On the Duality of Operating System Structures

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On the Duality of Operating System Structures

• Hugh C. Lauer, Xerox Corporation, Palo Alto, CA
• Roger M. Needham, Cambridge University,
  Cambridge, England

Tracy Wagner COP 6614 September 13, 2005
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Introduction

• Focus on OS Properties of
• Process
• Synchronization
• Interprocess Communication
• Two System Categories
• Message-oriented
• Procedure-oriented

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Message-oriented Systems

• Pass messages (events) easily and efficiently
  among processes
• Peripheral devices treated as processes
• Messages and message identifiers
• Queue messages at destination processes until
  action can be taken
• Synchronization and queuing implemented in
  message queues
• Processes operate on one message at a time

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Message-oriented Systems

• Specific communication paths
• Message Channels and Message Ports
• SendMessage, WaitForMessage, AwaitReply,
  SendReply operations
• Static number of processes and connections
• Process declarations
• CreateProcess operation
• Processes operate in a static context
• Priorities statically assigned
• Data structures passed by reference

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Producer/Consumer (Message-oriented)

• Consumer()
• int widget message m
• for(0 to N)
• sendMsg(producer, m) // send bufferSz
  empty msgs
• while (TRUE)
• awaitReply(producer, m) // get msg with new
  item
• extract_item(m, widget)
• sendMsg(producer, m) // reply with
  empty msg
• consume_item(widget) // consume the item
• Producer()
• int widget message m // message buffer
  
• while (TRUE)
• make_item(widget) // create
  a new item

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Procedure-oriented Systems

• Simple process creation/deletion
• Processes inherit priorities dynamically
  priorities associated with locks
• Global naming schemes important
• Procedure protection mechanisms
• Synchronization and queuing implemented by
  processes waiting for locks
• Control of and interrupts from peripheral devices
  by manipulation of locks and/or shared data

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Procedure-oriented Systems

• Efficient procedure calls
• Synchronous
• Asynchronous
• FORK and JOIN operations
• Synchronizing data structures
• Data shared directly among processes
• Condition variables
• WAIT, SIGNAL operations
• Processes typically have only one task

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Producer/Consumer (Procedure-oriented)

• monitor ProducerConsumer
• condition full, empty int count
• procedure enter()
• if (count N) wait(full) // if buffer is
  full, block
• put_item(widget) // put item in
  buffer
• count count 1
• if (count 1) signal(empty)// wake
  consumer
• procedure remove()
• if (count 0) wait(empty) // if buffer
  empty, block
• remove_item(widget) // remove
  from buffer
• count count - 1
• if (count N-1) signal(full)// wake
  producer

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Producer/Consumer (Procedure-oriented)

• Producer()
• while (TRUE)
• make_item(widget) // make a new item
  
• ProducerConsumer.enter // call enter in
  monitor
• Consumer()
• while (TRUE)
• ProducerConsumer.remove // call remove in
  monitor
• consume_item // consume an
  item

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System Comparison

• Message-oriented systems
• Passing messages only interaction
• Own address space
• Requests handled serially
• Procedure-oriented systems
• Procedural interaction
• Movement from context to context across module
  boundaries
• Depend on monitor locks and condition variables

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Mapping
Message-oriented
Procedure-oriented
Processes, CreateProcess
Monitors, NEW/START
Procedure identifiers, ENTRY procedure identifiers
Message Channels, Message Ports
SendMessage AwaitReply (immediate)
Simple procedure call

• SendMessage AwaitReply (delayed)

• FORKJOIN

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Mapping
Message-oriented
Procedure-oriented
SendReply
RETURN from procedure
Lock, ENTRY attribute
Main loop, WaitForMessage statement, case
statement
ENTRY procedure declarations
Arms of case statement selective

• Condition variables, WAIT, SIGNAL

• Waiting for messages

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Conclusions

• Models are Duals
• Similarity of Programs
• Identical Performance