Operating System Process Management: Representation, Control, and UNIX SVR4, Study notes of Operating Systems

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Chapter 3

Process Description and Control

Operating Systems: Internals and Design Principles, 6/E William Stallings Dave Bremer Otago Polytechnic, N.Z. ©2008, Prentice Hall

Roadmap

• (^) How are processes represented and controlled by the OS.
• (^) Process states which characterize the behaviour of processes.
• (^) Data structures used to manage processes.
• (^) Ways in which the OS uses these data structures to control process execution.
• (^) Discuss process management in UNIX SVR4.

Concepts

• (^) From earlier chapters we saw:
  • (^) Computer platforms consists of a collection of hardware resources
  • (^) Computer applications are developed to perform some task
  • (^) It is inefficient for applications to be written directly for a given hardware platform

Concepts cont…

• (^) OS provides an interface for applications to use
• (^) OS provides a representation of resources that can be requested and accessed by application

What is a “process”?

• (^) A program in execution
• (^) An instance of a program running on a computer
• (^) The entity that can be assigned to and executed on a processor
• (^) A unit of activity characterized by the execution of a sequence of instructions, a current state, and an associated set of system instructions

Process Elements

• (^) A process is comprised of:
  • (^) Program code (possibly shared)
  • (^) A set of data
  • (^) A number of attributes describing the state of the process

Process Control Block

• (^) Contains the process elements
• (^) Created and managed by the operating system
• (^) Allows support for multiple processes

Trace of the Process

• (^) The behavior of an individual process is shown by listing the sequence of instructions that are executed
• (^) This list is called a Trace
• (^) Dispatcher is a small program which switches the processor from one process to another

Trace from the

processes point of view:

• (^) Each process runs to completion

Trace from Processors

point of view

Timeout I/O TimeoutTimeout

Two-State Process Model

• (^) Process may be in one of two states
  • (^) Running
  • (^) Not-running

Queuing Diagram

Etc … processes moved by the dispatcher of the OS to the CPU then back to the queue until the task is competed

Process Creation

• (^) The OS builds a data structure to manage the process
• (^) Traditionally, the OS created all processes
  • (^) But it can be useful to let a running process create another
• (^) This action is called process spawning
  • (^) Parent Process is the original, creating, process
  • (^) Child Process is the new process

Process Termination

• (^) There must be some way that a process can indicate completion.
• (^) This indication may be:
  • (^) A HALT instruction generating an interrupt alert to the OS.
  • (^) A user action (e.g. log off, quitting an application)
  • (^) A fault or error
  • (^) Parent process terminating