Process vs Thread — What's the Difference?

A process is an independent running program with its own memory; a thread is a unit of execution inside a process that shares that memory with other threads. Threads are lighter but need careful synchronisation.

The concept

Each process has a private address space, so one crashing process does not corrupt another. Communicating between processes needs IPC such as pipes or sockets.

Threads in a process share the heap and globals, which makes sharing data easy and race conditions possible. Creating a thread is cheaper than starting a process.

Memory layout

Practical examples

Threads sharing data (needs a lock)

import threading
count = 0; lock = threading.Lock()
def work():
    global count
    for _ in range(100000):
        with lock: count += 1
ts = [threading.Thread(target=work) for _ in range(4)]
[t.start() for t in ts]; [t.join() for t in ts]

Separate processes

from multiprocessing import Process
p = Process(target=print, args=('hi from child',)); p.start(); p.join()

See them

ps aux | head
ps -T -p <pid>   # threads of a process

Common mistakes

• Sharing mutable data between threads without locks.
• Spawning thousands of processes instead of a pool.
• Assuming threads speed up CPU work in Python (GIL).

Key takeaways

• Process = isolated, thread = shared memory.
• Threads are cheaper, processes safer.
• Synchronise shared state.

FAQ

What is context switching?

The OS saving one thread or process's state and running another; processes cost more.

What is a thread pool?

A fixed set of reusable threads that run queued tasks.