1. Linux Overview COMS W4118 Spring 2008

2. Slides based on Phil Hutto, Silberschatz 2 History Linux is a modern, free operating system based on UNIX standards First developed as a small but self-contained kernel in 1991 by Linus Torvalds, with the major design goal of UNIX compatibility Its history has been one of collaboration by many users from all around the world, corresponding almost exclusively over the Internet It has been designed to run efficiently and reliably on common PC hardware, but also runs on a variety of other platforms The core Linux operating system kernel is entirely original, but it can run much existing free UNIX software, resulting in an entire UNIX- compatible operating system free from proprietary code Many, varying Linux Distributions including the kernel, applications, and management tools

3. Slides based on Phil Hutto, Silberschatz 3 Linux Lineage Multics: 1964 Corbato MIT, GE/Honeywell, Bell Labs UNIX: 1969 Thompson & Ritchie AT&T Bell Labs BSD: 1978 Berkeley Software Distribution Commercial Vendors: Sun, HP, IBM, SGI, DEC GNU: 1984 Richard Stallman, FSF POSIX: 1986 IEEE Portable Operating System unIX Minix: 1987 Andy Tanenbaum SVR4/Solaris: 1989 AT&T and Sun Linux: 1991 Linus Torvalds Intel 386 (i386) Open Source: GPL, LGPL, Cathedral and Bazaar

4. Slides based on Phil Hutto, Silberschatz 4 Linux Distributions Standard, precompiled sets of packages, or distributions, include the basic Linux system, system installation and management utilities, and ready-to- install packages of common UNIX tools The first distributions managed these packages by simply providing a means of unpacking all the files into the appropriate places; modern distributions include advanced package management Early distributions included SLS and Slackware Red Hat and Debian are popular distributions from commercial and noncommercial sources, respectively The RPM Package file format permits compatibility among certain Linux distributions

5. Slides based on Phil Hutto, Silberschatz 5 Linux Licensing The Linux kernel is distributed under the GNU General Public License (GPL), the terms of which are set out by the Free Software Foundation Anyone using Linux, or creating their own derivative of Linux, may not make the derived product proprietary; software released under the GPL may not be redistributed as a binary-only product

6. Slides based on Phil Hutto, Silberschatz 6 Linux Features Linux is a “UNIX-like” operating system that “aims at” standards compliance Linus Torvalds does not feel constrained to implement things that he things are “broken” (e.g. STREAMS) even if they are part of some standard Still most standard things are supported and Linux includes many innovative features Lot’s of sharing of features occurs between rival operating systems; if BSD or Windows or Mac OS comes up with a good idea, Linux will quickly incorporate a similar, often improved version

7. Slides based on Phil Hutto, Silberschatz 7 OS Components Kernel: core OS code loaded at boot Daemons: background servers that provide system services (e.g. init, inetd, ntpd, httpd…) Libraries: libc, libdl, etc. System software: compilers, linkers, loaders, debuggers, virus scanners, etc. System monitors: top, df, ps, vmstat, etc. Utilities: backup, compression, recovery, etc. Interfaces: windowing (GUI), shells (CLI)

8. Slides based on Phil Hutto, Silberschatz 8 Modules Modules were originally developed to support the conditional inclusion of device drivers Early OS kernels would need to either: 1) include code for all possible devices or 2) be recompiled to add support for a new device Modules provided a means to dynamically add pre-compiled device code to an existing kernel binary Modules are basically dynamically linked libraries for the kernel! Modules could also be unloaded if the device was no longer in use (not usually done with DLLs) Most modern OS kernels (e.g. Solaris, BSD, Windows) support kernel modules Linux uses kernel modules extensively as a structuring technique and allows file systems, network protocols, system calls, etc. to be defined in kernel modules

9. Slides based on Phil Hutto, Silberschatz 9 Linux Core Kernel CPU Management Interrupts, exceptions, context switching, timers Memory Management Memory addressing, allocation, page frame management, swapping Process Management Descriptors, threads, address spaces, lifecycle, synchronization, signals, inter-process communication Device Management Drivers, device caches and buffer, file systems, networking, disk and packet scheduling

10. Slides based on Phil Hutto, Silberschatz 10 Core Kernel Applications System Libraries (libc) System Call Interface Hardware Architecture-Dependent Code I/O Related Process Related Scheduler Memory Management IPC File Systems Networking Device Drivers Modules

11. Slides based on Phil Hutto, Silberschatz 11 Linux Source Tree Spend some time poking around in the source tree using lxr.linux.no You should familiarize yourself with the structure of the source tree and get an idea about which directories contain the most code The latest kernel contains 5 to 7 million lines of code (depending on how you count) By comparison, Open Office is about 5 million lines of code and the latest Debian release (kernel plus all included applications) is about 200 million lines of code! Take a look at the Wikipedia entry “Source lines of code” (SLOC) for more fun …

12. Slides based on Phil Hutto, Silberschatz 12 Architecture Dependencies Linux uses a clever technique to deal with architecture (CPU) dependencies (cf. device dependencies) Logically architecture dependencies comprise a hardware abstraction layer (HAL) but effected code is spread across hundreds of files One approach would be to use #ifdef macros but this leads to unreadable code Linux isolates and abstracts all architecture dependencies into small functions or macros (e.g. disable interrupts) Architecture-dependent versions of each function or macro reside in the arch/* subdirectories The kernel Makefile does all the work of linking the appropriate architecture dependent versions without any #ifdefs The only #ifdefs that appear in the kernel source are for optional capabilities (like support for multiple processors or hyper-threading or kernel modules) that can be compiled out during kernel configuration