OPERATING SYSTEM SUPPORT

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OPERATING SYSTEM SUPPORT

• DISTRIBUTED SYSTEMS
• CHAPTER 6
• Lawrence Heyman
• July 8, 2002

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CONTENTS

1. INTRODUCTION
2. THE OPERATING SYSTEM LAYER
3. PROTECTION
4. PROCESSES AND THREADS
5. COMMUNICATION AND INVOCATION
6. OPERATING SYSTEM ARCHITECTURE
7. SUMMARY

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OPERATING SYSTEM SUPPORT

• Important aspect of DS is resource sharing
• Client applications invoke operations
• Middleware provides remote invocations between
  processes at nodes of DS
• OS is layer below middleware
• OS supports middleware at nodes of a DS
• encapsulation
• protection
• invocation

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TASK OF OS

• Provide abstraction of physical resources
• Processors
• Memory
• Communications
• Storage media
• System call interface
• Files rather than data blocks
• Sockets rather than raw network access

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NETWORK OS vs DISTRIBUTED OS

• Multiple system images
• Autonomous nodes
• Remote login
• rlogin telnet
• Control at own node
• User scheduling

• Single system image
• Not autonomous
• OS controls all nodes
• Transparent access

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REASONS AGAINST DS

• Investment in current application software
• Preference for autonomy
• Combination of Middleware and Network offers
  balance

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WHAT DOES OS PROVIDE?

• OS running at a node provides abstractions of
  local hardware resourse
• Middleware uses these resources for remote
  invocations at the nodes
• Kernel and server processes manage resources and
  provide client interface
• Clients access resources

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PROTECTION

• Problems
• malicious code, bugs
• unanticipated behavior
• illegitimate access
• Solutions
• use of type-safe language
• hardware support at kernel level
• Price is speed

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OS COMPONENTS

• Process manager
• Thread manager
• Communications manager
• Memory manager
• Supervisor

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PROCESSES THREADS

• Process
• an execution environment with one or more threads
• Execution Environment
• unit of resource management
• local kernel-managed resources
• to which threads have access
• the protected domain in which threads exe
• Thread
• the OS abstraction of an activity

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PROCESS CREATION

• Creation of execution environment
• Address space
• Initialized contents
• Transparent to user
• Choice of target host node is policy decision
• transfer policy
• location policy
• sender-initiated
• receiver-initiated
• migration

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ADVANTAGES OF THREADS

• dynamically created and destroyed
• maximize concurrent execution
• maximize throughput (rps)
• overlap of computation with input/output
• concurrent processing on multiprocessors
• reduces bottlenecks

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MULTI-THREAD SERVERARCHITECTURES

• Worker-pool
• Thread-per-request
• Thread-per-connection
• Thread-per-object
• ( Various hybrids also possible )

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WORKER-POOL
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THREAD-PER-REQUEST
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THREAD-PER-CONNECTION
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THREAD-PER-OBJECT
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THREADS vs MULTIPLE PROCESSES

1. Switching to different thread within process
   cheaper than switching to thread in 2nd process
2. Threads within process may share resources and
   data efficiently compared to separate processes
3. Creating new thread within process is cheaper
   than creating new process
4. Threads within a process not protected from one
   another

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THREADS PROGRAMMING

• Threads programming is concurrent
• C - threads library
• Java methods
• Threads Lifetimes
• New thread created in SUSPENDED state
• Made RUNNABLE with start()
• Executes on run() method
• Ends on return from run() or destroy()
• Threads groups
• Assigned at creation
• Security

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THREADS PROGRAMMING 2

• Threads Synchronization
• Local variables are private (private stack)
• Synchronized through monitor construct so only
  one thread can execute at a time
• in Queue class
• in Object class
• Threads Scheduling
• Preemptive
• Non-preemptive

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THREADS PROGRAMMING 3

• Threads Implementation
• Many kernel-level implementations (NT, Solaris)
  are multi-level
• Creation and management system calls
• Schedule threads individually
• Threads run-time library
• Organizes multi-threaded processes
• Linked to user-level applications
• Kernel not involved here

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THREADS PROGRAMMING 4

• User-level threads
• Advantages over kernel-level threads
• Disadvantages without kernel support
• Combining user-level and kernel-level enables
  user-level code provide scheduling hints to
  kernels thread scheduler
• Advantages
• Disadvantage

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HIERARCHALEVENT-LEVEL SCHEDULING

• User-level scheduler requires kernel to notify it
  of scheduling-relevant events
• Each application process contains user-level
  scheduler
• manages threads in that process
• Kernel allocates virtual processors
• application requirements
• priority
• total demand

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ASSIGNMENT OF VIRTUAL PROCESSORS
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TYPES OF EVENTS

• Scheduler Activation call from kernel notifies
  process scheduler of an event
• Virtual Processor Added (ready thread)
• SA blocked
• SA unblocked
• SA preempted
• Advantages
• allocation based on user-level priorities
• kernel does not influence user-level schedulers
  behavior

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EVENTS SCHEDULING
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SUMMARY

• OS Support of middleware
• Implementing of resource management policy
• Encapsulation and protection of resources
• Allows concurrent sharing of resources
• Processes
• Execution environment
• Address space
• Communication interfaces
• Local resources, i.e., semaphores
• Threads kernel-level user-level
• Share the execution environment
• Cheap concurrency
• Parallel multiprocessors

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THE END