More on the auto-grader Very limited feedback: correct or incorrect Doesn’t say what is wrong Still have to write a test suite (except P0) Don’t rely on auto-grader feedback alone To get more useful feedback Come talk to us! We will provide many office hours every week (double office hours week before a deadline) Syllabus: project timelines Due at 6pm, accepted until 11:59:59pm Auto-grader clock is the one that counts Last submission to auto-grader is final 3 late days/group/semester Intended for unexpected problems No extensions Start early! Syllabus: project collaboration Ok, among groups C++ syntax, course concepts “What does this part of the handout mean?” Not ok, among groups Design/writing of another’s program Includes prior class solutions “How do I do this part of the handout?” We use automated similarity-detection software Just changing the variable names won’t save you If in doubt, ask me Syllabus: grades, exams Projects: 35% Midterm: 30% Tuesday, February 26 Final: 30% Friday, May 2, 7-10pm Participation: 5% Projects and exams The two are not independent Familiarity with projects is critical to doing well on exams I like to ask questions about projects on exams “Extend Project X to include this functionality” Know your project! You can assign roles to different people But each member must understand all aspects Syllabus: environment Linux/GNU environment Need to sign-up for term CS account Send email and I will do it for you [email protected] by January 15 Can login into linux.cs.duke.edu Use this account for all auto-grading Syllabus: getting help Newsgroup http://courses.duke.edu Office hours With me: W, 1:00 -- 3:00 With Amre: T, 12:00 – 2:00 Don’t email Amre or me directly Post to the newsgroup, which we monitor Grades from last semester 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1 2 3 4 5 6 Some kind of A 7 8 9 10 11 12 13 14 15 16 Some kind of B sko D Questions about the course? Goals for CPS 110 First part: demystify the operating system How does my computer start running? How does a program load into memory? Second part: demystify Internet systems How does my email know where to go? Why is Google so fast? Duke course view of computers CPS 1,6,100,108 What’s CPSmissing? 110 CPS 104 Applications Ideas high-level programming languages compiling, reading programs off disk, getting program into memory, reading keyboard, starting the computer, saving files, filenames, networking Hardware Assembly language program gates Thinking about interfaces Consider the Java language and its key word “interface” What is a Java object? List of methods and collection of internal state What is a Java interface? Set of methods associated with an object that a programmer can call What do those methods do? Invoke code (let the object do work on the caller’s behalf) Mutate the object’s public/private state Why are interfaces useful? They provide an “abstraction” or simplification Callers don’t have to know an object exact type OS terminology Key terms: interface, resource (cpu, mem, etc), abstraction, virtual Define an interface in terms of resources An interface is a set of primitives or operations Interfaces provides access to resources What do we mean by abstraction? How resources are presented to a client Can think of as an illusion that makes resources easier to program What does it mean to virtualize something? Provides an abstraction (simple way to manipulate resources) (mostly) disallow direct access to reality/resources What is an operating system? Program that runs on CPU, (mostly) like any other Applications OS Hardware “Virtual machine” Interface “Physical machine” Interface Virtual interface should be simpler than physical What is an operating system? Applications What interface does the OS present? “Virtual machine” Interface OS “Physical machine” Interface Hardware What interface does the hardware present? Instruction set: Load/store, mem, regs Hardware-software stack Applications Threads Virtual Memory System Calls Socket Traps Ether, 80211 OS Atomic Test/Set Hardware Page Tables OS vs user-level programs Familiar view Alternate view User program OS OS How do programs start? Tasks outside program? (net recv) How do prevent CPU hogging? User program User program OS runs first, calls program Programs run until they return control to OS (by themselves or forced by hardware) Then OS calls another program Key question: who calls whom? Functions of the OS 1. Illusionist Makes computer seem nicer than it really is Examples? Programs seem to have their own CPU AFS: single, unified file system Name data with human-readable names Directories Packets get lost; OS makes net look reliable Disk is slow; OS makes it look fast via caching Functions of the OS 1. Illusionist Makes computer seem nicer than it really is 2. Government Divides hardware resources among competing programs What hardware resources does the OS manage? Processor Memory Network Disk Functions of the OS 1. Illusionist Makes computer seem nicer than it really is 2. Government Divides hardware resources among competing programs Taxes programs (OS needs CPU, memory to run) Taken for granted when it works, cursed when it breaks Why study operating systems? Very few of you will ever write one … Illusionist, govn functions appear in many domains Google provides the illusion of a single web server Word does background spell checking Design principles Proper abstractions, caching, indirection Concurrency, naming, atomicity, authentication Protection, resource multiplexing (fairness) How does OS create the illusions we know/love? Hints for designing systems What is a system? Components, interconnections Interfaces, environment Systems do something for their environs Exhibit this behavior via interface Cleanly divides the world in two Parts of the system + the environment Systems from 10,000 feet Component Component Component Component System Environment aka “the client” Why is designing systems hard? 1. Emergent properties Can’t predict all component interactions Millennium bridge Synchronized stepping leads to swaying Swaying leads to more forceful synchronized stepping Leads to more swaying … 2. Propagation of effects 3. Incommensurate scaling 4. Trade-offs Why is designing systems hard? 1. Emergent properties 2. Propagation of effects Want a better ride so increase the tire size Need a larger trunk for the larger spare Need to move the back seat forward Need to make front seats thinner Leads to worse driver comfort than before 3. Incommensurate scaling 4. Trade-offs Why is designing systems hard? 1. Emergent properties 2. Propagation of effects 3. Incommensurate scaling Consider the giant mouse Weight ~ size3 (volume) Bone strength ~ size2 (cross section area) An elephant sized mouse is not sustainable 4. Trade-offs Why is designing systems hard? 1. 2. 3. 4. Emergent properties Propagation of effects Incommensurate scaling Trade-offs “Waterbed effect” Push on one end, and the other goes up Spam filters and smoke detectors False positives vs false negatives Why is designing systems hard? 1. Emergent properties 2. Propagation of effects 3. Incommensurate scaling 4. Trade-offs In the immortal words of HT Kung “Systems hard. Must work harder.” History of operating systems History dominated by two trends Increasingly inexpensive hardware Increased software complexity Microsoft embodies tension between these trends MS gained 90% market share by running on cheap hw Supporting all that hardware complicates the OS (3rd-party drivers responsible for vast majority of crashes) How is Apple’s strategy different? Jobs chooses the hardware you will run HW-to-app control reduces complexity, choice, discount First phase: single operator One goal: make it work Interactive (user has entire machine to herself) Users sign up, get room for two hours at a time “OS” is really just a library compiled into program What is wrong with this timeline? CPU utilization is awful Since CPUs were expensive, this mattered Second phase: batch processing Goal: improve CPU, I/O utilization Machine is no longer interactive Users submit program (stack of cards) to queue One job at a time, CPU idle during I/O, I/O idle during CPU OS is a batch monitor + library of services Loads program, runs program, prints results Loads next program … Second phase: batch processing Goal: improve CPU, I/O utilization Machine is no longer interactive Users submit program (stack of cards) to queue One job at a time, CPU idle during I/O, I/O idle during CPU What key OS function starts to matter now? Protection: programs must not corrupt monitor Programs must relinquish CPU to monitor Second phase: batch processing Goal: improve CPU, I/O utilization Machine is no longer interactive Users submit program (stack of cards) to queue One job at a time, CPU idle during I/O, I/O idle during CPU Why wasn’t protection an issue before? No batch monitor to corrupt Person in lab coat took CPU back from program Third phase: multi-program batch Goal: overlap CPU, I/O When one job is reading from disk, run another job on CPU Use DMA + interrupts to allow background I/O DMA: devices write to program memory Interrupts: devices can tell CPU the I/O is done Job 1 Job 2 Third phase: multi-program batch Goal: overlap CPU, I/O What are the OS’s new responsibilities? Switch between processes Manage multiple I/Os across devices Protect processes from each other Job 1 Job 2 Fourth phase: time-sharing Goal: keep efficiency, restore interactivity Key insight: humans are really just slow I/O devices Switch between programs during think-time Job 1 Job 2 Job 3 Increased complexity: • Many jobs • Outstanding reqs • Many job sources Fifth phase: personal computing What are PC operating systems most like? As PC prices dropped, single-operator became feasible OS was again just a library of services (MS-DOS) With one user, do jobs need to time-share? Early PC OSes could only do one thing at a time Everything waited while printing/loading a program (Mac < X) Need protection if I’m the only one using the PC? Protect me from myself (or my buggy software) Early PCs provided no protection (why Windows before XP, Mac before X were awful) PC operating systems are basically time-sharing OSes now Operating system complexity Windows XP > 40 million lines of code Most of this code is device drivers (not written by MS) Windows NT took 7 years to develop Only worked well years after it shipped Windows 2000 Shipped with 63,000 “potential known defects” Hot research area Simplify, automatically find OS bugs