netw 3005. reading for this lecture, you should have read chapter 2 (sections 1-9). netw3005...

NETW 3005

Reading

• For this lecture, you should have read Chapter 2 (Sections 1-9).

NETW3005 (Operating Systems) Lecture 02 - System Structure & Processes 2

Last lecture

• History and types of operating systems:– batch systems, multiprogramming systems,

time sharing systems, etc.

• Operating system tasks:– process management, storage

management, I/O device management, user interface.

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This lecture

• Hierarchical Structure in Operating Systems

• System calls and interrupts

• Representing processes in Operating Systems

• Overview of process scheduling.

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Hierarchical structure in OS

• An operating system, like any complex computer system, must be carefully designed.

• One central requirement is modularity.

• Distinction between system programs and application programs.

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Application programs

System programs

Kernel operations

Device drivers

Terminal drivers

Memory manager

Application programs

• Ones that ordinary users interact with:– Word-processors– Database packages– Web browsers– Compilers, editors, IDEs, etc– . . .

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System programs

• Provide a general-purpose lower-level function. System functions include:– file manipulation: create, delete, copy etc.– status info: date, available memory.– program loading and execution.– communication between processes.– Command interpreters (a.k.a. shell

programs). The set of system programs define the user interface.

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Other system functions

• Not strictly part of the OS, but often packaged with it.

• Programs to modify files (text editors, search, transform).

• Compilers, assemblers and interpreters.

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The set of system programs defines the user interface.

Degrees of modularity

• Different operating systems enforce different degrees of modularity.

• Ideally you want to oblige all system and application programs to talk to the hardware via the kernel.

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MS-DOS

NETW3005 (Operating Systems) 11COSC 243 (Operating Systems) 11

System programs

Kernel operations

Device drivers

Terminal drivers

Memory manager

Application programs

Lecture 02 - System Structure & Processes

Talking to the kernel: system calls

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System Program

Kernel

Hardware

‘‘read from the keyboard’’ ‘‘open a file’’‘‘write to the screen’’

System calls

• Written in the same language as kernel (typically C).

• Available to Assembler and (at least) some HLLs — C, Perl, etc.

• The set of system calls is termed the programmer interface to the system.

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A simple system program — copy

• cp file1 file2

• Open file1; Create file2.

• Loop: Read from file1; Write to file2.

• Close file1; Close file2.

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Types of system call (1)

• Process control– create, terminate, suspend, resume, abort.

• File manipulation– open, close, read, write

• Device manipulation– request, release, read, write.

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Types of system call (2)

• Housekeeping– get/set time or date– get/set attributes (process, device, file)

• Communications– set up link,– send/receive message

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InterruptsHow system calls are implemented

• CPU responds to interrupts no matter what else it happens to be doing.

• An interrupt transfers control to an appropriate module in the kernel.

• A system call transfers control to the kernel by generating an interrupt (sometimes called a trap in this context).

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Responding to an interrupt

• Effectively a Jump to Subroutine: – current instruction address (PC) is saved– control transferred to fixed address,

depending on the interrupt.

• The interrupt vector is an array of locations that hold the addresses of these routines, usually held in low memory.

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Implementation issues

• How do we guarantee that the interrupt-handing routine won’t affect the interrupted process?

• What happens if an interrupt occurs while an interrupt-handling routine is executing?

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Virtual machines

• System calls allow the OS to hide the low-level hardware from application programs.

• In a virtual machine these system calls are executed by a program which emulates the hardware.

• This hardware may, or may not be the same as the actual hardware.

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processes

hardware

kernel

processes processes processes

kernel1 kernel2 kernel3

hardware

VM1 VM2 VM3

virtual-machine implementation

Benefits of virtual machines

• Protection: users aren’t even aware there are other users.

• Good for operating systems R & D. (No down-times: just give a system programmer her own virtual machine.)

• A way of solving system-compatibility problems.

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Problems with virtual machines

• Speed: virtual machines are slower.

• Implementation is very difficult, e.g. resource allocation (particularly disc).

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Java

• Compiled Java code is called byte-code.

• It is designed to run on the Java Virtual Machine (JVM).

• Think about the command-line process of compiling and running a Java program as opposed to compiling and running a C++ program.

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Processes

• Recall: a process is not just a program – it is a dynamic entity.

• A given program (e.g. emacs) could be executing many times on a given machine – the machine must represent each execution as a separate process.

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Components of a process (1)

• Code section: the program code itself

• Data section: any global variables used by the program

• Process stack: any local variables currently being used (subroutine parameters, return addresses, etc.)

• Program counter: a pointer to some place in the program code.

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Components of a process (2)

• Contents of CPU registers.

• Memory management information.

• Accounting information: who owns the process, how long it’s been running, how much CPU time it’s used so far, etc.

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Process state

• The operating system keeps track of the state of each process.

• A process can be in any of the following states:– new– running– waiting/blocked– ready– terminated

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An Example from MacOS X

oucs1046: chandley$ ps –ax

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Macos X processes (1)

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PID TTY TIME CMD 1 ?? 1:26.15 /sbin/launchd10 ?? 0:03.03 /usr/libexec/kextd11 ?? 1:06.37 /usr/sbin/DirectoryService12 ?? 0:27.57 /usr/sbin/notifyd13 ?? 15:11.87 /usr/sbin/syslogd14 ?? 2:08.74 /usr/sbin/configd15 ?? 0:18.55 /usr/sbin/distnoted16 ?? 25:23.28 /usr/sbin/mDNSResponder –launchd20 ?? 0:03.56 /usr/sbin/securityd -i

Macos X processes (2)

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24 ?? 1:24.42 /usr/sbin/ntpd -n -g -p …25 ?? 0:02.00 /usr/sbin/cron26 ?? 17:58.14 /usr/sbin/update27 ?? 0:00.01 /sbin/SystemStarter31 ?? 0:00.03 /System/Library/CoreServices/…

Macos X processes (3)

• Produced 75 different processes.

• Several distinct classes– Daemons – security, cron, update, etc.– Core Services – Dock, Finder, etc.– Network and communications.– Application programs – Preview, Power-

Point, Word, Adobe Acrobat, etc.

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Scheduling: an overview

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Disk Job2, Job2, Job3, Job4, …

CPU

Main Memory

O.S.

Process 1

Process 2

Process 3

Process 4

Types of scheduler

• Long-term scheduler (batch systems): decides which jobs loaded onto the disk should be moved to main memory.

• Short-term scheduler (a.k.a. CPU scheduler): chooses how to allocate the CPU between the processes which are ready to execute.

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Scheduling queues

• The relationships between processes are represented by the O/S as queues.– Job queue: all the processes in the system

(including those on disk).– Ready queue: all the processes which are

ready to execute.– Device queue: all the processes waiting to

use a particular device. (One queue per device.)

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The Flow of Processes in an OS

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ready queue CPU

I/O request

I/O device queue

I/O device queue

I/O

I/O

fork

interrupt mechanism

Process creation

• A process is created (‘spawned’) by another process – we talk of parent and child processes.

• When you launch an application, the terminal process spawns the application process.

• Processes are able to ‘call’ other processes, just as a program can call functions.

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Differences

• A child process can (and often does) run concurrently with its parent.

• A child process needn’t be constrained to use the resources of its parent (although it may be, and often is).

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A process tree (1)

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root

daemons init

user1 user2 user3

system proc application

child child

A process tree (2)

• Note that users are treated as processes by the operating system.

• How does that work?

• Answer: users are really represented to the system as shell processes.

• An important notion: the root user.

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Homework

• The Unix command ps displays information about processes on the system. Read the man page for ps (i.e. do “man ps”), and try out some of the options.

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Next Lecture

Threads and Data SharingChapter 4 (Sections 1-4)