What is process and thread?

Yes, I have read many materials related to operating system. And I am still reading. But it seems all of them are describing the process and thread in a "abstract" way, which makes a lot of high level elabration on their behavior and logic orgnization. I am wondering what are they physically? In my opinion, they are just some in-memory "data structures" which are maintained and used by the kernel codes to facilitate the execution of program. For example, operating system use some process data structure (PCB) to describe the aspects of the process assigned for a certain program, such as its priority, its address space and so on. Is this all right?

First thing you need to know to understand the difference between a process and a thread, is a fact, that processes do not run, threads do.

So, what is a thread? Closest I can get explaining it is an execution state, as in: a combination of CPU registers, stack, the lot. You can see a proof of that, by breaking in a debugger at any given moment. What do you see? A call stack, a set of registers. That's pretty much it. That's the thread.

Now, then, what is a process. Well, it's a like an abstract "container" entity for running threads. As far as OS is concerned in a first approximation, it's an entity OS allocates some VM to, assigns some system resources to (like file handles, network sockets), &c.

How do they work together? The OS creates a "process" by reserving some resources to it, and starting a "main" thread. That thread then can spawn more threads. Those are the threads in one process. They more or less can share those resources one way or another (say, locking might be needed for them not to spoil the fun for others &c). From there on, OS is normally responsible for maintaining those threads "inside" that VM (detecting and preventing attempts to access memory which doesn't "belong" to that process), providing some type of scheduling those threads, so that they can run "one-after-another-and-not-just-one-all-the-time".

Normally when you run an executable like notepad.exe, this creates a single process. These process could spawn other processes, but in most cases there is a single process for each executable that you run. Within the process, there can be many threads. Usually at first there is one thread, which usually starts at the programs "entry point" which is the main function usually. Instructions are executed one by one in order, like a person who only has one hand, a thread can only do one thing at a time before it moves on to the next.

That first thread can create additional threads. Each additional thread has it's own entry point, which is usually defined with a function. The process is like a container for all the threads that have been spawned within it.

That is a pretty simplistic explanation. I could go into more detail but probably would overlap with what you will find in your textbooks.

EDIT: You'll notice there are lot's of "usually"'s in my explanation, as there are occasionally rare programs that do things drastically different.

One of the reasons why it is pretty much impossible to describe threads and processes in a non-abstract way is that they are abstractions.

Their concrete implementations differ tremendously.

Compare for example an Erlang Process and a Windows Process: an Erlang Process is very lightweight, often less than 400 Bytes. You can start 10 million processes on a not very recent laptop without any problems. They start up very quickly, they die very quickly and you are expected to be able to use them for very short tasks. Every Erlang Process has its own Garbage Collector associated with it. Erlang Processes can never share memory, ever.

Windows Processes are very heavy, sometimes hundreds of MiBytes. You can start maybe a couple of thousand of them on a beefy server, if you are lucky. They start up and die pretty slowly. Windows Processes are the units of Applications such as IDEs or Text Editors or Word Processors, so they are usually expected to live quite a long time (at least several minutes). They have their own Address Space, but no Garbage Collector. Windows Processes can share memory, although by default they don't.

Threads are a similar matter: an NPTL Linux Thread on x86 can be as small as 4 KiByte and with some tricks you can start 800000+ on a 32 Bit x86 machine. The machine will certainly be useable with thousands, maybe tens of thousands of threads. A .NET CLR Thread has a minimum size of about 1 MiByte, which means that just 4000 of those will eat up your entire address space on a 32 Bit machine. So, while 4000 NPTL Linux Threads is generally not a problem, you can't even start 4000 .NET CLR Threads because you will run out of memory before that.

OS Processes and OS Threads are also implemented very differently between different Operating Systems. The main two approaches are: the kernel knows only about processes. Threads are implemented by a Userspace Library, without any knowledge of the kernel at all. In this case, there are again two approaches: 1:1 (every Thread maps to one Kernel Process) or m:n (m Threads map to n Processes, where usually m > n and often n == #CPUs). This was the early approach taken on many Operating Systems after Threads were invented. However, it is usually deemed inefficient and has been replaced on almost all systems by the second approach: Threads are implemented (at least partially) in the kernel, so that the kernel now knows about two distinct entities, Threads and Processes.

One Operating System that goes a third route, is Linux. In Linux, Threads are neither implemented in Userspace nor in the Kernel. Instead, the Kernel provides an abstraction of both a Thread and a Process (and indeed a couple of more things), called a Task. A Task is a Kernel Scheduled Entity, that carries with it a set of flags that determine which resources it shares with its siblings and which ones are private.

Depending on how you set those flags, you get either a Thread (share pretty much everything) or a Process (share all system resources like the system clock, the filesystem namespace, the networking namespace, the user ID namespace, the process ID namespace, but do not share the Address Space). But you can also get some other pretty interesting things, too. You can trivially get BSD-style jails (basically the same flags as a Process, but don't share the filesystem or the networking namespace). Or you can get what other OSs call a Virtualization Container or Zone (like a jail, but don't share the UID and PID namespaces and system clock). Since a couple of years ago via a technology called KVM (Kernel Virtual Machine) you can even get a full-blown Virtual Machine (share nothing, not even the processor's Page Tables). [The cool thing about this is that you get to reuse the highly-tuned mature Task Scheduler in the kernel for all of these things. One of the things the Xen Virtual Machine has often criticized for, was the poor performance of its scheduler. The KVM developers have a much superior scheduler than Xen, and the best thing is they didn't even have to write a single line of code for it!]

So, on Linux, the performance of Threads and Processes is much closer than on Windows and many other systems, because on Linux, they are actually the same thing. Which means that the usage patterns are very different: on Windows, you typically decide between using a Thread and a Process based on their weight: can I afford a Process or should I use a Thread, even though I actually don't want to share state? On Linux (and usually Unix in general), you decide based on their semantics: do I actually want to share state or not?

One reason why Processes tend to be lighter on Unix than on Windows, is different usage: on Unix, Processes are the basic unit of both concurrency and functionality. If you want to use concurrency, you use multiple Processes. If your application can be broken down into multiple independent pieces, you use multiple Processes. Every Process does exactly one thing and only that one thing. Even a simple one-line shell script often involves dozens or hundreds of Processes. Applications usually consist of many, often short-lived Processes.

On Windows, Threads are the basic units of concurrency and COM components or .NET objects are the basic units of functionality. Applications usually consist of a single long-running Process.

Again, they are used for very different purposes and have very different design goals. It's not that one or the other is better or worse, it's just that they are so different that the common characteristics can only be described very abstractly.

Pretty much the only few things you can say about Threads and Processes are that:

• Threads belong to Processes
• Threads are lighter than Processes
• Threads share most state with each other
• Processes share significantly less state than Threads (in particular, they generally share no memory, unless specifically requested)

I would say that :

A process has a memory space, opened files,..., and one or more threads.

A thread is an instruction stream that can be scheduled by the system on a processor.

Have a look at the detailed answer I gave previously here on SO. It gives an insight into a toy kernel structure responsible for maintaining processes and the threads...

Hope this helps, Best regards, Tom.

We have discussed this very issue a number of times here. Perhaps you will find some helpful information here:

What is the difference between a process and a thread

Process vs Thread

Thread and Process

A process is a container for a set of resources used while executing a program.

A process includes the following:

• Private virtual address space
• A program.
• A list of handles.
• An access token.
• A unique process ID.
• At least one thread.
• A pointer to the parent process, whether or not the process still exists or not.

That being said, a process can contain multiple threads.

Processes themselves can be grouped into jobs, which are containers for processes and are executed as single units.

A thread is what windows uses to schedule execution of instructions on the CPU. Every process has at least one.

I have a couple of pages on my wiki you could take a look at:

Process

Thread

Threads are memory structures in the scheduler of the operating system, as you say. Threads point to the start of some instructions in memory and process these when the scheduler decides they should be. While the thread is executing, the hardware timer will run. Once it hits the desired time, an interrupt will be invoked. After this, the hardware will then stop execution of the current program, and will invoke the registered interrupt handler function, which will be part of the scheduler, to inform that the current thread has finished execution.

Physically:

• Process is a structure that maintains the owning credentials, the thread list, and an open handle list

• A Thread is a structure containing a context (i.e. a saved register set + a location to execute), a set of PTEs describing what pages are mapped into the process's Virtual Address space, and an owner.

This is of course an extremely simplified explanation, but it gets the important bits. The fundamental unit of execution on both Linux and Windows is the Thread - the kernel scheduler doesn't care about processes (much). This is why on Linux, a thread is just a process who happens to share PTEs with another process.

A process is a area in memory managed by the OS to run an application. Thread is a small area in memory within a process to run a dedicated task.

Processes and Threads are abstractions - there is nothing physical about them, or any other part of an operating system for that matter. That is why we call it software.

If you view a computer in physical terms you end up with a jumble of electronics that emulate what a Turing Machine does. Trying to do anything useful with a raw Truing Machine would turn your brain to Jell-O in five minutes flat. To avoid that unpleasant experience, computer folks developed a set of abstractions to compartmentalize various aspects of computing. This lets you focus on the level of abstraction that interests you without having to worry about all the other stuff supporting it. Some things have been cast into circuitry (eg. adders and the like) which makes them physical but the vast majority of what we work with is based on a set abstractions. As a general rule, the abstractions we use have some form of mathematical underpinning to them. This is why stacks, queues and "state" play such an important role in computing - there is a well founded set of mathematics around these abstractions that let us build upon and reason about their manipulation.

The key is realizing that software is always based on a composite of abstract models of "things". Those "things" don't always relate to anything physical, more likely they relate some other abstraction. This is why you cannot find a satisfactory "physical" basis for Processes and Threads anywhere in your text books.

Several other people have posted links to and explanations about what threads and processes are, none of them point to anything "physical" though. As you guessed, they are really just a set of data structures and rules that live within the larger context of an operating system (which in turn is just more data structures and rules...)

Software is like an onion, layers on layers on layers, once you peal all the layers (abstractions) away, nothing much is left! But the onion is still very real.

It's kind of hard to give a short answer which does this question justice.

And at the risk of getting this horribly wrong and simplying things, you can say threads & processes are an operating-system/platform concept; and under-the-hood, you can define a single-threaded process by,

• Low-level CPU instructions (aka, the program).
• State of execution--meaning instruction pointer (really, a special register), register values, and stack
• The heap (aka, general purpose memory).

In modern operating systems, each process has its own memory space. Aside shared memory (only some OS support this) the operating system forbids one process from writing in the memory space of another. In Windows, you'll see a general protection fault if a process tries.

So you can say a multi-threaded process is the whole package. And each thread is basically nothing more than state of execution.

So when a thread is pre-empted for another (say, on a uni-processor system), all the operating system has to do in principle is save the state of execution of the thread (not sure if it has to do anything special for the stack) and load in another.

Pre-empting an entire process, on the other hand, is more expensive as you can imagine.

Edit: The ideas apply in abstracted platforms like Java as well.

They are not physical pieces of string, if that's what you're asking. ;)

As I understand it, pretty much everything inside the operating system is just data. Modern operating systems depend on a few hardware requirements: virtual memory address translation, interrupts, and memory protection (There's a lot of fuzzy hardware/software magic that happens during boot, but I'm not very familiar with that process). Once those physical requirements are in place, everything else is up to the operating system designer. It's all just chunks of data.

The reason they only are mentioned in an abstract way is that they are concepts, while they will be implemented as data structures there is no universal rule how they have to be implemented.
This is at least true for the threads/processes on their own, they wont do much good without a scheduler and an interrupt timer.
The scheduler is the algorithm by which the operating system chooses the next thread to run for a limited amount of time and the interrupt timer is a piece of hardware which periodically interrupts the execution of the current thread and hands control back to the scheduler.

Forgot something: the above is not true if you only have cooperative threading, cooperative threads have to actively yield control to the next thread, which can get ugly with one thread polling for results of an other thread, which waits for the first to yield.
These are even more lightweight than other threads as they don't require support of the underlying operating system to work.

I had seen many of the answers but most of them are not clear enough for an OS beginner.

In any modern day operating system, one process has a virtual CPU, virtual Memory, Virtual I/O.

Virtual CPU : if you have multiple cores the process might be assigned one or more of the cores for processing by the scheduler.

Virtual I/O : I/O might be shared between various processes. Like for an example keyboard that can be shared by multiple processes. So when you type in a notepad you see the text changing while a key logger running as daemon is storing all the keystrokes. So the process is sharing an I/O resource.

Virtual Memory : http://en.wikipedia.org/wiki/Virtual_memory you can go through the link.

So when a process is taken out of the state of execution by the scheduler it's state containing the values stored in the registers, its stack and heap and much more are saved into a data structure.

So now when we compare a process with a thread, threads started by a process shares the Virtual I/O and Virtual Memory assigned to the process which started it but not the Virtual CPU. So there might be multiple thread being started by a process all sharing the same virtual Memory and Virtual I/O bu but having different Virtual CPUs.

So you understand the need for locking the resource of a process be it statically allocated (stack) or dynamically allocated(heap) as the virtual memory space is shared between threads of a process.

Also each thread having its own Virtual CPU can run in parallel in different cores and significantly reduce the completion time of a process(reduction will be observable only if you have managed the memory wisely and there are multiple cores).

A thread is controlled by a process, a process is controlled by the operating system

Process doesn't share memory between each other - since it works in so called "protected flat model", on other hand threads shares the same memory.

With the Windows, at least once you get past Win 3.1, the operating system (OS) contains multiple process each with its own memory space and can't interact with other processes without the OS.

Each process has one or more threads that share the same memory space and do not need the OS to interact with other threads.

Process is a container of threads.

Well, I haven't seen an answer to "What are they physically", yet. So I give it a try.

Processes and Thread are nothing phyical. They are a feature of the operating system. Usally any physical component of a computer does not know about them. The CPU does only process a sequential stream of opcodes. These opcodes might belong to a thread. Then the OS uses traps and interrupts regain control, decide which code to excecute and switch to another thread.

Process is one complete entity e.g. and exe file or one jvm. There can be a child process of a parent process where the exe file run again in a separate space. Thread is a separate path of execution in the same process where the process is controlling which thread to execute, halt etc.

Trying to answer this question relating to Java world.

A process is an execution of a program but a thread is a single execution sequence within the process. A process can contain multiple threads. A thread is sometimes called a lightweight process.

For example:

Example 1: A JVM runs in a single process and threads in a JVM share the heap belonging to that process. That is why several threads may access the same object. Threads share the heap and have their own stack space. This is how one thread’s invocation of a method and its local variables are kept thread safe from other threads. But the heap is not thread-safe and must be synchronized for thread safety.

Example 2: A program might not be able to draw pictures by reading keystrokes. The program must give its full attention to the keyboard input and lacking the ability to handle more than one event at a time will lead to trouble. The ideal solution to this problem is the seamless execution of two or more sections of a program at the same time. Threads allows us to do this. Here Drawing picture is a process and reading keystroke is sub process (thread).