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On The Duality of Operating System
Structures
Hugh Lauer, Xerox
Roger Needham, Cambridge University
1979
Presented by Catherine Vilhauer
CS533 - Concepts of Operating Systems
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Introduction
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Classic paper by Lauer and Needham(1979)
Operating Systems fall into two categories
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These two systems are duals
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o
o
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Message-oriented systems
Procedure-oriented systems
can be mapped to one another
are logically identical
performance is also identical
As a result, which you choose based on architecture
constraints
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What I’m Going to Cover
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Message-Oriented System
Procedure-Oriented System
Comparison of the Two
Argument for Duality
Evidence?
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Message-Oriented Systems
What are they?
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Pass messages (or events) easily and efficiently
among processes
Queue messages at destination process
Primitive operations for processes
Pre-emption occurs when higher priority message
received
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Message-Oriented System
What’s a good design?
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Specific communication paths (message channels,
sockets etc)
Relatively static number of processes and
connections
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Relatively static context for each process
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Deletion and creation of processes can be difficult
virtual memories or address spaces in one-to-one
correspondence with process.
Processes rarely share data in memory
Processes tend to be associated with system
resources
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Message-Oriented Systems
Characteristics
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Synchronization implemented in message queues
Data structures passed by reference in messages
Peripheral devices treated as processes
Priorities statically assigned at time of design
Processes operate on small number of messages at a
time and complete those operations before looking
at message queue again
Procedural interfaces or global naming schemes not
useful
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Message-Oriented Systems
Canonical Model
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Idealized Operating System kernel contains:
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Messages and message identifiers
Message channels and Message Ports
Four message transmission operations
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SendMessage(MessageChannel, MessageBody)
AwaitReply(MessageID)
WaitForMessage(set of MessagePorts)
SendReply(MsgId, MsgBody)
Process Declarations
Create Process
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An Example: SEDA
Message-Oriented System
SEDA
Message Queues
YES – each stage has a message queue
Data structures passed by reference
YES
Peripheral Devices treated as processes
YES
Processes that complete operation
before looking at message queue
No procedural interfaces
YES – A SEDA stage operates like this
YES
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Procedure-Oriented Systems
What are they?
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Oriented around procedure
Efficient context switching
Synchronization via locks, mutexes and semaphores
Process may have to wait for a lock
Preemption occurs when release performed on a lock
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Procedure-Oriented Systems
What’s a good design?
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Global data protected and accessed by procedural
interfaces which do synchronization in controlled
ways
Process creation and deletion easy
Process has just one task, but wanders over system
Encoded in common / global data structures
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Procedure-Oriented Systems
Characteristics
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Synchronization - queues of processes waiting for
locks
Data shared directly among processes
Control of and interrupts from devices manipulating locks and / or shared data in memory
Processes inherit priorities dynamically
Global naming schemes
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Procedure-Oriented System
Canonical Model
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Idealized Operating System contains:
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Procedures
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Procedure call facilities
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local data
algorithms
parameters
results
synchronous
asynchronous
Modules and monitors
Condition Variables
Fork and Join
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Comparison of the Two Systems
Synchronization
Shared Data
Peripheral
Devices
Priorities
Global Naming
Schemes
Message-Oriented
System
Message queues
Procedure-Oriented System
Not useful
Useful
Queues of processes waiting for
locks
Data structures passed Data shared dirctly among
by reference in msgs
processes
Treated as processes
Control of and interrupts from
devices - manipulating locks /
shared data in memory
Statically assigned
Dynamically assigned
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An Example: KNOT
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Knot platform built by Behren, Condit and Brewer in
following paper
Interesting that Brewer was co-author of SEDA
paper arguing for event-based system 2 years
before
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The Duality Mapping
Message-Oriented System
Procedure-Oriented System
Processes, CreateProcess
Monitors, NEW/START External
Message Channels Message
Procedure identifiers ENTRY
Ports
Procedure identifiers simple
SendMessage; AwaitReply (immediate)
Procedure Call
SendMessage; AwaitReply (delayed)
FORK / JOIN
Main loop of resource manager,
WaitForMessage, case statement
Arms of the case statement selective
Lock, ENTRY attribute
ENTRY procedure declarations
Waiting for messages
Condition variables, WAIT, SIGNAL
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The Duality Argument
Similarity of Programs
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Can transform one system to the other
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If written in exact style shown on previous slide
Logic unchanged
Monitor
Procedure
Calls
Process
declaration
SendMessage
AwaitReply
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The Duality Argument
Performance is Preserved
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Performance of both systems the same
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Execution time of program themselves
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computational overhead of primitive operations they call
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But since we have already shown that the programs are
identical, then the execution time of each system will be
exactly the same too.
The authors assert that the two canonical models can be made
execute as efficiently as the other
GEC 4080 and MESA - different systems, but same speed
queuing and waiting times (sharing resources)
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can make basi operations of the two models behave identically
with scheduling operations.
Will happen just as quickly and in the same order in both
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Evidence?
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Not much empirical evidence
Cambridge CAP Computer
Hugely influential and still considered a classic
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Conclusions
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No inherent different between the two styles
Produce systems of similar structure and
performance
Computational complexity is similar
Why choose one over the other?
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Machine architecture
Programming environment
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