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Modern Operating Systems by Herbert Bos and Andrew...
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Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 134
SEC. 2.2
separate them; this is where threads come in. First we will look at the classical
thread model; after that we will examine the Linux thread model, which blurs the
line between processes and threads.
One way of looking at a process is that it is a way to group related resources
A process has an address space containing program text and data, as well
as other resources. These resources may include open files, child processes, pend-
ing alarms, signal handlers, accounting information, and more.
By putting them
together in the form of a process, they can be managed more easily.
The other concept a process has is a thread of execution, usually shortened to
The thread has a program counter that keeps track of which instruc-
tion to execute next. It has registers, which hold its current working variables. It
has a stack, which contains the execution history, with one frame for each proce-
dure called but not yet returned from. Although a thread must execute in some
process, the thread and its process are different concepts and can be treated sepa-
rately. Processes are used to group resources together; threads are the entities
scheduled for execution on the CPU.
What threads add to the process model is to allow multiple executions to take
place in the same process environment, to a large degree independent of one anoth-
er. Having multiple threads running in parallel in one process is analogous to hav-
ing multiple processes running in parallel in one computer.
In the former case, the
threads share an address space and other resources.
In the latter case, processes
share physical memory, disks, printers, and other resources. Because threads have
some of the properties of processes, they are sometimes called
lightweight pro-
The term
is also used to describe the situation of allowing
multiple threads in the same process.
As we saw in Chap. 1, some CPUs have
direct hardware support for multithreading and allow thread switches to happen on
a nanosecond time scale.
In Fig. 2-11(a) we see three traditional processes. Each process has its own ad-
dress space and a single thread of control.
In contrast, in Fig. 2-11(b) we see a sin-
gle process with three threads of control. Although in both cases we have three
threads, in Fig. 2-11(a) each of them operates in a different address space, whereas
in Fig. 2-11(b) all three of them share the same address space.
When a multithreaded process is run on a single-CPU system, the threads take
turns running.
In Fig. 2-1, we saw how multiprogramming of processes works. By
switching back and forth among multiple processes, the system gives the illusion
of separate sequential processes running in parallel. Multithreading works the same
way. The CPU switches rapidly back and forth among the threads, providing the
illusion that the threads are running in parallel, albeit on a slower CPU than the
real one. With three compute-bound threads in a process, the threads would appear
to be running in parallel, each one on a CPU with one-third the speed of the real
Different threads in a process are not as independent as different processes. All
threads have exactly the same address space, which means that they also share the

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