Processes, Threads, Concurrency, Multithreading, and Multiprocessing - Full Notes with Python Examples

Table of Contents

• Processes
• Threads
• Processes vs Threads
• Concurrency vs Parallelism
• Conclusion
• References

Processes

A process is a running instance of a program, containing its own memory space, program counter, registers, and stack (TutorialsPoint, n.d.). Each process is isolated, and processes communicate via inter-process communication (IPC) mechanisms like pipes, sockets, or shared memory (GeeksforGeeks, 2024).

Example in Python:

import os

def child_process():
    print(f"Child process ID: {os.getpid()}")

if __name__ == "__main__":
    # method to create a child process only available in Unix/Linux
    pid = os.fork()
    if pid == 0:
        child_process()
    else:
        print(f"Parent process ID: {os.getpid()}")

Here, fork() creates a new process (child).

Processes have a lifecycle: New, Ready, Running, Waiting, and Terminated (TutorialsPoint, n.d.).

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Threads

A thread is the smallest unit of CPU execution inside a process. Multiple threads can exist within the same process, sharing memory and resources but having independent execution flows (GeeksforGeeks, 2025).

Example in Python:

import threading

def thread_function():
    print(f"Thread running: {threading.current_thread().name}")

if __name__ == "__main__":
    t = threading.Thread(target=thread_function)
    t.start()
    t.join()

Threads are lightweight compared to processes since they share address space (GeeksforGeeks, 2025).

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Processes vs Threads

Feature	Process	Thread
Memory	Separate memory space	Shared memory within process
Communication	IPC mechanisms (slow)	Direct memory access (fast)
Overhead	High (context switch is heavy)	Low (lightweight switching)
Fault Tolerance	Crash isolated	Crash may affect all threads
Creation	Heavyweight (more resources)	Lightweight (less resources)
Scheduling	OS-level scheduling	User-level scheduling

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Concurrency vs Parallelism

Concurrency is the ability to manage multiple tasks at the same time, but not necessarily simultaneously. Tasks start, run, and complete in overlapping time periods (GeeksforGeeks, 2024).

Parallelism is when tasks literally run at the same time, typically on multiple cores (Splunk, 2025).

Aspect	Concurrency	Parallelism
Definition	Multiple tasks in progress (may not run together)	Multiple tasks running at the same instant
Hardware	Can work on single or multiple cores	Requires multiple cores/processors
Example	Multitasking on a single CPU	Tasks split across multiple CPUs/cores
Focus	Structure and coordination	Speed and simultaneous execution
Python Usage	threading, asyncio	multiprocessing, joblib

Illustration:

• Concurrency: A single cashier multitasking between customers.
• Parallelism: Multiple cashiers serving different customers simultaneously.

Python Example for Concurrency (threads):

import threading
import time

def task(name):
    print(f"Starting {name}")
    time.sleep(2)
    print(f"Ending {name}")

if __name__ == "__main__":
    t1 = threading.Thread(target=task, args=("Thread-1",))
    t2 = threading.Thread(target=task, args=("Thread-2",))
    t1.start()
    t2.start()
    t1.join()
    t2.join()

Python Example for Parallelism (multiprocessing):

import multiprocessing
import time

def task(name):
    print(f"Starting {name}")
    time.sleep(2)
    print(f"Ending {name}")

if __name__ == "__main__":
    p1 = multiprocessing.Process(target=task, args=("Process-1",))
    p2 = multiprocessing.Process(target=task, args=("Process-2",))
    p1.start()
    p2.start()
    p1.join()
    p2.join()

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Conclusion

Concurrency and parallelism are core concepts for modern operating systems and programming.

• Concurrency is about dealing with lots of things at once.
• Parallelism is about doing lots of things at the same time.

Threads offer lightweight concurrency, while processes offer heavyweight isolation and true parallelism (GeeksforGeeks, 2024).

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References

GeeksforGeeks. (2024, September 15). Difference between Multiprocessing and Multithreading. GeeksforGeeks. https://www.geeksforgeeks.org/difference-between-multiprocessing-and-multithreading/

GeeksforGeeks. (2024, December 28). Concurrency in Operating System. GeeksforGeeks. https://www.geeksforgeeks.org/concurrency-in-operating-system/

GeeksforGeeks. (2025, February 21). Thread in Operating System. GeeksforGeeks. https://www.geeksforgeeks.org/thread-in-operating-system/

Panigrahi, K. K. (2022, December 16). Difference Between Process and Thread. TutorialsPoint. https://www.tutorialspoint.com/difference-between-process-and-thread

Splunk. (2025, April 23). Concurrency in Programming and Computer Science: The Complete Guide. Splunk. https://www.splunk.com/en_us/blog/learn/concurrency.html

TutorialsPoint. (n.d.). Operating System - Operations on Processes. TutorialsPoint. https://www.tutorialspoint.com/operating_system/os_operations_on_processes.htm