Operating SystemsECE344

Ashvin GoelECE

University of Toronto

OS Design

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Design of Operating Systems

OS manages h/w resourceso Allows running many programs at the same timeo Allows programs to interact with h/w devices

Hardware

OS Kernel

Program1 UIBackground

tasksProgram2

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Key Ideas in OS Design

How does OS allow running many programs?o Provides virtualization mechanism that allows programs to

share resources securely

How does OS allow programs to interact with h/w?o Provides a h/w abstraction, implemented via system calls

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Virtualization

OS provides the illusion that each program is running on its own machineo Program thinks it has full access to CPU, memory, disko This is called virtualization because there is one physical

machine, but the OS provides the illusion of multiple virtual machines

Hardware

OS Kernel

Program1 UIBackground

tasksProgram2

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Benefits of Virtualization

Virtual machines provide resource isolationo Program is not aware that other programs exist, i.e., each

program is isolated from otherso A program cannot accidentally overwrite another program’s

memory or files, causing it to crasho Ideally, if a program uses too much memory, disk, etc., only its

performance degrades

What are the properties of an “ideal” virtual machine?

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Implementing Virtualization

OS implements virtual machines by virtualizing each physical resource

Processor -> Threadso OS provides the illusion that a large number of virtual

processors, called threads, are available to programs Run cpu.c

Memory -> Virtual Memoryo OS provides the illusion that each program has access to a

large amount of contiguous, private memory Run mem.c

Disk -> Fileso OS provides the illusion that a large number of virtual disks,

called files, are available to programs

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Concurrency

Thread abstraction is powerful because it allows a program to perform several tasks in parallel (or concurrently)o E.g., a web server can serve multiple web requests, by using

a thread per request

However, running threads concurrently raises some challenging problemso Run threads-race.c with args 100, 1000, 10000, 100000o Run threads-sync.c

The OS runs multiple programs concurrently, and thus must handle these problems as well

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System Calls

Programs request h/w access via system callso Similar to procedure calls, but a little more complicatedo System calls provide an abstraction of h/w, making it easier to

write programs

Set of system calls is OS API

Examples:o Create/destroy processo Allocate/deallocate memoryo Read/write a file

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System Call Example

Program calls read() syscall to read a file from disko Execution goes via a library that generates a trapo Trap invokes the kernel, which accesses disko Kernel returns results to program

Will discuss traps, and need for traps later

Hardware

OS Kernel

Program1 Library

trap

read return results

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OS Interface

OS Kernel

Video Card

CPU

Monitor

Printer

Disk

MemoryNetwork

Application layer

system calls: thread_create(), read(), write(), thread_join() ...

Device Mgmt File System Network Comm.

Process MgmtProtection Security

Hardware layer

virtual machine interface

physical machine interface

Thread scheduler Memory management

OS interface

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Summary

Two important OS conceptso OS isolates applications using virtualizationo OS abstracts h/w with a simpler interface

Virtualizationo Each program thinks it owns the machineo Programs don’t need to worry about other running programs

E.g., a program cannot corrupt other programs

Abstractiono Simpler interface to h/w implemented via system callso Eases programing, improves portability