What exactly is a process? In the original Unix implementations, a process was any executing program. For each program, the kernel kept track of

• The current location of execution (such as waiting for a system call to return from the kernel), often called the program's context

• Which files the program had access to

• The program's credentials (which user and group owned the process, for example)

• The program's current directory

• Which memory space the program had access to and how it was laid out

A process was also the basic scheduling unit for the operating system. Only processes were allowed to run on the CPU.

Complicating Things with Threads

Although the definition of a process may seem obvious, the concept of threads makes all of this less clear-cut. A thread allows a single program to run in multiple places at the same time. All the threads created (or spun off) by a single program share most of the characteristics that differentiate processes from each other. For example, multiple threads that originate from the same program share information on open files, credentials, current directory, and memory image. As soon as one of the threads modifies a global variable, all the threads see the new value rather than the old one.

Many Unix implementations (including AT&T's canonical System V release) were redesigned to make threads the fundamental scheduling unit for the kernel, and a process became a collection of threads that shared resources. As so many resources were shared among threads, the kernel could switch between threads in the same process more quickly than it could perform a full context switch between processes. This resulted in most Unix kernels having a two-tiered process model that differentiates between threads and processes.

The Linux Approach

Linux took another route, however. Linux context switches had always been extremely fast (on the same order of magnitude as the new "thread switches" introduced in the two-tiered approach), suggesting to the kernel developers that rather than change the scheduling approach Linux uses, they should allow processes to share resources more liberally.

Under Linux, a process is defined solely as a scheduling entity and the only thing unique to a process is its current execution context. It does not imply anything about shared resources, because a process creating a new child process has full control over which resources the two processes share. This model allows the traditional Unix process management approach to be retained while allowing a traditional thread interface to be built outside the kernel.

Luckily, the differences between the Linux process model and the two-tiered approach surface only rarely. Usually, we use the term process to refer to a set of (normally one) scheduling entities which share fundamental resources, and a thread is each of those individual scheduling entities. When a process consists of a single thread, we often use the terms interchangeably.