8/10/2019 dc-os arch

1/14

1

Operating System Architecture andDistributed Systems

Let us look into the architecture of akernel suitable for a distributed system.

A key principle of DS is openness andwith this in mind let us examine themajor kernel architectures:

Monolithic kernels

Micro-kernels

8/10/2019 dc-os arch

2/14

2

Open DS and System Software

A open DS should make it possible to: Run only that system software at each computer

that is necessary for its particular role in the systemarchitecture. For example, system software needs ofPDA and dedicated server are different and loadingredundant modules wastes memory resources.

Allow the software (and the computer) implementingany 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 usersor applications.

Introduce new services without harming the integrityof existing ones.

8/10/2019 dc-os arch

3/14

3

Separating Mechanisms and Policiesin OS and DS

A Guiding principle of OS design: The separation of fixed resource management mechanisms

from resource management policies, which vary fromapplication to application and service to service.

For example, an ideal scheduling system would providemechanisms that enable a multimedia application suchas videoconferencing to meet its real-time demandswhile coexisting with a non-real-time application suchas web browsing.

That is kernel would provide only the most basic

mechanisms upon which the general resourcemanagement tasks at a node are carried out.

Server modules would be dynamically loaded asrequired, to implement the required RM policies for thecurrently running applications.

8/10/2019 dc-os arch

4/14

4

OS/Kernel Architecture

The two key examples of kernel design approaches are: Monolithic Microkernel

Basically these two designs differ primarily in thedecision as to what functionality belongs in the kernel

and what is left to server processes that can bedynamically loaded to run on top of it.

In Literature, we find predominantly 3 types of OS: Monolithic OS Layered OS Microkernel-based OS

The first two may be put under the same category asmonolithic. The chambers 20thcentury dictionary definition of monolithic

is: a pillar, column, of a single stone: anything that resembling

a monolithic, massiveness.

8/10/2019 dc-os arch

5/14

5

Monolithic kernel and microkernel

Monolithic Kernel Microkernel

Serv er: Dy namically loaded server program:Kernel code and data:

.......

.......

Key:

S4

S1 .......

S1 S2 S3

S2 S3 S4

8/10/2019 dc-os arch

6/14

6

Application

Programs

Application

Programs

System Services

Hardware

User Mode

Kernel Mode

Monolithic Operating System

Better application Performance

Difficult to extend Ex: MS-DOS

8/10/2019 dc-os arch

7/147

Traditional OS

OS Designer

OS

Hardware

User Mode

Kernel Mode

Application

Programs

Application

Programs

8/10/2019 dc-os arch

8/148

Disadvantages of Monolithic OS

It is massive:

It performs all basic OS functions and takes up inthe order of megabytes of code and data

It is undifferentiated: It is coded in a non-modular way (traditionally)

although modern ones are much more layered.

It is intractable:

Altering any individual software component to adaptit to changing requirements is difficult.

8/10/2019 dc-os arch

9/149

New trend in OS design: Separating

mechanisms and policies

User Mode

Kernel Mode

Hardware

Microkernel

(very basic functions)

ServersApplicationPrograms

Application

Programs

8/10/2019 dc-os arch

10/1410

Micro-kernel

Compared to monolithic, microkernel designprovides only the most basic abstractions,

principally address space, threads and local IPC.

All other system services are provided byservers that are dynamically loaded precisely onthose computers in the DS that require them.

Clients access these system services using thekernels message-based invocationmechanisms.

8/10/2019 dc-os arch

11/1411

Microkernel/Client Server OS

Tiny OS kernel providing basic primitive (process, memory, IPC)

Traditional services becomes subsystems

OS = Microkernel + User Subsystems

Client

Application

OS

Emulators

File

Server

Network

Server

Display

Server

Microkernel

Hardware

User

Kernel

Send

Reply

Ex: Mach , PARAS, Cho rus , etc.

8/10/2019 dc-os arch

12/1412

The role of the microkernel

MK appears as a layer between H/W andsystem systems.

Middleware

Language

support

subsystem

Language

support

subsystem

OS emulation

subsystem....

Microkernel

Hardware

The microkernel supports middleware via subsystems

8/10/2019 dc-os arch

13/1413

Few Popular Microkernel Systems

MACH, CMUIt supports different OS emulators

including Unix and OS/2.

PARAS, C-DACChorus

QNX,

Windows NToriginal design.

8/10/2019 dc-os arch

14/14

14

Comparison: Monolithic andMicro-kernel OS Design

The chief advantages of a MK-based OS:

Extensibility and its ability to enforce modularitybehind memory protection boundaries

A relative small kernel is more likely to be free ofbugs than one that is larger and complex.

The advantage of a monolithic OS:

Relative efficiency with which operations can be

invoked is high because even invocation to aseparate user-level address space on the same nodeis more costly.