Process vs Thread: Definitions, Differences, and Analogy

A process is an independent execution entity that the operating system treats as the smallest unit for resource allocation and scheduling, while a thread is the basic unit of CPU scheduling that exists within a process, sharing most of the process's resources but possessing its own execution context such as a program counter, registers, and stack.

The main differences between processes and threads are:

Process is the smallest unit of resource allocation; thread is the smallest unit of program execution.

Each process has its own independent address space; threads share the same address space of their parent process.

Switching between threads consumes less CPU time than switching between processes.

Creating a thread incurs less overhead than creating a process.

Threads require far fewer resources than processes.

Threads communicate more easily by sharing global and static variables; inter‑process communication requires explicit mechanisms (IPC).

Failure of one process does not affect others, while failure of a single thread can terminate the entire process.

Processes demand higher protection and have larger overhead, whereas threads have lower protection requirements, lower overhead, and higher efficiency.

To strengthen understanding, the article offers a simple analogy: a process is like a train and a thread is like a carriage.

A thread moves only within its process, just as a carriage cannot move independently of the train.

A process can contain multiple threads, similar to a train having many carriages.

Data sharing between different processes is difficult, like passengers finding it hard to move from one train to another.

Data sharing between threads of the same process is easy, like moving from one carriage to another within the same train.

Processes consume more computing resources than threads, just as running multiple trains consumes more resources than adding carriages.

If a thread crashes, the whole process crashes, akin to a fire in one carriage affecting the entire train.

Processes can be distributed across multiple machines and are suited for multi‑core environments, whereas threads of a single process run on the same core.

Memory used by a process can be locked; when one thread holds a shared memory segment, other threads must wait—similar to a locked restroom on a train.

Memory can be limited by semaphores, like a restaurant car on a train that only allows a certain number of people at a time.

Reference: https://www.zhihu.com/question/25532384