Books / Think OS / Chapter 2


Abstraction and virtualization

Before we talk about processes, I want to define a few words:

  • Abstraction: An abstraction is a simplified representation of something complicated. For example, if you drive a car, you understand that when you turn the wheel left, the car goes left, and vice versa. Of course, the steering wheel is connected to a sequence of mechanical and (often) hydraulic systems that turn the wheels, and the wheels interact with the road in ways that can be complex, but as a driver, you normally don’t have to think about any of those details. You can get along very well with a simple mental model of steering. Your mental model is an abstraction.

    Similarly, when you use a web browser, you understand that when you click on a link, the browser displays the page the link refers to. The software and network communication that make that possible are complex, but as a user, you don’t have to know the details.

    A large part of software engineering is designing abstractions like these that allow users and other programmers to use powerful and complicated systems without having to know about the details of their implementation.

  • Virtualization: An important kind of abstraction is virtualization, which is the process of creating a desirable illusion.

    For example, many public libraries participate in inter-library collaborations that allow them to borrow books from each other. When I request a book, sometimes the book is on the shelf at my local library, but other times it has to be transferred from another collection. Either way, I get a notification when it is available for pickup. I don’t need to know where it came from, and I don’t need to know which books my library has. As a whole, the system creates the illusion that my library has every book in the world.

    The collection physically located at my local library might be small, but the collection available to me virtually includes every book in the inter-library collaboration.

    As another example, most computers are only connected to one network, but that network is connected to others, and so on. What we call the Internet is a collection of networks and a set of protocols that forward packets from one network to the next. From the point of view of a user or programmer, the system behaves as if every computer on the Internet is connected to every other computer. The number of physical connections is small, but the number of virtual connections is very large.

The word “virtual” is often used in the context of a virtual machine, which is software that creates the illusion of a dedicated computer running a particular operating system, when in reality the virtual machine might be running, along with many other virtual machines, on a computer running a different operating system.

In the context of virtualization, we sometimes call what is really happening “physical”, and what is virtually happening either “logical” or “abstract.”


One of the most important principles of engineering is isolation: when you are designing a system with multiple components, it is usually a good idea to isolate them from each other so that a change in one component doesn’t have undesired effects on other components.

One of the most important goals of an operating system is to isolate each running program from the others so that programmers don’t have to think about every possible interaction. The software object that provides this isolation is a process.

A process is a software object that represents a running program. I mean “software object” in the sense of object-oriented programming; in general, an object contains data and provides methods that operate on the data. A process is an object that contains the following data:

  • The text of the program, usually a sequence of machine language instructions.

  • Data associated with the program, including static data (allocated at compile time) and dynamic data (allocated at run time).

  • The state of any pending input/output operations. For example, if the process is waiting for data to be read from disk or for a packet to arrive on a network, the status of these operations is part of the process.

  • The hardware state of the program, which includes data stored in registers, status information, and the program counter, which indicates which instruction is currently executing.

Usually one process runs one program, but it is also possible for a process to load and run a new program.

It is also possible, and common, to run the same program in more than one process. In that case, the processes share the same program text but generally have different data and hardware states.

Most operating systems provide a fundamental set of capabilities to isolate processes from each other:

  • Multitasking: Most operating systems have the ability to interrupt a running process at almost any time, save its hardware state, and then resume the process later. In general, programmers don’t have to think about these interruptions. The program behaves as if it is running continuously on a dedicated processor, except that the time between instructions is unpredictable.

  • Virtual memory: Most operating systems create the illusion that each process has its own chunk of memory, isolated from all other processes. Again, programmers generally don’t have to think about how virtual memory works; they can proceed as if every program has a dedicated chunk of memory.

  • Device abstraction: Processes running on the same computer share the disk drive, the network interface, the graphics card, and other hardware. If processes interacted with this hardware directly, without coordination, chaos would ensue. For example, network data intended for one process might be read by another. Or multiple processes might try to store data in the same location on a hard drive. It is up to the operating system to maintain order by providing appropriate abstractions.

As a programmer, you don’t need to know much about how these capabilities are implemented. But if you are curious, you will find a lot of interesting things going on under the metaphorical hood. And if you know what’s going on, it can make you a better programmer.

UNIX processes

While I write this book, the process I am most aware of is my text editor, emacs. Every once in a while I switch to a terminal window, which is a window running a UNIX shell that provides a command-line interface.

When I move the mouse, the window manager wakes up, sees that the mouse is over the terminal window, and wakes up the terminal. The terminal wakes up the shell. If I type make in the shell, it creates a new process to run Make, which creates another process to run LaTeX and then another process to display the results.

If I need to look something up, I might switch to another desktop, which wakes up the window manager again. If I click on the icon for a web browser, the window manager creates a process to run the web browser. Some browsers, like Chrome, create a new process for each window and each tab.

And those are just the processes I am aware of. At the same time there are many other processes running in the background. Many of them are performing operations related to the operating system.

The UNIX command ps prints information about running processes. If you run it in a terminal, you might see something like this:

  PID TTY          TIME CMD
 2687 pts/1    00:00:00 bash
 2801 pts/1    00:01:24 emacs
24762 pts/1    00:00:00 ps

The first column is the unique numerical process ID. The second column is the terminal that created the process; “TTY” stands for teletypewriter, which was the original mechanical terminal.

The third column is the total processor time used by the process, in hours, minutes, and seconds. The last column is the name of the running program. In this example, bash is the name of the shell that interprets the commands I type in the terminal, emacs is my text editor, and ps is the program generating this output.

By default, ps lists only the processes associated with the current terminal. If you use the -e flag, you get every process (including processes belonging to other users, which is a security flaw, in my opinion).

On my system there are currently 233 processes. Here are some of them:

  PID TTY          TIME CMD
    1 ?        00:00:17 init
    2 ?        00:00:00 kthreadd
    3 ?        00:00:02 ksoftirqd/0
    4 ?        00:00:00 kworker/0:0
    8 ?        00:00:00 migration/0
    9 ?        00:00:00 rcu_bh
   10 ?        00:00:16 rcu_sched
   47 ?        00:00:00 cpuset
   48 ?        00:00:00 khelper
   49 ?        00:00:00 kdevtmpfs
   50 ?        00:00:00 netns
   51 ?        00:00:00 bdi-default
   52 ?        00:00:00 kintegrityd
   53 ?        00:00:00 kblockd
   54 ?        00:00:00 ata_sff
   55 ?        00:00:00 khubd
   56 ?        00:00:00 md
   57 ?        00:00:00 devfreq_wq

init is the first process created when the operating system starts. It creates many of the other processes, and then sits idle until the processes it created are done.

kthreadd is a process the operating system uses to create new threads. We’ll talk more about threads later, but for now you can think of a thread as kind of a process. The k at the beginning stands for kernel, which is the part of the operating system responsible for core capabilities like creating threads. The extra d at the end stands for daemon, which is another name for processes like this that run in the background and provide operating system services. In this context, “daemon” is used in the sense of a helpful spirit, with no connotation of evil.

Based on the name, you can infer that ksoftirqd is also a kernel daemon; specifically, it handles software interrupt requests, or “soft IRQ”.

kworker is a worker process created by the kernel to do some kind of processing for the kernel.

There are often multiple processes running these kernel services. On my system at the moment, there are 8 ksoftirqd processes and 35 kworker processes.

I won’t go into more details about the other processes, but if you are interested you can search for more information about them. You should run ps on your system and compare your results to mine.

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