linux overview coms w4118 spring 2008. slides based on phil hutto, silberschatz 2 history linux is a...

LinuxOverview

COMS W4118

Spring 2008

Slides based on Phil Hutto, Silberschatz2

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


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


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


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


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


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)


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


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


Core KernelApplications

System Libraries (libc)

System Call Interface

Hardware

Architecture-Dependent Code

I/O Related Process RelatedScheduler

Memory Management

IPC

File Systems

Networking

Device Drivers

Mod

ules


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 …


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

linux overview coms w4118 spring 2008. slides based on phil hutto, silberschatz 2 history linux is a...

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