Process and Threads

Lecture 3

Process Concept

• An operating system executes a variety of programs

  • Textbook uses the terms jobs and program used interchangeably

• Process - a program in execution

  • Process execution proceeds in a sequential fashion

• A process contains

  • The program code, also called text section

  • Current activity including program counter, processor registers

  • Stack containing temporary data

  • Function parameters, return addresses, local variables

  • Data section containing global variables

  • Heap containg memory dynamically allocated during run time

• Program is passive entity stored on disk (Execuable file), process is active

  • _Progrem becomes process when executable file loaded into memory__

• One program -> can be several processes

  Process =? Program

  • More to a process than just a program:

    • I run emacs/Notepad on lectures.txt, you run it on homework.java - Same program, Different processes

  • Less to a process than a program:

    • cc/cpp stars up processes to handle different stages of the compilation process cc1, cc2, and ld

  Process State

  • As a proces executes, it changes state

    • new: The process is being created

    • running: Instructions are being executed

    • waiting: The process is waiting for some event to occur

    • ready: The process is waiting to be assigned to a processor

    • terminated: The process has finished execution

  

  Process Control Block (PCB)

  • Information associated with each process (also called task control block)

    • Process state - running, waiting, etc

    • Program counter - location of instruction to next execute

    • CPU registers - contents of all process - centric registers

    • CPU scheduling information -memory allocated to the process, page tables, value of the base, etc

    • Accounting information -CPU used, clock time elapsed since start, time limits

    • I/O status information -I/O devices allocated to process, list of open files

  Data Structures

  • Hash function can create a hash map

    

    • Bitmap - string of n binary digits representing the status of n items

    • Linux data structures defined in include files:

      • Linux/list.h

      • Linux/kfifo.h

      • Linux/rbtree.h

    Process Scheduling Queues

    • Job Queue - set of all processes in the system

    • Ready Queue - set of all processes residing in main memory, ready and wating to execute

    • Device Queue - set of proceses waiting for an I/O device

    • Process migration between the various queues

    • Queue Structures - typically linked list, circular list, etc.