What is a thread really

Question

I have been trying to find a good definition  and get an understanding  of what a thread really is   It seems that I must be missing something obvious  but every time I read about what a thread is  it s almost a circular definition  a la  a thread is a thread of execution  or   a way to divide into running tasks   Uh uh  Huh   It seems from what I have read that a thread is not really something concrete  like a process is  It is in fact just a concept  From what I understand of the way this works  a processor executes some commands for a program  which has been termed a thread of execution   then when it needs to switch to processing for some other program for a bit  it stores the state of the program it s currently executing for somewhere  Thread Local Storage  and then starts executing the other program s instructions  And back and forth  Such that  a thread is really just a concept for  one of the paths of execution  of a program that is currently running   Unlike a process  which really is something - it is a conglomeration of resources  etc   As an example of a definition that didn t really help me much        From Wikipedia       A thread in computer science is short for a thread of execution  Threads are a way for a program to divide  termed  split   itself into two or more simultaneously  or pseudo-simultaneously  running tasks  Threads and processes differ from one operating system to another but  in general  a thread is contained inside a process and different threads in the same process share same resources while different processes in the same multitasking operating system do not     So am I right  Wrong  What is a thread really   Edit  Apparently a thread is also given its own call stack  so that is somewhat of a concrete thing

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A thread is an execution context  which is all the information a CPU needs to execute a stream of instructions   Suppose you re reading a book  and you want to take a break right now  but you want to be able to come back and resume reading from the exact point where you stopped  One way to achieve that is by jotting down the page number  line number  and word number  So your execution context for reading a book is these 3 numbers   If you have a roommate  and she s using the same technique  she can take the book while you re not using it  and resume reading from where she stopped  Then you can take it back  and resume it from where you were   Threads work in the same way  A CPU is giving you the illusion that it s doing multiple computations at the same time  It does that by spending a bit of time on each computation  It can do that because it has an execution context for each computation  Just like you can share a book with your friend  many tasks can share a CPU   On a more technical level  an execution context  therefore a thread  consists of the values of the CPU s registers   Last  threads are different from processes  A thread is a context of execution  while a process is a bunch of resources associated with a computation  A process can have one or many threads   Clarification  the resources associated with a process include memory pages  all the threads in a process have the same view of the memory   file descriptors  e g   open sockets   and security credentials  e g   the ID of the user who started the process

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A thread is an independent set of values for the processor registers  for a single core    Since this includes the Instruction Pointer  aka Program Counter   it controls what executes in what order   It also includes the Stack Pointer  which had better point to a unique area of memory for each thread or else they will interfere with each other   Threads are the software unit affected by control flow  function call  loop  goto   because those instructions operate on the Instruction Pointer  and that belongs to a particular thread   Threads are often scheduled according to some prioritization scheme  although it s possible to design a system with one thread per processor core  in which case every thread is always running and no scheduling is needed    In fact the value of the Instruction Pointer and the instruction stored at that location is sufficient to determine a new value for the Instruction Pointer   For most instructions  this simply advances the IP by the size of the instruction  but control flow instructions change the IP in other  predictable ways   The sequence of values the IP takes on forms a path of execution weaving through the program code  giving rise to the name  thread

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Processes are like two people using two different computers  who use the network to share data when necessary   Threads are like two people using the same computer  who don t have to share data explicitly but must carefully take turns   Conceptually  threads are just multiple worker bees buzzing around in the same address space   Each thread has its own stack  its own program counter  etc   but all threads in a process share the same memory   Imagine two programs running at the same time  but they both can access the same objects   Contrast this with processes   Processes each have their own address space  meaning a pointer in one process cannot be used to refer to an object in another  unless you use shared memory    I guess the key things to understand are    Both processes and threads can  run at the same time   Processes do not share memory  by default   but threads share all of their memory with other threads in the same process  Each thread in a process has its own stack and its own instruction pointer

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In order to define a thread formally  we must first understand the boundaries of where a thread operates    A computer program becomes a process when it is loaded from some store into the computer s memory and begins execution  A process can be executed by a processor or a set of processors  A process description in memory contains vital information such as the program counter which keeps track of the current position in the program  i e  which instruction is currently being executed   registers  variable stores  file handles  signals  and so forth    A thread is a sequence of such instructions within a program that can be executed independently of other code  The figure shows the concept    Threads are within the same process address space  thus  much of the information present in the memory description of the process can be shared across threads    Some information cannot be replicated  such as the stack  stack pointer to a different memory area per thread   registers and thread-specific data  This information suffices to allow threads to be scheduled independently of the program s main thread and possibly one or more other threads within the program    Explicit operating system support is required to run multithreaded programs  Fortunately  most modern operating systems support threads such as Linux  via NPTL   BSD variants  Mac OS X  Windows  Solaris  AIX  HP-UX  etc  Operating systems may use different mechanisms to implement multithreading support     Here  you can find more information about the topic  That was also my information-source   Let me just add a sentence coming from Introduction to Embedded System by Edward Lee and Seshia      Threads are imperative programs that run concurrently and share a memory space  They can access each others    variables   Many practitioners in the field use the term    threads    more narrowly to refer to particular ways of constructing programs that share memory   others  to broadly refer to any mechanism where imperative programs run concurrently  and  share memory  In this broad sense   threads exist in the form of interrupts on almost all microprocessors  even without any operating system at all  bare iron

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A thread is nothing more than a memory context  or how Tanenbaum better puts it  resource grouping  with execution rules  It s a software construct  The CPU has no idea what a thread is  some exceptions here  some processors have hardware threads   it just executes instructions   The kernel introduces the thread and process concept to manage the memory and instructions order in a meaningful way

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I am going to use a lot of text from the book Operating Systems Concepts by ABRAHAM SILBERSCHATZ  PETER BAER GALVIN and GREG GAGNE along with my own understanding of things    Process  Any application resides in the computer in the form of text  or code       We emphasize that a program by itself is not a process  A program is a   passive entity  such as a file containing a list of instructions stored on disk    often called an executable file     When we start an application  we create an instance of execution  This instance of execution is called a process   EDIT  As per my interpretation  analogous to a class and an instance of a class  the instance of a class being a process     An example of processes is that of Google Chrome   When we start Google Chrome  3 processes are spawned          The browser process is responsible for managing the user interface as   well as disk and network I O  A new browser process is created when   Chrome is started  Only one browser process is created            Renderer processes contain logic for rendering web pages  Thus  they   contain the logic for handling HTML  Javascript  images  and so forth    As a general rule  a new renderer process is created for each website   opened in a new tab  and so several renderer processes may be active   at the same time           A plug-in process is created for each type of plug-in  such as Flash   or QuickTime  in use  Plug-in processes contain the code for the   plug-in as well as additional code that enables the plug-in to   communicate with associated renderer processes and the browser   process    Thread  To answer this I think you should first know what a processor is  A Processor is the piece of hardware that actually performs the computations   EDIT   Computations like adding two numbers  sorting an array  basically executing the code that has been written    Now moving on to the definition of a thread       A thread is a basic unit of CPU utilization  it comprises a thread ID  a program   counter  a register set  and a stack    EDIT  Definition of a thread from intel s website       A Thread  or thread of execution  is a software term for the basic ordered sequence of instructions that can be passed through or processed by a single CPU core    So  if the Renderer process from the Chrome application sorts an array of numbers  the sorting will take place on a thread thread of execution   The grammar regarding threads seems confusing to me   My Interpretation of Things  A process is an execution instance  Threads are the actual workers that perform the computations via CPU access  When there are multiple threads running for a process  the process provides common memory    EDIT   Other Information that I found useful to give more context  All modern day computer have more than one threads  The number of threads in a computer depends on the number of cores in a computer    Concurrent Computing    From Wikipedia       Concurrent computing is a form of computing in which several computations are executed during overlapping time periods   concurrently   instead of sequentially  one completing before the next starts   This is a property of a system   this may be an individual program  a computer  or a network   and there is a separate execution point or  thread of control  for each computation   process      So  I could write a program which calculates the sum of 4 numbers     1   3     4   5    In the program to compute this sum  which will be one process running on a thread of execution  I can fork another process which can run on a different thread to compute  4   5  and return the result to the original process  while the original process calculates the sum of  1   3

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This was taken from a Yahoo Answer      A thread is a coding construct   unaffect by the architecture of an   application  A single process   frequently may contain multiple   threads  Threads can also directly   communicate with each other since they   share the same variables        Processes are independent execution   units with their own state   information  They also use their own   address spaces and can only interact   with other processes through   interprocess communication mechanisms    However  to put in simpler terms threads are like different  tasks    So think of when you are doing something  for instance you are writing down a formula on one paper   That can be considered one thread   Then another thread is you writing something else on another piece of paper   That is where multitasking comes in   Intel processors are said to have  hyper-threading   AMD has it too  and it is meant to be able to perform multiple  threads  or multitask much better   I am not sure about the logistics of how a thread is handled   I do recall hearing about the processor going back and forth between them  but I am not 100  sure about this and hopefully somebody else can answer that

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The answer varies hugely across different systems and different implementations  but the most important parts are    A thread has an independent thread of execution  i e  you can context-switch away from it  and then back  and it will resume running where it was   A thread has a lifetime  it can be created by another thread  and another thread can wait for it to finish   It probably has less baggage attached than a  process     Beyond that  threads could be implemented within a single process by a language runtime  threads could be coroutines  threads could be implemented within a single process by a threading library  or threads could be a kernel construct   In several modern Unix systems  including Linux which I m most familiar with  everything is threads -- a process is merely a type of thread that shares relatively few things with its parent  i e  it gets its own memory mappings  its own file table and permissions  etc   Reading man 2 clone  especially the list of flags  is really instructive here

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A thread is a set of  CPU instructions which can be executed  But in order to better understand the thread we have to have some knowledge about how the CPU and RAM works  A computer follows instructions and manipulates data  RAM is the place where those instructions and data on which the CPU will operate on  are saved  Within the CPU some memory cells called registers exist  Those registers contain numbers on which the CPU performs simple mathematical operations  An example can be the following     Add the number in register  3 to the number in register  1      The collection of all operations a CPU can do is called instruction set  Each operation in the instruction set is assigned a number  so computer code is essentially a sequence of numbers representing CPU operations  These operations are stored as numbers in the RAM  The CPU works in a never-ending loop  always fetching and executing an instruction from memory  At the core of this cycle is the PC register  or Program Counter  It s a special register that stores the memory address of the next instruction to be executed  The CPU will      Fetch the instruction at the memory address given by the PC  Increment the PC by 1  Execute the instruction  Go back to step 1   The CPU can be instructed to write a new value to the PC  causing the execution to branch  or  quot jump quot  to somewhere else in the memory  And this branching can be conditional  For instance  a CPU instruction could say   quot set PC to address  200 if register  1 equals zero quot   This allows computers to execute stuff like this  if   x   0     compute this   else     compute that    Resources taken from Computer Science Distilled were used

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Unfortunately  threads do exist  A thread is something tangible  You can kill one  and the others will still be running  You can spawn new threads     although each thread is not it s own process  they are running separately inside the process  On multi-core machines  2 threads could run at the same time   http   en wikipedia org wiki Simultaneous multithreading   http   www intel com intelpress samples mcp samplech01 pdf

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Let me explain the difference between process and threads first   A process can have  1  N  number of threads  A small explanation on virtual memory and virtual processor   Virtual memory  Used as a swap space so that a process thinks that it is sitting on the primary memory for execution   Virtual processor  The same concept as virtual memory except this is for processor  To a process  it will look it s the only thing that is using the processor   OS will take care of allocating the virtual memory and virtual processor to a process and performing the swap between processes and doing execution   All the threads within a process will share the same virtual memory  But  each thread will have their individual virtual processor assigned to them so that they can be executed individually   Thus saving the memory as well as utilizing the CPU to its potential

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