Processes and Threads

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Processes and Threads

• Ch 2.1 and 2.2
• Tuesday, January 23s, 2007

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Todays Schedule

• Return HW 1 and Quiz 1
• Sec 1.7 - O/S Stuctures
• Sec 2.1 Processes
• Sec 2.2 - Threads

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Approaches to OS

• Monolithic
• Virtual Machines
• Client-Server (microkernel)

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Monolithic OS Architecture

• OS written as a set of procedures
• Each procedure has well-defined interface
  (parameters)
• Very little structure or information hiding

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Virtual Machine OS Architecture

• Replicate the hardware within software
• Trap OS calls and translate/handle them
• Breaks down if you make direct I/O calls
• Slow(Windows running DOS applications)
• Other approach JVM (define an alternate
  instruction set and translate to various OS)

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Client-Server OS Architecture

• Implement as much as possible in user-level
  modules
• The OS is as small as possible
• It only serves to translate and message pass
  OS-level calls to service processes

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Processes - The Process Model

• Multiprogramming of four programs
• Conceptual model of 4 independent, sequential
  processes
• Only one program active at any instant

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Process Creation

• Principal events that cause process creation
• System initialization
• Execution of a process creation system
• User request to create a new process
• Initiation of a batch job

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Process Termination

• Conditions which terminate processes
• Normal exit (voluntary)
• Error exit (voluntary)
• Fatal error (involuntary)
• Killed by another process (involuntary)

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Process Hierarchies

• Parent creates a child process, child processes
  can create its own process
• Forms a hierarchy
• UNIX calls this a "process group"
• Windows has no concept of process hierarchy
• all processes are created equal

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Process States

• Possible process states
• running
• blocked
• ready
• Transitions between states shown

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Conceptualizing Process States

• Lowest layer of process-structured OS
• handles interrupts, scheduling
• Above that layer are sequential processes

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Process table entry Items

• Fields of a process table entry

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The Thread Model

• (a) Three processes each with one thread
• (b) One process with three threads

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Process vs. Thread Items

• Items shared by all threads in a process
• Items private to each thread

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The Thread Model (3)

• Each thread has its own stack

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Thread Usage

• A word processor with three threads

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Thread Usage (2)

• A multithreaded Web server

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Thread Usage (3)

• Rough outline of code for previous slide
• (a) Dispatcher thread
• (b) Worker thread

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Implementing Threads in User Space

• A user-level threads package

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Implementing Threads in the Kernel

• A threads package managed by the kernel

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Tuesday, January 23

• Read part of Chapter 2.3 Interprocess
  Communication pages 100 - 106
• Do not worry about code fragments, reading for
  understanding
• IPC
• Critical Region
• Difficulties solving problem