Dr. Christian Vecchiola Postdoctoral Research Fellow [email protected] Cloud Computing and Distributed Systems (CLOUDS) Lab Dept. of Computer Science

Dr. Christian VecchiolaPostdoctoral Research Fellow

[email protected]

Cloud Computing and Distributed Systems (CLOUDS) Lab

Dept. of Computer Science and Software Engineering

The University of Melbourne

Operating System Architectureand Distributed Systems

Most concepts aredrawn from Chapter 6© Pearson Education

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sOutline

Introduction OS / Kernel Models

– Monolithic Operating Systems– Layered Systems– Traditional Operating Systems– Micro-kernel Operating Systems

Micro-kernel Operating Systems– Observations– Architecture– Client Server Model– Comparison

Hybrid Approaches Summary

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sIntroduction

Why Operating Systems?– OSes are a fundamental component of today’s

computing systems.– They offer a set of services for:• Managing the hardware resources of devices• Scheduling the execution of the applications• Simplifying the development of applications

– OSes constitute a common sub-stratus for• (almost) all the applications ….• …. even distributed system middleware

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sIntroduction

A little bit of history…– Pre-OS software development (before 60s)• Single task model (one application running)• Developers had to:

– Program the system from booting the hardware up..– Manage every aspect of the system

• Assembly language• Very challenging • Model:

Machine + Program + Data = Application

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sIntroduction

A little bit of history…– The OS era (60s onwards)• Evolution from single task to multi-tasking• There exist a common program that

– .. is loaded before any other program– .. manages the hardware resources of the machine– .. schedules the execution of user applications (one or more)– .. provides a higher level interface to the system

• Development of compilers• Model:

Application + OS +Machine = Execution

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sIntroduction

What does an OS do?– Manages the hardware resources

• Memory• CPU• Disks

– Provides access to connected devices (drivers)• Printers• Cameras• Scanners…

– Manages and Schedules applications• Simple user applications• Services

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sIntroduction

Ehy… are we studying DS…?– OSes are a fundamental component in DS– They provide the basic services for what

concerns• Network connectivity• Machine resources management• Concurrency and IPC• Access to the file system

– Distributed Systems are based on middleware• it is “just another” application for the OS• it might be a fundamental part of the OS

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sIntroduction

DS-wanted features of an OS– Let us look into the architecture of a kernel

suitable for a distributed system. – A key principle of DS is openness and with this

in mind let us examine the major kernel architectures:• Monolithic kernels• Layered architecture-based kernels• Micro-kernels

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sIntroduction

DS-wanted features of an OS– An open DS should make it possible to:

• Run only that system software on each computer that is necessary for its particular role in the system architecture:– Avoiding redundant modules on capability-limited devices.– Optimizing the behavior of specific components in different scenarios

(server, client, ….)

• Allow the software (and the computer) implementing any particular service to be changed independent of other facilities.

• Allow for alternatives of the same services to be provided, when this is required to suit different users or applications.

• Introduce new services without harming the integrity of existing ones.

–Which features are required by an OS to expose these properties?

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sIntroduction

A Guiding principle of OS design

– For example, an ideal scheduling system would provide mechanisms that enable a multimedia application such as videoconferencing to meet its real-time demands while coexisting with a non-real-time application such as web browsing.

– That is kernel would provide only the most basic mechanisms upon which the general resource management tasks at a node are carried out.

– Server modules would be dynamically loaded as required, to implement the required RM policies for the currently running applications.

Separation of fixed resource management “mechanisms“ from resource management “policies”, which vary from application to application and service to service.

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sOS / Kernel Models

Main Design Principles– The two key examples of kernel design

approaches are:• Monolithic• Microkernel

– Key difference: what does belong to the kernel?– Three main models:

• Monolithic OS• Layered OS• Microkernel-based OS

– The first two can be considered monolithic.

The chambers 20th century dictionary definition of monolithic is: a pillar, column, of a single stone: anything that resembling a monolithic, massiveness.

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s Monolithic Kernel vs Micro-Kernel

OS / Kernel Models

Monolithic Kernel Micro-Kernel

Server:

Dynamically loaded server program:

Kernel code and data:

.......

.......

.......S2 S3 S4

S1

S2 S3 S4S1

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sOS / Kernel Models

Operating System Models– Serve as frameworks that unify capabilities, services and

tasks to be performed– Three approaches to building OS....

• Monolithic OS• Layered OS• Microkernel based OS

– Client server OS– Suitable for distributed systems

– Simplicity, flexibility and high performance are crucial for OS.

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sOS / Kernel Models

Monolithic Kernels

– Better application performance– Hard to extend– Example: MS-DOS

ApplicationPrograms

System Services

Hardware

User Mode

Kernel Mode

ApplicationPrograms

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sOS / Kernel Models

Layered Operating Systems

User Mode

Kernel Mode

Memory & I/O Device Mgmt

Process Scheduler

ApplicationPrograms

ApplicationPrograms

System Services

Hardware

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s Traditional Operating Systems

OS / Kernel Models

OS Designer

OS

User Mode

Kernel Mode

ApplicationPrograms

ApplicationPrograms

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sMicro-Kernel Operating Systems

Observations–Monolithic Operating Systems are:• Massive: they perform all basic OS functions and

take up in the order of megabytes of code and data.• Undifferentiated: they are coded in a non-modular

way (traditionally) although modern ones are much more layered.• Intractable: altering any individual software

component to adapt it to changing requirements is difficult.

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Observations– New trend in Operating System Design

User Mode

Kernel Mode

Hardware

Microkernel(very basic functions)

ServersApplicationPrograms

ApplicationPrograms

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Micro-Kernel Operating Systems– Compared to monolithic, microkernel design provides

only the most basic abstractions, • principally address space, threads and local IPC.

– All other system services are provided by servers that are dynamically loaded precisely those computers in the DS that require them.

– Clients access these system services using the kernel’s message based invocation mechanisms.

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Client Server Model

– Tiny OS kernel providing basic primitive (process, memory, IPC)– Traditional services becomes subsystems– OS = Microkernel + User Subsystems– Examples: Mach, PARAS, and Chorus

ClientApplication

OS Emulators

FileServer

NetworkServer

DisplayServer

Microkernel

Hardware

User

Kernel

SendReply

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Architecture– Micro-Kernels are a layer between H/W and system

systems. If performance is goal, rather than portability, then middleware may use facilities of the kernel directly.

– Micro-Kernels support middleware as subsystems.

Middleware

Languagesupport

subsystem

Languagesupport

subsystem

OS emulationsubsystem ....

Microkernel

Hardware

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Examples of Micro-Kernels–MACH, CMU• It supports different OS emulators (Unix and OS/2).• (Mach only) base for OS X

– PARAS, C-DAC– Chorus– QNX, – (Windows NT) • combination of layered and microkernel..• .. but massive code.

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Comparison–Micro-Kernel main advantages:• Extensibility and its ability to enforce modularity

behind memory protection boundaries• A relative small kernel is more likely to free of bugs

than one that is larger and complex.

–Monolithic OS main advantage:• Relative efficiency with which operations can be

invoked is high because even invocation to a separate user-level address space on the same node is more costly.

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sHybrid Approaches

Which is the trend today?–Many modern OS follow hybrid approach in OS

structure. E.g., Windows NT.– Pure microkernel OSs such as Chorus & Mach

have changed over a time to allow servers to be loaded dynamically into the kernel address space or into a user-level address space.

– Some OSs such as SPIN used event-based model as a mechanism for interaction between modules grafted into the kernel address space.

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sSummary

What did we learn?– Operating system provides various types of

facilities to support middleware for distributed system: • encapsulation, • protection, • concurrent access • management of node resources.

– New OS designs provide flexibility in terms of separating mechanisms from policies.

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sAdditional References

Tanenbaum, A.S, Modern Operating Systems, 2nd /3rd Editions, Prentice Hall, ISBN 013-031-3580 / 0136-006-639.

Silbershatz, A., Galvin, P.B., Gagne, Modern Operating Systems, 8th Editions, Wiley, ISBN 0470-128-720.

Documents

Dr. Christian Vecchiola Postdoctoral Research Fellow [email protected] Cloud Computing and Distributed Systems (CLOUDS) Lab Dept. of Computer Science