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fork in multi-threaded program
If I have an application which employs fork() and might be developed as multithreaded, what are the thumb rules/guidelines to consider to safely program this kind of applications?
The basic thumb rules, according to various internet articles like ( http://www.linuxprogrammingblog.com/threads-and-fork-think-twice-before-using-them , fork in multi-threaded program ) are:
(Main) Process[0] Monothread --> fork() --> (Child) Process[1] Multithreaded: OK!
If Process[1] crashes or messes around with memory it won't touch address space of Process[0] (unless you use shared R/W memory... but this is another topic of its own).In Linux by default all fork()ed memory is Copy On Write. Given that Process[0] is monothreaded, when we invoke fork() all possible mutual exclusion primitives should be generally in an unlocked state.
(Main) Process[0] Multithreaded --> fork() --> (Child) Process[1] Mono/Multithread: BAD!
If you fork() a Multithreaded process your mutexes and many other thread synchronization primitives will likely be in an undefined state in Process[1]. You can work around with pthread_atfork() but if you use libraries you might as well roll a dice and hope to be lucky. Because generally you don't (want to) know the implementation details of libraries.
The advantages of fork() into a multithreaded process are that you could manipulate/read/aggregate your data quicker (in the Child process), without having to care about stability of the process you fork() from (Main). This is useful if your main process has a dataset of a lot of memory and you don't want to duplicate/reload it to safely process the data in another process (Child). This way the original process is stable and independent from the data aggregation/manipulation process (fork()ed).
Of course this means that the original process will generally be slower than it might be if developed in multithreaded fashion. But again, this is the price you might want to be paying for more stability.
If instead your main process is multithreaded, refrain from using fork(). It's going to be a proper mess to implement it in a stable way.
Cheers
On Linux, threads are implemented in terms of processes. In other words, threads are really just a fork() with mostly shared memory, instead of completely copy-on-write memory. What this means, is that when you use fork() in a thread (main or other), you end up copying the entire shared memory space of all of the threads, and the thread specific storage of the thread you call fork() from.
Now all of this sounds good, but that doesn't mean that this is what will happen or work well. If you want to make a cloned process, try to do a fork before starting any other threads, and then use read-only virtual memory to keep the forked process up to date with current memory values.
So although it may work, I just suggest testing, and try to find another way first. And be prepared for a lot of:
Segmentation fault
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).
I have a count variable that should get counted up by a few processes I forked and used/read by the mother process.
I tried to create a pointer in my main() function of the mother process and count that pointer up in the forked children. That does not work! Every child seems to have it's own copy even though the address is the same in every process.
What is the best way to do that?
Each child gets its own copy of the parent processes memory (at least as soon as it trys to modify anything). If you need to share betweeen processes you need to look at shared memory or some similar IPC mechanism.
BTW, why are you making this a community wiki - you may be limiting responses by doing so.
2 processes cannot share the same memory. It is true that a forked child process will share the same underlying memory after forking, but an attempt to write to this would cause the operating system to allocate a new writeable space for it somewhere else.
Look into another form of IPC to use.
My experience is, that if you want to share information between at least two processes, you almost never want to share just some void* pointer into memory. You might want to have a look at
Boost Interprocess
which can give you an idea, how to share structured data (read "classes" and "structs") between processes.
No, use IPC or threads. Only file descriptors are shared (but not the seek pointer).
You might want to check out shared memory.
the pointers are always lies in the same process. It's private to the process, relative to the process's base address. There different kind of IPC mechanisms available in any operating systems. You can opt for Windows Messaging, Shared memory, socket, pipes etc. Choose one according to your requirement and size of data. Another mechanism is to write data in target process using Virtual memory APIs available and notify the process with corresponding pointer.
One simple option but limited form of IPC that would work well for a shared count is a 'shared data segment'. On Windows this is implemented using the #pragma data_seg directive.
See this article for an example.
What is the common theory behind thread communication? I have some primitive idea about how it should work but something doesn't settle well with me. Is there a way of doing it with interrupts?
Really, it's just the same as any concurrency problem: you've got multiple threads of control, and it's indeterminate which statements on which threads get executed when. That means there are a large number of POTENTIAL execution paths through the program, and your program must be correct under all of them.
In general the place where trouble can occur is when state is shared among the threads (aka "lightweight processes" in the old days.) That happens when there are shared memory areas,
To ensure correctness, what you need to do is ensure that these data areas get updated in a way that can't cause errors. To do this, you need to identify "critical sections" of the program, where sequential operation must be guaranteed. Those can be as little as a single instruction or line of code; if the language and architecture ensure that these are atomic, that is, can't be interrupted, then you're golden.
Otherwise, you idnetify that section, and put some kind of guards onto it. The classic way is to use a semaphore, which is an atomic statement that only allows one thread of control past at a time. These were invented by Edsgar Dijkstra, and so have names that come from the Dutch, P and V. When you come to a P, only one thread can proceed; all other threads are queued and waiting until the executing thread comes to the associated V operation.
Because these primitives are a little primitive, and because the Dutch names aren't very intuitive, there have been some ther larger-scale approaches developed.
Per Brinch-Hansen invented the monitor, which is basically just a data structure that has operations which are guaranteed atomic; they can be implemented with semaphores. Monitors are pretty much what Java synchronized statements are based on; they make an object or code block have that particular behavir -- that is, only one thread can be "in" them at a time -- with simpler syntax.
There are other modeals possible. Haskell and Erlang solve the problem by being functional languages that never allow a variable to be modified once it's created; this means they naturally don't need to wory about synchronization. Some new languages, like Clojure, instead have a structure called "transactional memory", which basically means that when there is an assignment, you're guaranteed the assignment is atomic and reversible.
So that's it in a nutshell. To really learn about it, the best places to look at Operating Systems texts, like, eg, Andy Tannenbaum's text.
The two most common mechanisms for thread communication are shared state and message passing.
THe most common way for threads to communicate is via some shared data structure, typically a queue. Some threads put information into the queue while others take it out. The queue must be protected by operating system facilities such as mutexes and semaphores. Interrupts have nothing to do with it.
If you're really interested in a theory of thread communications, you may want to look into formalisms like the pi Calculus.
To communicate between threads, you'll need to use whatever mechanism is supplied by your operating system and/or runtime. Interrupts would be unusually low level, although they might be used implicitly if your threads communicate using sockets or named pipes.
A common pattern would be to implement shared state using a shared memory block, relying on an os-supplied synchronization primitive such as a mutex to spare you from busy-waiting when your read from the block. Remember that if you have threads at all, then you must have some kind of scheduler already (whether it's native from the OS or emulated in your language runtime). So this scheduler can provide synchronization objects and a "sleep" function without necessarily having to rely on hardware support.
Sockets, pipes, and shared memory work between processes too. Sometimes a runtime will give you a lighter-weight way of doing synchronization for threads within the same process. Shared memory is cheaper within a single process. And sometimes your runtime will also give you an atomic message-passing mechanism.