Announcements • Prerequisite Forms CS 3204 • My office hours: M/W 3-5pm Operating Systems • Vijay: T/Th 2-4pm Lecture 2 Godmar Back CS 3204 Spring 20061/20/2006 2 Outline for today Why are OS interesting? • Motivation for teaching OS • OS are “magic” – Most people don’t understand them – including • Brief history sysadmins and computer scientists! • A survey of core issues OS address • OS are incredibly complex systems • What you should get out of this class – “Hello, World” – program really 1 million lines of code • Studying OS is learning how to deal with complexity – Abstractions (+interfaces) – Modularity (+structure) – Iteration (+learning from experience) CS 3204 Spring 20061/20/2006 3 CS 3204 Spring 20061/20/2006 4 What does an OS do? OS vs Kernel • Software layer that sits • Can take a wider view or a narrower definition what an OS is between applications • Wide view: Windows, Linux, Mac OSX are operating and hardware systems gcc csh X11 – Includes system programs, system libraries, servers, shells, GUI etc. • Narrow definition: • Performs services Operating System – OS often equated with the kernel. – Abstracts hardware – The Linux kernel; the Windows executive – the special piece of Hardware software that runs with special privileges and actually controls CPU Memory Network Disk – Provides protection the machine. – Manages resources • In this class, usually mean the narrow definition. • In real life, always take the wider view. (Why?) CS 3204 Spring 20061/20/2006 5 CS 3204 Spring 20061/20/2006 6 1 Evolution of OS Evolution of OS (II) • OSs as a library • Invent multiprogramming – Abstracts away hardware, provide neat – First multi-programmed batch systems, then time- interfaces sharing systems • Makes software portable; allows software evolution • Idea: – Single user, single program computers – Load multiple programs in memory • No need for protection: no malicious users, no – Do something else while one program is waiting, don’t interactions between programs sit idle (see next slide) – Disadvantages of uniprogramming model • Complexity increases: • Expensive – What if programs interfer with each other (wild writes) • Poor utilization – What if programs don’t relinguish control (infinite loop) CS 3204 Spring 20061/20/2006 7 CS 3204 Spring 20061/20/2006 8 Single Program vs Protection Multiprogramming • Multiprogramming requires isolation • OS must protect/isolate applications from each other, and OS from applications • This requirement is absolute – In Pintos also: if one application crashes, kernel should not! Bulletproof. • Three techniques –Preemption – Interposition – Privilege CS 3204 Spring 20061/20/2006 9 CS 3204 Spring 20061/20/2006 10 Protection #1: Preemption Protection #2: Interposition • Resource can be given to program and access • OS hides the hardware can be revoked • Application have to go through OS to – Example: CPU, Memory, Printer, “abstract” resources: files, sockets access resources • CPU Preemption using interrupts • OS can interpose checks: – Hardware timer interrupt invokes OS, OS checks if – Validity (Address Translation) current program should be preempted, done every – Permission (Security Policy) 1ms in Linux – Solves infinite loop problem! – Resource Constraints (Quotas) • Q.: Does it work with all resources equally? CS 3204 Spring 20061/20/2006 11 CS 3204 Spring 20061/20/2006 12 2 Protection #3: Privilege OS as a Resource Manager • Two fundamental modes: • OS provides illusions, examples: – “kernel mode” – privileged – every program is run on its own CPU • aka system, supervisor or monitor mode • Intel calls its PL0, Privilege Level 0 on x86 – every program has all the memory of the – “user mode” – non-privileged machine (and more) • PL3 on x86 – every program has its own I/O terminal • Bit in CPU – controls operation of CPU • “Stretches” resources – Protection operations can only be performed in kernel – Possible because resource usage is bursty, mode. Example: hlt typically – Carefully control transitions between user & kernel mode • Increases utilization CS 3204 Spring 20061/20/2006 13 CS 3204 Spring 20061/20/2006 14 Resource Management (2) Resource Management (3) • Multiplexing increases complexity • OS must decide who gets to use what • Car Analogy (by Rosenblum): – Dedicated road per car would be incredibly inefficient, so cars resource share freeway. Must manage this. – (abstraction) different lanes per direction • Approach 1: have admin (boss) tell it – (synchronization) traffic lights • Approach 2: have user tell it – (increase capacity) build more roads • More utilization creates contention – What if user lies? What if user doesn’t know? – (decrease demand) slow down • Approach 3: figure it out through feedback – (backoff/retry) use highway during off-peak hours – (refuse service, quotas) force people into public transportation – Problem: how to tell power users from – (system collapse) traffic jams resource hogs? CS 3204 Spring 20061/20/2006 15 CS 3204 Spring 20061/20/2006 16 Goals for Resource Management • Fairness – Assign resources equitably • Differential Responsiveness – Cater to individual applications’ needs • Efficiency – Maximize throughput, minimize response time, support as many apps as you can • These goals are often conflicting. – All about trade-offs CS 3204 Spring 20061/20/2006 17 3.