Threads and Processes

High-Level Understanding of a Process

• A process is an instance of a computer program that is being executed
• A thread is a basic unit of CPU utilization
• A task is a set of instructions allocated to memory
• A thread is what the CPU actually runs
• A process is what gets loaded in memory
• A process will include one or more threads
• This is because threads are what the CPU actually runs (not processes)
• Tasks and processes are used synonymously sometimes

Defining the Structure of a Process

• The process model is based on two independent concepts:

  1. Resource grouping
  2. Execution

Resource Grouping

• In terms of resource grouping, a process is something that groups related resources together

• Specifically, a process has the foklowing:

  • An address space containing program text and data
  • Information about open files
  • Information about child processes
  • And other resources

Execution

• In terms of execution, a process refers to a primary thread of execution
• A thread of execution refers to a set of threads
• A process doesn't perfectly translate to a set of threads, but we can think of it that way for now

• Specifically, a thread has the following:

  • A program counter that keeps track of which instruction to execute next
  • A set of registers that hold its current working variables
  • A stack that contains the thread execution history

Summarizing Processes and Threads

• A process refers to a program that is being executed
• A child process is a process created by another process (i.e. the parent process)
• A thread is the basic unit to which the operating system allocates processor time
• A thread can execute any code from the process
• Once a process is created by the operating system, a single thread will be initialized within that process
• This initialized thread is called the primary thread
• Each process can create additional threads from any of its threads

Differentiating between Processes and Threads

• A major difference between a thread and process is:

  • A process runs in a separate memory space

    • Compared to other processes

  • A thread runs in a shared memory space

    • Compared to other threads of the same process
• Essentially, processes are used to group resources together
• On the other hand, threads are the entities scheduled for execution on the CPU

• For a process, the operating system:

  • Allocates memory for its instricutions and data

• For a thread, the operating system:

  • Handles scheduling
  • Allocates CPU time

References

• Python Essential Reference
• Python in a Nuteshell
• Lecture Slides about the Modern Process and Thread
• Threads and Concurrency
• Illustrating the Thread Context
• Hyperthreading and Hardware Threads
• Hyperthreading and the Operating System
• Defining the Architecture of Hyperthreading
• Defining a Process and a Thread
• Another Definition of a Process
• Illustrating a Process and Thread
• Difference between Threads, Tasks, and Processes