1. Operating System Architecture and Distributed Systems
Most concepts are
drawn from Chapter 6
© Pearson Education
Dr. Christian Vecchiola
Postdoctoral Research Fellow
Cloud Computing and Distributed Systems (CLOUDS) Lab
Dept. of Computer Science and Software Engineering
The University of Melbourne

2. Outline Hybrid Approaches Introduction OS / Kernel Models Summary
Monolithic Operating Systems
Layered Systems
Traditional Operating Systems
Micro-kernel Operating Systems
Observations
Architecture
Client Server Model
Comparison
Hybrid Approaches
Summary

3. Introduction 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

4. Introduction 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

5. Introduction 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

6. Introduction 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

7. Introduction 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

8. Introduction 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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9. Introduction 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?

10. Introduction 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.

11. OS / 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.

12. OS / Kernel Models Monolithic Kernel vs Micro-Kernel S1 ....... S1 S2
Server:
Kernel code and data:
Dynamically loaded server program:

13. OS / 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.

14. OS / Kernel Models Monolithic Kernels System Services Hardware
Better application performance
Hard to extend
Example: MS-DOS
Application
Programs
Application
Programs
User Mode
Kernel Mode
System Services
Hardware

15. Memory & I/O Device Mgmt
OS / Kernel Models
Layered Operating Systems
Application
Programs
Application
Programs
User Mode
Kernel Mode
System Services
Memory & I/O Device Mgmt
Process Scheduler
Hardware

16. OS / Kernel Models Traditional Operating Systems OS User Mode
Application
Programs
Application
Programs
User Mode
Kernel Mode OS
OS Designer

17. Micro-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.

18. Micro-Kernel Operating Systems
Observations
New trend in Operating System Design
User Mode
Kernel Mode
Hardware
Microkernel
(very basic functions)
Servers
Application
Programs

19. 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.

20. Micro-Kernel Operating Systems
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
Client
Application
OS Emulators
File
Server
Network
Display
Microkernel
Hardware
User
Kernel
Send
Reply

21. Micro-Kernel Operating Systems
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
Language
support
subsystem
OS emulation
....
Microkernel
Hardware

22. Micro-Kernel Operating Systems
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.

23. Micro-Kernel Operating Systems
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.

24. Hybrid 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.

25. Summary 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.
Car

26. Additional References
Tanenbaum, A.S, Modern Operating Systems, 2nd /3rd Editions,  Prentice Hall,
ISBN /
Silbershatz, A., Galvin, P.B., Gagne, Modern Operating Systems, 8th Editions,  Wiley,
ISBN