Operating System Architecture and Distributed Systems
Some concepts are drawn from Chapter 7 ©
Distinguished Professor
Cloud Computing and Distributed Systems (CLOUDS) Laboratory School of Computing and Information Systems
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The University of Melbourne, Australia http://www.cloudbus.org/652
http://www.buyya.com/microkernel/chap2.pdf
Operating System Architecture and Distributed Systems (DS)
Explore 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
Open DS and System Software
A open DS should make it possible to:
Run only that (“specific” components of) system software at each computer that is necessary for its particular role in the system architecture.
For example, system software needs of laptops and dedicated servers are different and loading redundant modules wastes memory resources.
Allow the software implementing any particular service to be changed independent of other facilities.
Allow for alternatives of the same service to be provided, when this is required to suit different users or applications.
Introduce new services without harming the integrity
of existing ones.
Separating Mechanisms and Policies in OS and DS
A Guiding Principle of OS design:
The separation of fixed resource management “mechanisms“ from resource management “policies”, which vary from application to application and service to service.
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.
OS/Kernel Architecture
The two key examples of kernel design approaches are:
Monolithic Microkernel
These two designs differ primarily in the decision as to what functionality belongs in the kernel and what is left to server processes that can be dynamically loaded to run on top of it.
In literature, we find predominantly 3 types of OSs:
Monolithic OS
Layered OS
Microkernel-based OS
Monolithic kernel and microkernel
Ke y: Server:
Monolithic Kernel
Kernel code and data:
Microkernel Dynamically loaded server program:
S1 S2 S3 S4
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.
Monolithic Operating System
Application Programs
Application Programs
System Services
Better application Performance Difficult to extend
User Mode Kernel Mode
Ex: MS-DOS
Layered OS
Application Programs
Application Programs
System Services Memory & I/O Device Mgmt
Process Schedule Hardware
User Mode Kernel Mode
Easier to enhance
Each layer of code access lower level interface
Low-application performance
Traditional OS
User Mode Kernel Mode
OS Designer
Application Programs
Application Programs
Disadvantages of Monolithic OS
It is massive:
It performs all basic OS functions and takes up in
the 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 adapt it to changing requirements is difficult.
New trend in OS design: Separating mechanisms and policies
User Mode Kernel Mode
Microkernel
(very basic functions)
Application Programs
Application Programs
Micro-kernel / μ-kernel
Compared to monolithic, microkernel design provides only the most basic abstractions,
address space, threads and local IPC.
All other system services are provided by servers that are dynamically loaded precisely on those computers in the DS that require them.
Clients access these system services using the kernel’s message-based invocation mechanisms.
Microkernel/Client Server OS
Microkernel
User Kernel
Client Application
OS Emulators
File Server
Network Server
Display Server
Tiny OS kernel providing basic primitive (process, memory, IPC) Traditional services becomes subsystems
OS = Microkernel + User Subsystems
Ex: Mach, QNX, Windows NT!
The role of the microkernel (MK)
MK appears as a layer between H/W and a layer of major system components (subsystems). If performance, rather than portability is goal, then middleware may use facilities of MK directly.
Middlew are
Language support
Language support subsystem
Mic rokernel
OS emulation subsystem
The microkernel supports middleware viasubsystems
Few Popular Microkernel Systems
MACH, CMU ( University) supports OS emulators such as Unix and OS/2.
PARAS (C-DAC, India) for PARAM Supercomputers
ChorusOS (Sun, USA) Realtime OS (RTOS)
seL4 (created by NICTA/Data61, Australia)
phones (e.g., BlackBerry).
Intel x86, MIPS, PowerPC, StrongARM..
Windows NT – original design.
QNX – Unix-like RTOS (Canada, BlackBerry) used in a variety of devices including cars and mobile
seL4: Made in Australia!
Comparison: Monolithic and Micro-kernel OS Design
The main advantages of a MK-based OS:
A relative small kernel is more likely to be free of
bugs than one that is larger and complex.
Extensibility and its ability to enforce modularity behind memory protection boundaries
The advantage of a monolithic OS:
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.
Hybrid Approaches
Many modern OSs follow hybrid approach in OS structure. E.g., Windows NT.
Pure microkernel OSs such as Chorus & Mach have changed over 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) use event-based model as a mechanism for interaction between modules grafted into the kernel address space.
OSs provide various types of facilities/services to support middleware for distributed system:
encapsulation, protection, and concurrent access and management of node resources.
Three types of OS:
Monolithic OS
Layered OS
Microkernel-based OS
S designs provide flexibility in terms of separating mechanisms from policies.
Microkernel based systems are flexible
Quite popular model for OS design for embedded systems
optimized Kernels like nanokernel or picokernel 20
Additional References
, The Design of PARAS
Microkernel, Centre for Development of
Advanced Computing (C-DAC), 1998.
http://www.buyya.com/microkernel/chap2.pdf
, , June Andronick, seL4 in Australia: From Research to Real-World Trustworthy Systems, Communications of the ACM, April 2020.
https://cacm.acm.org/magazines/2020/4/243 641-sel4-in-australia/fulltext
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