04-1.Proceses and Threads - Process concepts

이 글은 서울대학교 평생교육원 열린강좌 중, 홍성수 교수님의 운영체제 강의 및 pt자료를 정리한 내용임을 알려드립니다. 
서울대학교 평생교육원 열린강좌 - 운영체제 (홍성수 교수님)

Process and Threads

Agenda

1. Process Concepts
2. Process Scheduling
3. Context Switching
4. Process Creation and Termination
5. MultiThreading
6. Conclusion
   

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1. Process concepts

Process Concepts (1)

• What is a process and why is it useful?

  • 프로세스란, OS위에서 program을 구동시키는 단위.
  • runtime system의 수행주체.
  • 자원사용의 주체.

• Why?

  • With many things happening at once in a system, need some way of separating them all out cleanly.
  • Important concept : “Decomposition”
    
    • Solve a hard problem by chopping it into serveral simpler problems that can be solved separately.
    • SW의 복잡도를 감소하는 2가지 방법
      
      • abstraction
      • decomposition : 복잡한 문제를 단순한 여러개로 분할하여 해결하는 방법

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Process Concepts (2)

• What?

  • Definition of a process
    
    • Program in execution.
    • An execution stream in the context of a particular process state.
    • 특정 프로세스의 상태에서 실행되는 execution stream이다.

  • What is an “execution stream” and what is a “process state”?

    • Process state is everything that can affect, or be affected by the process
      
      • code, data, values, open files, etc.
    • Execution strea is a sequence of instructions performed in a process state
      
      • Key simplifying feature of a process
      • Only one thing happens at a time within a process.

  • Execution Stream

    • 프로세스가 지금까지 수행된 모든 명령어들의 순서.
  • Program과 Process의 차이
    
    • Program이 수행중이라는 것은?
      
      • 저장매체에 저장된 수동적인 Code Sequence => process
      • cpu, memory, i/o device 등 더 많은 자원을 필요로함. (능동적인)

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Process Concepts (3)

• Process state of context
  
  • Collection of three types of contexts
    
    • Memory context
      
      • code segment, data segment, stack segment, heap
      • code : 기계어
      • data : 프로그램 전역변수
      • stack : 지역변수, function call 등의 정보 저장.
    • Hardware context
      
      • CPU registers, I/O registers
    • System context
      
      • Process table, open file table, page table.
  • Realization of the notion of process.

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Process Concepts (4)

• Multiprogramming vs. multiprocessing
  
  • Uniprogramming
    
    • Only one process in memory at a time.
    • Mostly old PC OS
    • Makes some parts of OS easier, bu others hard.
  • Multiprogramming
    
    • memory관점에서 여러개의 active job이 수행되는 것(load 되는것)
    • Multiple processes in memory.
    • Most systems support multiprogramming
  • Multiprocessing
    
    • Multiple processes are running together at the same time
    • CPU is multiplexed.
  • Swapping
    
    • 메모리부족 문제를 해결하기 위해,
    • cpu를 사용하지 않는 프로세스의 데이터를 메모리에서 다른 저장장치로 내보내고,
    • cpu를 사용할 프로세스의 데이터를 메모리로 로드하는 것.

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Process Concepts (5)

• Why is the process useful?

• Design-time entity vs. run-time entity

  • Accepting the system requirements and
  • Generating a collection of tasks
    
    • Design by decomposition
  • Task is a design-time entity
  • Process is a run-time entity
    
    • Target of CPU scheduling and resource allocation

  • Implementation is

    • A mapping from design-time entities onto run-time entities.

  • Run-time entity : OS가 관리하는 단위, 수행의 주체, 자원사용의 주체

  • SW system의 개발
    
    1. 설계 –> 요구사항 명세서 –> 설계 (Decomposition) —> “a set of Task”
    2. 구현 –> {Tasks} –> 구현 –> {Programs} –> “Processes”
       
       • 설계단계의 Task들이 1:1로 Process로 mapping된다.
       • OS가 Task들을 하나의 Process로 처리가능
  • Process는 실세계의 무엇과 비슷할까?
    
    • OS가 정부라면, Process는 시민과 유사
    • 수행의 주체 + 자원사용의 주체

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Process Control Block

• With Multiprocessing, OS must keep track of processes
  
  • For each process, a process control block (PCB) holds
    
    • Execution state( saved registers, etc. )
    • Scheduling information ( priority )
    • Accounting and other misc, information (open files )
  • System-wide table of PCB
    
    • Process table
  • UNIX
    
    • Fixed-size array of PCB’s

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State Transition (1)

• As a process executes, it changes state
  
  • New
    
    • Process is being created.
  • Running
    
    • Instructions are being executed.
  • Waiting
    
    • Process is waiting for some event to occur
  • Ready
    
    • Process is waiting to be assigned to CPU
  • Terminated
    
    • Process has finished execution.

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State Transition (3)

• State transitions and scheduling queues
  
  • Queues at different state
    
    • Ready queue(PCB사용)
      
      • set of all processes residing in main memory. –> cpu를 받지 못함.
        –> Synchrous I/O시 waiting to ready 신호 수신시
        다수의 waiting proess가 있을 수 있음.
        다만 이유가 다를 수 있고,
        각 이유에 따른 queue에 저장됨.
      • ready and waiting to execute
    • Device queues ( I/O waiting queues )
      
      • Set of processes waiting for an I/O device
  • State transition
    
    • Migrating processes between various queues.