Computer Systems and Systems Software Lecture 15

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Computer Systems and Systems Software - Lecture 15

• In this lecture we will look at UNIX Linux
• History
• Overall design
• User/Programmer Interface
• Process Management

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History

• First developed in 1969 by Ken Thompson and
  Dennis Ritchie of the Research Group at Bell
  Laboratories incorporated features of other
  operating systems, especially MULTICS.
• The third version was written in C, which was
  developed at Bell Labs specifically to support
  UNIX.
• The most influential of the non-Bell Labs and
  non-ATT UNIX development groups University of
  California at Berkeley (Berkeley Software
  Distributions).

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• 4BSD UNIX resulted from DARPA funding to develop
  a standard UNIX system for government use.
• Developed for the VAX, 4.3BSD is one of the most
  influential versions, and has been ported to many
  other platforms.
• Several standardization projects seek to
  consolidate the variant flavors of UNIX leading
  to one programming interface to UNIX.

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Linux origins

• 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.
• 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.

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Linux system

• Linux uses many tools developed as part of
  Berkeleys BSD operating system, MITs X Window
  System, and the Free Software Foundations GNU
  project.
• The main system libraries were started by the GNU
  project, with improvements provided by the Linux
  community.
• Linux networking-administration tools were
  derived from 4.3BSD code recent BSD derivatives
  such as FreeBSD have borrowed code from Linux in
  return.
• The Linux system is maintained by a loose network
  of developers.

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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 Slackware. Red Hat
  is common distributions from commercial source.

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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
  derivate of Linux, may not make the derived
  product proprietary software released under the
  GPL may not be redistributed as a binary-only
  product.

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Design Principles

• Linux is a multiuser, multitasking system with a
  full set of UNIX-compatible tools.
• Its file system adheres to traditional UNIX
  semantics, and it fully implements the standard
  UNIX networking model.
• Main design goals are speed, efficiency, and
  standardization.
• Linux is designed to be compliant with the
  relevant POSIX standards

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UNIX/Linux Components

• Like most UNIX implementations, Linux is composed
  of three main bodies of code the most important
  distinction is between the kernel and all other
  components.
• The kernel is responsible for maintaining the
  important abstractions of the operating system.
• Kernel code executes internel mode with full
  access to all the physical resources of the
  computer.
• All kernel code and data structures are kept in
  the same single address space.
• Provides file system, CPU scheduling, memory
  management, and other OS functions.

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• The system libraries define a standard set of
  functions through which applications interact
  with the kernel, and which implement much of the
  operating-system functionality that does not need
  the full privileges of kernel code.
• The system programs use the kernel-supported
  system calls to provide useful functions, such as
  compilation and file manipulation.

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UNIX/Linux overall structure

• System User User Compilers
• management Processes Utility
• programs Programs
• --------------------------------------------------
  -------------------------
• Shells
• --------------------------------------------------
  -------------------------
• System Shared Libraries
• _______________________________________
• Linux Kernel
• --------------------------------------------------
  ----------------
• Loadable kernel modules

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Linux kernel modules

• Sections of kernel code that can be compiled,
  loaded, and unloaded independently of the rest of
  the kernel.
• A kernel module may typically implement a device
  driver, a file system, or a networking protocol.
• The module interface allows third parties to
  write and distribute, on their own terms, device
  drivers or file systems that could not be
  distributed under the GPL.
• Kernel modules allow a Linux system to be set up
  with a standard, minimal kernel, without any
  extra device drivers built in. Linux then allows
  additional modules to be loaded and integrated
  with the base system - tailored to underlying
  hardware and needs of system

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System Calls

• System calls define the programmer interface to
  UNIX
• The set of systems programs commonly available
  defines the user interface.
• The programmer and user interface define the
  context that the kernel must support.
• Roughly three categories of system calls in UNIX.
• File manipulation (same system calls also support
  device manipulation)
• Process control
• Information manipulation.

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User Interface - Shells

• Shell the user process which executes programs
  (also called command interpreter).
• Called a shell, because it surrounds the kernel.
• The shell indicates its readiness to accept
  another command by typing a prompt, and the user
  types a command on a single line.
• A typical command is an executable binary object
  file.
• The shell travels through the search path to find
  the command file, which is then loaded and
  executed.

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Shells and Commands (Cont.)

• The directories /bin and /usr/bin are almost
  always in the search path.
• The shell usually suspends its own execution
  until the command completes.
• Programmers and users mainly deal with already
  existing systems programs the needed system
  calls are embedded within the program and do not
  need to be obvious to the user.
• The most common systems programs are file or
  directory oriented e.g. mkdir, rmdir, cd, pwd,
  ls, cp, mv, rm

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Process Control

• A process is a program in execution.
• Process control system calls
• fork creates a new process that is a copy of the
  parent process
• execve is used after a fork to replace the child
  processess virtual memory space (program code
  and data) with program code and data of a program
  file passed as parameter to execve call
• exit terminates a process
• A parent may wait for a child process to
  terminate wait provides the process id of a
  terminated child so that the parent can tell
  which child terminated.

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Illustration of Process Control Calls
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Process Control (Cont.)

• Processes communicate via pipes queues of bytes
  between two processes that are accessed by a file
  descriptor.
• All user processes are descendants of one
  original process, init.
• init forks a getty process initialises terminal
  line parameters and passes the users login name
  to login.
• login sets the numeric user identifier of the
  process to that of the user
• executes a shell which forks subprocesses for
  user commands.

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Process Control (Cont.)

• setuid bit allows programs to sets the effective
  user identifier of the process to the user
  identifier of the owner of a file being accessed,
  and leaves the real user identifier as it was.
• setuid scheme allows certain processes to have
  more than ordinary privileges when accessing
  certain system resources while still being
  executable by ordinary users e.g. changing
  password involves process invoked by user
  changing password file.

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Process Management

• Representation of processes is a major design
  problem for operating system.
• UNIX is distinct from other systems in that
  multiple processes can be created and manipulated
  with ease.
• These processes are represented in UNIX by
  various control blocks.
• Control blocks associated with a process are
  stored in the kernel.
• Information in these control blocks is used by
  the kernel for process control and CPU scheduling.

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Process Control Blocks

• The most basic data structure associated with
  processes is the process structure.
• unique process identifier (PID)
• scheduling information (e.g., priority)
• pointers to other control blocks
• The virtual address space of a user process is
  divided into text (program code), data, and stack
  segments.

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System Data Segment

• Most ordinary work is done in user mode system
  calls are performed in system mode.
• The system and user phases of a process never
  execute simultaneously.
• a kernel stack (rather than the user stack) is
  used for a process executing in system mode.
• The user structure contains information about
  environment of process and resources allocated to
  it e.g. Owner user id, and open file descriptors
• The kernel stack and the user structure together
  compose the system data segment for the process.

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• Every process with sharable text has a pointer
  from its process structure to a text structure.
• always resident in main memory.
• records how many processes are using the text
  segment
• records where the page table for the text segment
  can be found on disk when it is swapped.

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Finding parts of a process using process
structure
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Allocating a New Process Structure

• fork allocates a new process structure for the
  child process, and copies the user structure.
• new page table is constructed
• new main memory is allocated for the data and
  stack segments of the child process
• copying the user structure preserves open file
  descriptors, user and group identifiers, signal
  handling, etc.

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Allocating a New Process Structure

• vfork does not copy the data and stack to the new
  process the new process simply shares the page
  table of the old one.
• new user structure and a new process structure
  are still created
• commonly used by a shell to execute a command and
  to wait for its completion
• A parent process can use vfork to produce a child
  process the child uses execve to change its
  virtual address space, so there is no need for a
  copy of the parent.

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• Using vfork with a large parent process saves CPU
  time, but can be dangerous since any memory
  change occurs in both processes until execve
  occurs.
• execve creates no new process or user structure
  rather the text and data of the process are
  replaced.

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CPU Scheduling

• Every process has a scheduling priority
  associated with it larger numbers indicate lower
  priority.
• Negative feedback in CPU scheduling makes it
  difficult for a single process to take all the
  CPU time.
• Process aging is employed to prevent starvation.
• When a process chooses to relinquish the CPU, it
  goes to sleep on an event.

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• When that event occurs, the system process that
  knows about it calls wakeup with the address
  corresponding to the event, and all processes
  that had done a sleep on the same address are put
  in the ready queue to be run.