Lecture 4 Multiprogramming and Time-sharing Multi-task

Outline

1. Multiprogramming and Cooperative Scheduling

2. Time-sharing Multi-task and Preemptive Scheduling
3. The Concept of Process
4. Process Model

History

The widespread use of OS began during the transition from mainframes to minicomputers.

• OS/360 is a multiprogramming batch OS in the mainframe (System/360) era
• PDP series minicomputers from DEC
• Many companies may have their own computers
• Multiprogramming becomes common

Multiprogramming

• Multiple executable programs in the memory
• The programs share the processor

Job

• One-time execution process of an application

Terms appeared in history: Job, Multiprogramming

• Job and Multiprogramming are concepts used by IBM for multiprogramming

Cooperative scheduling

• Executables program voluntarily relinquish the processor
• OS does not interrupt the running program
• OS is responsible for selecting the next program to take the processor for execution

Outline

1. Multiprogramming and Cooperative Scheduling

2. Time-sharing Multi-task and Preemptive Scheduling

3. The Concept of Process
4. Process Model

History

The widespread use of minicomputers led to a demand for time-sharing and multi-task capabilities, resulting in the development of time-sharing multi-user OS.

• Widely use of DEC's PDP and VAX minicomputers
• DEC's VMX operating system
• MIT's CTSS operating system
• AT&T's UNIX operating system

Time-sharing Multi-task from user's perspective

Time-sharing Multi-task: from user's perspective

• Multiple executable programs residing in memory
• Each executable program takes the processor in a time-sharing way
• The OS allocates CPU to each executable program by time slices
• Process: one-time execution process of the application

Time-sharing Multi-task from the perspective of OS

• Process: A dynamic execution process of a program with certain independent functions on a data collection. Also known as Task.
• Process Switching: switch from the process corresponding to one application to the process corresponding to another application.

Job, Task and Process

Terms appeared in history: Job, Task, Process

• Task and Process are concepts proposed by Multics and UNIX, respectively, for time-sharing multi-task
• A process is one-time execution process of an application. In OS context, task and process are the same meaning
• A job (goal) is a whole formed by a group of related program execution processes (processes, tasks) for the same goal

Reference: Difference between Job, Task and Process

Preemptive Scheduling

• Process passively relinquishes processor
• The process uses the processor in turn according to time slice, which is a "pause-continue" execution process
• Based on the clock interrupt mechanism of hardware, OS can interrupt the running program at any time
• The OS is responsible for selecting the next program to take the processor for execution

Outline

1. Multiprogramming and Cooperative Scheduling
2. Time-sharing Multi-task and Preemptive Scheduling

3. The Concept of Process

4. Process Model

Process Switching

Process Features

• Dynamic
  • Execution starts --> Pause --> Continue --> Execution ends
• Concurrency
  • Multiple processes are executing during a time period
• Limited independence
  • Processes do not need to be aware of each other's existence in the system

Composition of Process and Program

Composition of Process and Program

Relationship between Task (Process) and Program

• A task is an abstraction of a program in the execution of OS

  • program = file (static & executable)
  • task = program in execution = program + execution status

• Multiple executions of the same program lead to different tasks

  • E.g., multiple executions of the command "ls" lead to multiple tasks

• Resources required for task execution

  • Memory: store code and data
  • CPU: execute instructions

Differences between Task (Process) and Program

• Tasks are dynamic, programs are static
  • A program is a collection of ordered codes
  • A task is the execution of a program
• Tasks are temporary, programs are permanent
  • A task is a state-changing process
  • Programs can be saved permanently
• Tasks and programs have different compositions
  • The task consists of program codes, data and process control blocks

Process Status

A process contains all status information about a running program

• Control Flow for task execution
  • Code content and code execution location (code segment)
• Data accessed by the task
  • Memory read and written by tasks (heap, stack, data segment)
  • Registers read and written by tasks
    • general-purpose registers

Process Status

A process contains all status information about a running program

• OS manages the data related to the task (task context )
  • General purpose registers required for task switching
  • Status registers required for task switching (PC, etc.)
  • Other information: the stack address of the task, etc.
  • Other resources: …

Task Control Block (TCB)

The core data structure of OS for managing tasks, also known as process control block (PCB)

• OS manages and controls the information used by the process
• OS uses TCB to describe the basic information and changing status of the process
• TCB is the only symbol for a process
• Each task has a corresponding TCB in OS

Process Control Block Managed by OS

Outline

1. Multiprogramming and Cooperative Scheduling
2. Time-sharing Multi-task and Preemptive Scheduling
3. The Concept of Process

4. Process Model

Process State: Created and Ready

• Created --> Ready
  • When was it created?
  • How to create?
    

Process State: Running

• Created --> Ready --> Running
  • The kernel selects a ready task
  • How to run?

Process State: Waiting

• Created --> Ready --> Running --> Waiting
  • The reason why the task enters the waiting status
    • Itself
    • The outside world

Process State Transition: Waking up

• Created --> Ready --> Running --> Waiting --> Waking up
  • The reason for waking up the task
    • Self: wake up naturally?
    • Outside: be Woke up?

Process State Transition: Preempted

• Created --> Ready --> Running --> Preempted
  • Why is the task preempted?

Process State: Exited

• Created --> Ready --> Running --> ...... --> Exited
  • Why did the task exit?
    • Voluntary?
    • Forced?

Three-State Process Model

Process State Transition and System Call

• Created --> Ready --> Running --> ...... --> Exited
• Preempted, waiting, waking up

What system calls are involved?

• exit, sleep
• ...
  

Process State Transition and Process Switching

• Created --> Ready --> Running --> ...... --> Exited
• Preempted, waiting, waking up

During the lifecycle of a task, when does a task switch occur?

Process Switching