What is the difference between concurrent programming and parallel programming

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What is the difference between concurrent programming and parallel programing  I asked google but didn t find anything that helped me to understand that difference  Could you give me an example for both   For now I found this explanation  http   www linux-mag com id 7411 - but  concurrency is a property of the program  vs  parallel execution is a property of the machine  isn t enough for me - still I can t say what is what

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Just sharing an example that helps to highlight the distinction:

Parallel Programming: Say you want to implement the merge-sort algorithm. Each time that you divide the problem into two sub-problems, you can have two threads that solve them. However, in order to do the merge step you have to wait for these two threads to finish since merging requires both sub-solutions. This "mandatory wait" makes this a parallel program.

Concurrent Program: Say you want to compress n text files and generate a compressed file for each of them. You can have from 2 (up to n) threads that each handle compressing a subset of the files. When each thread is done, it's just done, it doesn't have to wait or do anything else. So, since different tasks are performed in an interleaved manner in "any arbitrary order" the program is concurrent but not parallel.

As someone else mentioned, every parallel program is concurrent (has to be in fact), but not the other way around.

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I believe concurrent programming refers to multithreaded programming which is about letting your program run multiple threads  abstracted from hardware details   Parallel programming refers to specifically designing your program algorithms to take advantage of available parallel execution  For example  you can execute in parallel two branches of some algorithms in expectation that it will hit the result sooner  on average  than it would if you first checked the first then the second branch

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https   joearms github io published 2013-04-05-concurrent-and-parallel-programming html  Concurrent   Two queues and one coffee machine   Parallel   Two queues and two coffee machines

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Parallel programming happens when code is being executed at the same time and each execution is independent of the other  Therefore  there is usually not a preoccupation about shared variables and such because that won t likely happen   However  concurrent programming consists on code being executed by different processes threads that share variables and such  therefore on concurrent programming we must establish some sort of rule to decide which process thread executes first  we want this so that we can be sure there will be consistency and that we can know with certainty what will happen  If there is no control and all threads compute at the same time and store things on the same variables  how would we know what to expect in the end  Maybe a thread is faster than the other  maybe one of the threads even stopped in the middle of its execution and another continued a different computation with a corrupted  not yet fully computed  variable  the possibilities are endless  It s in these situations that we usually use concurrent programming instead of parallel

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In programming  concurrency is the composition of independently   executing processes  while parallelism is the simultaneous execution   of  possibly related  computations     - Andrew Gerrand -   And      Concurrency is the composition of independently executing   computations  Concurrency is a way to structure software  particularly   as a way to write clean code that interacts well with the real world    It is not parallelism       Concurrency is not parallelism  although it enables parallelism  If   you have only one processor  your program can still be concurrent but   it cannot be parallel  On the other hand  a well-written concurrent   program might run efficiently in parallel on a multiprocessor  That   property could be important       - Rob Pike -   To understand the difference  I strongly recommend to see this Rob Pike one of Golang creators  s video  Concurrency Is Not Parallelism

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They re two phrases that describe the same thing from  very slightly  different viewpoints  Parallel programming is describing the situation from the viewpoint of the hardware -- there are at least two processors  possibly within a single physical package  working on a problem in parallel  Concurrent programming is describing things more from the viewpoint of the software -- two or more actions may happen at exactly the same time  concurrently    The problem here is that people are trying to use the two phrases to draw a clear distinction when none really exists  The reality is that the dividing line they re trying to draw has been fuzzy and indistinct for decades  and has grown ever more indistinct over time   What they re trying to discuss is the fact that once upon a time  most computers had only a single CPU  When you executed multiple processes  or threads  on that single CPU  the CPU was only really executing one instruction from one of those threads at a time  The appearance of concurrency was an illusion--the CPU switching between executing instructions from different threads quickly enough that to human perception  to which anything less than 100 ms or so looks instantaneous  it looked like it was doing many things at once   The obvious contrast to this is a computer with multiple CPUs  or a CPU with multiple cores  so the machine is executing instructions from multiple threads and or processes at exactly the same time  code executing one can t doesn t have any effect on code executing in the other   Now the problem  such a clean distinction has almost never existed  Computer designers are actually fairly intelligent  so they noticed a long time ago that  for example  when you needed to read some data from an I O device such as a disk  it took a long time  in terms of CPU cycles  to finish  Instead of leaving the CPU idle while that happened  they figured out various ways of letting one process thread make an I O request  and let code from some other process thread execute on the CPU while the I O request completed   So  long before multi-core CPUs became the norm  we had operations from multiple threads happening in parallel   That s only the tip of the iceberg though  Decades ago  computers started providing another level of parallelism as well  Again  being fairly intelligent people  computer designers noticed that in a lot of cases  they had instructions that didn t affect each other  so it was possible to execute more than one instruction from the same stream at the same time  One early example that became pretty well known was the Control Data 6600  This was  by a fairly wide margin  the fastest computer on earth when it was introduced in 1964--and much of the same basic architecture remains in use today  It tracked the resources used by each instruction  and had a set of execution units that executed instructions as soon as the resources on which they depended became available  very similar to the design of most recent Intel AMD processors   But  as the commercials used to say  wait--that s not all  There s yet another design element to add still further confusion  It s been given quite a few different names  e g    Hyperthreading    SMT    CMP    but they all refer to the same basic idea  a CPU that can execute multiple threads simultaneously  using a combination of some resources that are independent for each thread  and some resources that are shared between the threads  In a typical case this is combined with the instruction-level parallelism outlined above  To do that  we have two  or more  sets of architectural registers  Then we have a set of execution units that can execute instructions as soon as the necessary resources become available  These often combine well because the instructions from the separate streams virtually never depend on the same resources   Then  of course  we get to modern systems with multiple cores  Here things are obvious  right  We have N  somewhere between 2 and 256 or so  at the moment  separate cores  that can all execute instructions at the same time  so we have  clear-cut case of real parallelism--executing instructions in one process thread doesn t affect executing instructions in another   Well  sort of  Even here we have some independent resources  registers  execution units  at least one level of cache  and some shared resources  typically at least the lowest level of cache  and definitely the memory controllers and bandwidth to memory    To summarize  the simple scenarios people like to contrast between shared resources and independent resources virtually never happen in real life  With all resources shared  we end up with something like MS-DOS  where we can only run one program at a time  and we have to stop running one before we can run the other at all  With completely independent resources  we have N computers running MS-DOS  without even a network to connect them  with no ability to share anything between them at all  because if we can even share a file  well  that s  a shared resource  a violation of the basic premise of nothing being shared    Every interesting case involves some combination of independent resources and shared resources  Every reasonably modern computer  and a lot that aren t at all modern  has at least some ability to carry out at least a few independent operations simultaneously  and just about anything more sophisticated than MS-DOS has taken advantage of that to at least some degree    The nice  clean division between  concurrent  and  parallel  that people like to draw just doesn t exist  and almost never has  What people like to classify as  concurrent  usually still involves at least one and often more different types of parallel execution  What they like to classify as  parallel  often involves sharing resources and  for example  one process blocking another s execution while using a resource that s shared between the two   People trying to draw a clean distinction between  parallel  and  concurrent  are living in a fantasy of computers that never actually existed

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Different people talk about different kinds of concurrency and parallelism in many different specific cases  so some abstractions to cover their common nature are needed   The basic abstraction is done in computer science  where both concurrency and parallelism are attributed to the properties of programs  Here  programs are formalized descriptions of computing  Such programs need not to be in any particular language or encoding  which is implementation-specific  The existence of API ABI ISA OS is irrelevant to such level of abstraction  Surely one will need more detailed implementation-specific knowledge  like threading model  to do concrete programming works  the spirit behind the basic abstraction is not changed   A second important fact is  as general properties  concurrency and parallelism can coexist in many different abstractions   For the general distinction  see the relevant answer for the basic view of concurrency v  parallelism   There are also some links containing some additional sources    Concurrent programming and parallel programming are techniques to implement such general properties with some systems which expose programmability  The systems are usually programming languages and their implementations   A programming language may expose the intended properties by built-in semantic rules  In most cases  such rules specify the evaluations of specific language structures  e g  expressions  making the computation involved effectively concurrent or parallel   More specifically  the computational effects implied by the evaluations can perfectly reflect these properties   However  concurrent parallel language semantics are essentially complex and they are not necessary to practical works  to implement efficient concurrent parallel algorithms as the solutions of realistic problems   So  most traditional languages take a more conservative and simpler approach  assuming the semantics of evaluation totally sequential and serial  then providing optional primitives to allow some of the computations being concurrent and parallel  These primitives can be keywords or procedural constructs   functions   supported by the language  They are implemented based on the interaction with hosted environments  OS  or  bare metal  hardware interface   usually opaque  not able to be derived using the language portably  to the language  Thus  in this particular kind of high-level abstractions seen by the programmers  nothing is concurrent parallel besides these  magic  primitives and programs relying on these primitives  the programmers can then enjoy less error-prone experience of programming when concurrency parallelism properties are not so interested   Although primitives abstract the complex away in the most high-level abstractions  the implementations still have the extra complexity not exposed by the language feature  So  some mid-level abstractions are needed  One typical example is threading  Threading allows one or more thread of execution  or simply thread  sometimes it is also called a process  which is not necessarily the concept of a task scheduled in an OS  supported by the language implementation  the runtime   Threads are usually preemptively scheduled by the runtime  so a thread needs to know nothing about other threads  Thus  threads are natural to implement parallelism as long as they share nothing  the critical resources   just decompose computations in different threads  once the underlying implementation allows the overlapping of the computation resources during the execution  it works  Threads are also subject to concurrent accesses of shared resources  just access resources in any order meets the minimal constraints required by the algorithm  and the implementation will eventually determine when to access  In such cases  some synchronization operations may be necessary  Some languages treat threading and synchronization operations as parts of the high-level abstraction and expose them as primitives  while some other languages encourage only relatively more high-level primitives  like futures promises  instead   Under the level of language-specific threads  there come multitasking of the underlying hosting environment  typically  an OS   OS-level preemptive multitasking are used to implement  preemptive  multithreading  In some environments like Windows NT  the basic scheduling units  the tasks  are also  threads   To differentiate them with userspace implementation of threads mentioned above  they are called kernel threads  where  kernel  means the kernel of the OS  however  strictly speaking  this is not quite true for Windows NT  the  real  kernel is the NT executive   Kernel threads are not always 1 1 mapped to the userspace threads  although 1 1 mapping often reduces most overhead of mapping  Since kernel threads are heavyweight  involving system calls  to create destroy communicate  there are non 1 1 green threads in the userspace to overcome the overhead problems at the cost of the mapping overhead  The choice of mapping depending on the programming paradigm expected in the high-level abstraction  For example  when a huge number of userspace threads expected being concurrently executed  like Erlang   1 1 mapping is never feasible   The underlying of OS multitasking is ISA-level multitasking provided by the logical core of the processor  This is usually the most low-level public interface for programmers  Beneath this level  there may exist SMT  This is a form of more low-level multithreading implemented by the hardware  but arguably  still somewhat programmable - though it is usually only accessible by the processor manufacturer  Note the hardware design is apparently reflecting parallelism  but there is also concurrent scheduling mechanism to make the internal hardware resources being efficiently used   In each level of  threading  mentioned above  both concurrency and parallelism are involved  Although the programming interfaces vary dramatically  all of them are subject to the properties revealed by the basic abstraction at the very beginning

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If you program is using threads  concurrent programming   it s not necessarily going to be executed as such  parallel execution   since it depends on whether the machine can handle several threads   Here s a visual example  Threads on a non-threaded machine           --  --  --                        gt ---- --  --  --  -- ---- gt  gt    Threads on a threaded machine        ------               gt -------------- gt  gt    The dashes represent executed code  As you can see  they both split up and execute separately  but the threaded machine can execute several separate pieces at once

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Classic scheduling of tasks can be  serial  parallel or concurrent    Serial  tasks must be executed one after the other in a known tricked order or it will not work  Easy enough  Parallel  tasks must be executed at the same time or it will not work    Any failure of any of the tasks - functionally or in time - will result in total system failure  All tasks must have a common reliable sense of time     Try to avoid this or we will have tears by tea time  Concurrent  we do not care  We are not careless  though  we have analysed it and it doesn t matter  we can therefore execute any task using any available facility at any time  Happy days    Often  the available scheduling changes at known events which we call a state change   People often think this is about software  but it is in fact a systems design concept that pre-dates computers  software systems were a little slow in the uptake  very few software languages even attempt to address the problem  You might try looking up the transputer language occam if you are interested   Succinctly  systems design addresses the following    the verb - what you are doing  operation or algorithm  the noun - what you are doing it to  data or interface  when - initiation  schedule  state changes how - serial  parallel  concurrent where - once you know when things happen  you can say where they can happen and not before  why - is this the way to do it  Are there other ways  and more importantly  a better way  What happens if you don t do it    Good luck

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Concurrent programming is in a general sense to refer to environments in which the tasks we define can occur in any order  One   task can occur before or after another  and some or all tasks can be   performed at the same time     Parallel programming is to specifically refer to the simultaneous  execution of concurrent tasks on different processors  Thus  all   parallel programming is concurrent  but not all concurrent programming   is parallel       Source  PThreads Programming - A POSIX Standard for Better Multiprocessing  Buttlar  Farrell  Nichols

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Interpreting the original question as parallel concurrent computation instead of programming    In concurrent computation two computations both advance independently of each other  The second computation doesn t have to wait until the first is finished for it to advance  It doesn t state however  the mechanism how this is achieved  In single-core setup  suspending and alternating between threads is required  also called pre-emptive multithreading    In parallel computation two computations both advance simultaneously - that is literally at the same time  This is not possible with single CPU and requires multi-core setup instead   Images from article   Parallel vs Concurrent in Node js     versus

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I found this content in some blog  Thought it is useful and  relevant    Concurrency and parallelism are NOT the same thing  Two tasks T1 and T2 are concurrent if the order in which the two tasks are executed in time is not predetermined   T1 may be executed and finished before T2  T2 may be executed and finished before T1  T1 and T2 may be executed simultaneously at the same instance of time  parallelism   T1 and T2 may be executed alternatively      If two concurrent threads are scheduled by the OS to run on one single-core non-SMT non-CMP processor  you may get concurrency but not parallelism  Parallelism is possible on multi-core  multi-processor or distributed systems   Concurrency is often referred to as a property of a program  and is a concept more general than parallelism   Source  https   blogs oracle com yuanlin entry concurrency vs parallelism concurrent programming

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Concurrency and Parallelism Source  In a multithreaded process on a single processor  the processor can switch execution resources between threads  resulting in concurrent execution   In the same multithreaded process in a shared-memory multiprocessor environment  each thread in the process can run on a separate processor at the same time  resulting in parallel execution   When the process has fewer or as many threads as there are processors  the threads support system in conjunction with the operating environment ensure that each thread runs on a different processor   For example  in a matrix multiplication that has the same number of threads and processors  each thread  and each processor  computes a row of the result

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Concurrent programming regards operations that appear to overlap and is primarily concerned with the complexity that arises due to non-deterministic control flow  The quantitative costs associated with concurrent programs are typically both throughput and latency  Concurrent programs are often IO bound but not always  e g  concurrent garbage collectors are entirely on-CPU  The pedagogical example of a concurrent program is a web crawler  This program initiates requests for web pages and accepts the responses concurrently as the results of the downloads become available  accumulating a set of pages that have already been visited  Control flow is non-deterministic because the responses are not necessarily received in the same order each time the program is run  This characteristic can make it very hard to debug concurrent programs  Some applications are fundamentally concurrent  e g  web servers must handle client connections concurrently  Erlang  F  asynchronous workflows and Scala s Akka library are perhaps the most promising approaches to highly concurrent programming   Multicore programming is a special case of parallel programming  Parallel programming concerns operations that are overlapped for the specific goal of improving throughput  The difficulties of concurrent programming are evaded by making control flow deterministic  Typically  programs spawn sets of child tasks that run in parallel and the parent task only continues once every subtask has finished  This makes parallel programs much easier to debug than concurrent programs  The hard part of parallel programming is performance optimization with respect to issues such as granularity and communication  The latter is still an issue in the context of multicores because there is a considerable cost associated with transferring data from one cache to another  Dense matrix-matrix multiply is a pedagogical example of parallel programming and it can be solved efficiently by using Straasen s divide-and-conquer algorithm and attacking the sub-problems in parallel  Cilk is perhaps the most promising approach for high-performance parallel programming on multicores and it has been adopted in both Intel s Threaded Building Blocks and Microsoft s Task Parallel Library  in  NET 4

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I will try to explain it in my own style  it might not be in computer terms but it gives you the general idea   Let s take an example  say Household chores  cleaning dishes  taking out trash  mowing the lawn etc  also we have 3 people threads  A  B  C to do them  Concurrent  The three individuals start different tasks independently i e     A -- gt  cleaning dishes B -- gt  taking out trash  C -- gt  mowing the lawn    Here  the order of tasks are indeterministic and responses depends on the amount of work  Parallel  Here if we want to improve the throughput we can assign multiple people to the single task  for example  cleaning dishes we assign two people  A soaping the dishes and B washing the dishes which might improve the throughput   cleaning the dishes   A -- gt  soaping the dishes B -- gt  washing the dishes   so on  Hope this gives an idea  now move on to the technical terms which are explained in the other answers

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Although there isn   t complete agreement on the distinction between the terms parallel and concurrent  many authors make the following distinctions    In concurrent computing  a program is one in which multiple tasks can be in progress at any instant  In parallel computing  a program is one in which multiple tasks cooperate closely to solve a problem    So parallel programs are concurrent  but a program such as a multitasking operating system is also concurrent  even when it is run on a machine with only one core  since multiple tasks can be in progress at any instant   Source  An introduction to parallel programming  Peter Pacheco

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I understood the difference to be   1  Concurrent - running in tandem using shared resources 2  Parallel - running side by side using different resources  So you can have two things happening at the same time independent of each other  even if they come together at points  2  or two things drawing on the same reserves throughout the operations being executed  1

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In the view  from a processor  It can be described by this pic     In the view  from a processor  It can be described by this pic

[language-agnostic] What is the difference between concurrent programming and parallel programming?

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