Operating System Organization

Andy Wang

COP 5611

Advanced Operating Systems

Outline

Organizing operating systemsSome microkernel examplesObject-oriented organizations

Spring

Organization for multiprocessors

Operating System Organization

What is the best way to design an OS?Put another way, what are the important

software characteristics of an OS?Decide on those, then design to match

them

Important OS Software Characteristics

Correctness and simplicityPower and completenessPerformanceExtensibility and portabilitySuitability for distributed and parallel

systemsCompatibility with existing systemsSecurity and fault tolerance

Common OS Organizations

Monolithic Virtual machine Layered designsKernel designsMicrokernelsObject-Oriented

Note that individual OS components can be organized these ways

Monolithic OS Design

Build OS as single combined moduleHopefully using data abstraction,

compartmentalized function, etc.

OS lives in its own, single address spaceExamples

DOSearly Unix systemsmost VFS file systems

Pros/Cons of Monolithic OS Organization

+ Highly adaptable (at first . . .)

+ Little planning required

+ Potentially good performance

– Hard to extend and change

– Eventually becomes extremely complex

– Eventually performance becomes poor

– Highly prone to bugs

Virtual Machine Organizations

A base OSprovides services in a very generic way

One or more other OSes live on top of the base systemUsing the services it providesTo offer different views of system to users

Examples - IBM’s VM/370, the Java interpreter

Pros/Cons of VM Organizations

+ Allows multiple OS personalities on a single machine

+ Good OS development environment

+ Can provide good portability of applications

– Significant performance problems

– Especially if more than 2 layers

– Lacking in flexibility

Layered OS Design

Design tiny innermost layer of softwareNext layer out provides more functionality

Using services provided by inner layer

Continue adding layers until all functionality required has been provided

ExamplesMulticsFlukelayered file systems and comm. protocols

Pros/Cons of Layered Organization

+ More structured and extensible

+ Easy model

– Layer crossing can be expensive

– In some cases, multiple layers unnecessary

– Duplicate caching/consistency issues

Kernel OS Designs

Similar to layers, but only two OS layersKernel OS servicesNon-kernel OS services

Move certain functionality outside kernelfile systems, libraries

Unlike VMs, kernel doesn’t stand aloneExamples - Most modern Unix systems

Pros/Cons of Kernel OS Organization

+ Advantages of layering, without disadvantage of many layers

+ Easier to demonstrate correctness

– Not as general as layering

– Offers no organizing principle for other parts of OS, user services

– Kernels tend to grow to monoliths

Microkernel OS Design

Like kernels, only less soTry to include only small set of required

services in the microkernelMoves even more out of innermost OS

partLike parts of VM, IPC, paging, etc.

Examples - Mach, Amoeba, Plan 9, Windows NT, Chorus

Pros/Cons of Microkernel Organization

+ Those of kernels, plus:

+ Minimizes code for most important OS services

+ Offers model for entire system

– Microkernels tend to grow into kernels

– Requires very careful initial design choices

– Serious danger of bad performance

Object-Oriented OS Design

Design internals of OS as set of privileged objects, using OO methods

Sometimes extended into application spaceTends to lead to client/server style of

computingExamples

Mach (internally)Spring (totally)

Pros/Cons of OO OS Organization

+ Offers organizational model for entire system

+ Easily divides system into pieces

+ Good hooks for security

– Can be a limiting model

– Must watch for performance problems

Micro-ness is in the eye of the beholderMachAmoebaPlan 9Windows NT

Some Important Microkernel Designs

Mach

Mach didn’t start life as a microkernelBecame one in Mach 3.0

Object-oriented internallyDoesn’t force OO at higher levels

Microkernel focus is on communications facilities

Much concern with parallel/distributed systems

Mach Model

Kernelspace

UserspaceSoftware

emulationlayer

4.3BSDemul.

SysVemul.

HP/UXemul.

otheremul.

Userprocesses

Microkernel

What’s In the Mach Microkernel?

Tasks & ThreadsPorts and Port SetsMessagesMemory ObjectsDevice SupportMultiprocessor/Distributed Support

Mach Tasks

An execution environment providing basic unit of resource allocation

Contains Virtual address spacePort setOne or more threads

Mach Task Model

Processport

Bootstrapport

Exceptionport

Registeredports

Addressspace

Thread

Process

Use

r sp

ace

Ker

nel

Mach Threads

Basic unit of Mach executionRun in context of one taskAll threads in one task share its resourcesUnix process similar to Mach task with

single thread

Task and Thread Scheduling

Very flexibleControllable by kernel or user-level

programsThreads of single task can run in parallel

On single processor and multiple processorsLocal and global schedulers for multicore

machines

User-level scheduling can extend to multiprocessor scheduling

Mach Ports

Basic Mach object reference mechanismKernel-protected communication channel

Tasks communicate by sending messages to ports

Threads in receiving tasks pull messages off a queue

Ports are location independentPort queues protected by kernel; bounded

Port Rights

Mechanism by which tasks control who may talk to their ports

Kernel prevents messages being set to a port unless the sender has its port rights

Port rights also control which single task receives on a port

Port Sets

A group of ports sharing a common message queue

A thread can receive messages from a port setThus servicing multiple ports

Messages are tagged with the actual portA port can be a member of at most one

port set

Mach Messages

Typed collection of data objectsUnlimited size

Sent to particular portMay contain actual data or pointer to dataPort rights may be passed in a messageKernel inspects messages for particular

data types (like port rights)

Mach Memory Objects

A source of memory accessible by tasksMay be managed by user-mode external

memory managera file managed by a file server

Accessed by messages through a portKernel manages physical memory as

cache of contents of memory objects

Mach Device Support

Devices represented by portsMessages control the device and its data

transferActual device driver outside the kernel in

an external object

Mach Multiprocessor and Distributed System SupportMessages and ports can extend across

processor/machine boundariesLocation transparent entities

Kernel manages distributed hardwarePer-processor data structures, but also

structures shared across the processorsIntermachine messages handled by a

server that knows about network details

Mach’s NetMsgServer

User-level capability-based networking daemon

Handles naming and transport for messages

Provides world-wide name service for ports

Messages sent to off-node ports go through this server

NetMsgServer in Action

User space

Kernel space

Sender

User process

NetMsgServer

User space

Kernel space

Receiver

User process

NetMsgServer

Mach and User Interfaces

Mach was built for the UNIX community UNIX programs don’t know about ports,

messages, threads, and tasksHow do UNIX programs run under Mach?Mach typically runs a user-level server that

offers UNIX emulationEither provides UNIX system call semantics

internally or translates it to Mach primitives

Amoeba

Amoeba presents transparent distributed computing environment (a la timesharing)

Major componentsprocessor poolsserver machinesX-terminalsgateway servers for off-LAN communications

Microkernel runs everywhere

Amoeba Diagram

Server pool

Workstations

Specialized servers

Gateway

LAN

WAN

Amoeba’s Basic Primitives

ProcessesThreadsLow level memory managementRPCI/O

Amoeba Software Model

Addressspace

Thread

Process

Use

r sp

ace

Ker

nel

Process mgmt.Memory mgmt.Comm’sI/O

Amoeba Processes

Similar to Mach processesProcess has multiple threads

But each thread has a dedicated portion of a shared address space

Thread scheduling by microkernel

Amoeba Memory Management

Amoeba microkernel supports concept of segmentsTo avoid the heavy cost of fork across machine

boundaries

A segment is a set of memory blocksSegments can be mapped in/out of

address spaces

Remote Procedure Call

Fundamental Amoeba IPC mechanismAmoeba RPC is thread-to-threadMicrokernel handles on/off machine

invocation of RPC

Plan 9

Everything in Plan 9 is a file system (almost)ProcessesFilesIPCDevices

Only a few operations are required for filesText-based interface

Plan 9 Basic Primitives

TerminalsCPU serversFile systemsChannels

File Systems in Plan 9

File systems consist of a hierarchical tree Can be persistent or temporaryCan represent simple or complex entitiesCan be implemented

In the kernel as a driverAs a user level processBy remote servers

Sample Plan 9 File Systems

Device file systems - Directory containing data and ctl file

Process file systems - Directory containing files for memory, text, control, etc.

Network interface file systems

Plan 9 Channels and Mounting

A channel is a file descriptorSince a file can be anything, a channel is a

general pointer to anything

Plan 9 provides 9 primitives on channelsMounting is used to bring resources into a

user’s name spaceUsers start with minimal name space,

build it up as they go along

Typical User Operation in Plan 9User logs in to a terminal

Provides bitmap display and input

Minimal name space is set up on loginMounts used to build spacePooled CPU servers used for compute

tasksSubstantial caching used to make

required files local

Windows NT

More layered than some microkernel designs

NT Microkernel provides base servicesExecutive builds on base services via

modules to provide user-level servicesUser-level services used by

privileged subsystems (parts of OS)true user programs

Windows NT Diagram

Hardware

MicrokernelExecutive

UserProcesses

ProtectedSubsystems

User Mode

Kernel Mode

Win32 POSIX

NT Microkernel

Thread schedulingProcess switchingException and interrupt handlingMultiprocessor synchronizationOnly NT part not preemptible or pageable

All other NT components runs in threads

NT Executive

Higher level services than microkernelRuns in kernel mode

but separate from the microkernel itselfease of change and expansion

Built of independent modulesall preemptible and pageable

NT Executive Modules

Object managerSecurity reference monitorProcess managerLocal procedure call facility (a la RPC)Virtual memory managerI/O manager

Typical Activity in NT

Hardware

KernelExecutive

Client Process

Win32ProtectedSubsystem

Windows NT Threads

Executable entity running in an address space

Scheduled by kernelHandled by kernel’s dispatcherKernel works with stripped-down view of

thread - kernel thread objectMultiple process threads can execute on

distinct processors--even Executive ones

Microkernel Process Object

A proxy for the real processMicrokernel’s interface to the real processContains pointers to the various resources

owned by the processe.g., threads and address spaces

Alterable only by microkernel calls

Microkernel Thread Objects

Proxies for the real thread One per thread

Contains minimal information about threadPriorities, dispatching state

Used by the microkernel for dispatching

Microkernel Process and Thread Object Diagram

Kernel Process

Kernel Thread

Kernel Thread

Other Microkernel Process Information

Kernel Process

Kernel Thread

Kernel Thread

Virtual Address SpaceDescriptors

Object Table

ProcessObject

ThreadObjects

More On Microkernels

Microkernels were the research architecture of the 80s

But few commercial systems really use microkernels

To some extent, “microkernel” is now a dirty word in OS design

Why?