Difference between revisions of "Concurrency"

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''Note: you may want to read [[Parallelism vs. Concurrency]], as the terms have historically been conflated.''
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[[Category:GHC|Concurrency]]
  
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[[Category:Parallel]]
  
   
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This page contains notes and information about how to write concurrent programs in Haskell.
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This page contains notes and information about how to write concurrent programs in Haskell.
   
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For practicality, the content is GHC-centric at the moment, although this may change as Haskell evolves.
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If you're more interested in performance than non-determinism, learn about [[parallelism]] first.
  
   
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== Concurrency in GHC ==
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== Overview ==
   
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GHC provides multi-scale support for parallel programming, from very fine-grained, small "sparks", to coarse-grained explicit threads and locks, along with other models of concurrent and parallel programming, including actors, CSP-style concurrency, nested data parallelism and Intel Concurrent Collections. Synchronization between tasks is possible via messages, regular Haskell variables, MVar shared state or transactional memory.
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GHC provides multi-scale support for concurrent programming, from very fine-grained, small tasks to coarse-grained explicit threads and locks, along with other models of concurrent programming including actors, CSP-style concurrency, and Intel Concurrent Collections. Synchronization between tasks is possible via messages, regular Haskell variables, <code>MVar</code>-based shared state or transactional memory.
   
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== Getting started ==
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* See the [[Parallel/Reading|reading list for parallelism in Haskell]].
  
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The concurrent programming models in GHC can be divided into the following forms:
  
   
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# Manage simultaneous I/O actions (eg. multiple connections on a web server)
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* Lightweight Haskell threads, explicit synchronization with STM or MVars. See the paper "Tackling the Awkward Squad" below.
  
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#: Start with Concurrent Haskell (<code>forkIO</code>, <code>MVar</code>)
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* [http://www.cs.kent.ac.uk/projects/ofa/chp/ CHP]: CSP-style concurrency for Haskell.
  
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#: [[Parallel/Reading|Learn more about concurrency]], then try using [[Applications_and_libraries/Network#Libraries|network protocol libraries]] like HTTP or zeromq.
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# Work with clusters or do distributed programming
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#: Look out for [[Parallel/Research|ongoing research]] into distributed Haskell.
   
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== Digging deeper ==
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The most important (as of 2010) to get to know are the basic "concurrent Haskell" model of threads using forkIO and MVars, the use of transactional memory via STM.
  
   
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* '''Software Transactional Memory''' (STM) is a newer way to coordinate concurrent threads. There's a separate [[Software transactional memory|Wiki page devoted to STM]].
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=== Starting points ===
  
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: STM was added to GHC 6.4, and is described in the paper [https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.67.3686&rep=rep1&type=pdf Composable memory transactions]. The paper [https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.64.1678&rep=rep1&type=pdf Lock-free data structures using STM in Haskell] gives further examples of concurrent programming using STM.
   
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* '''Foreign function interface'''. If you are calling foreign functions in a concurrent program, you need to know about ''bound threads''. They are described in a Haskell workshop paper, [https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.80.4811&rep=rep1&type=pdf Extending the Haskell Foreign Function Interface with Concurrency]. The GHC Commentary [http://darcs.haskell.org/ghc/docs/comm/rts-libs/multi-thread.html Supporting multi-threaded interoperation] contains more detailed explanation of cooperation between FFI calls and multi-threaded runtime.
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* '''Basic concurrency: forkIO and MVars'''.
 
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* '''Software Transactional Memory''' (STM) is a new way to coordinate concurrent threads. There's a separate [[Software transactional memory|Wiki page devoted to STM]].
  
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: STM was added to GHC 6.4, and is described in the paper [http://research.microsoft.com/~simonpj/papers/stm/index.htm Composable memory transactions]. The paper [http://research.microsoft.com/~simonpj/papers/stm/lock-free.htm Lock-free data structures using Software Transactional Memory in Haskell] gives further examples of concurrent programming using STM.
  
   
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== GHC concurrency specifics ==
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* '''Foreign function interface'''. If you are calling foreign functions in a concurrent program, you need to know about ''bound threads''. They are described in a Haskell workshop paper, [http://research.microsoft.com/~simonpj/Papers/conc-ffi/index.htm Extending the Haskell Foreign Function Interface with Concurrency]. The GHC Commentary [http://darcs.haskell.org/ghc/docs/comm/rts-libs/multi-thread.html Supporting multi-threaded interoperation] contains more detailed explanation of cooperation between FFI calls and multi-threaded runtime.
  
   
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You get access to concurrency operations by importing the library [http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurrent.html Control.Concurrent].
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=== Using concurrency in GHC ===
  
   
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== Community ==
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* You get access to concurrency operations by importing the library [http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurrent.html Control.Concurrent].
  
   
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* Ask questions on [[Mailing lists|Haskell Cafe]]
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* The GHC manual gives a few useful flags that control scheduling (not usually necessary) [http://www.haskell.org/ghc/docs/latest/html/users_guide/sec-using-parallel.html#parallel-rts-opts RTS options].
  
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* See what [https://groups.google.com/group/concurrent-haskell concurrent-haskell] researchers and developers are working on
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* Follow [http://twitter.com/#!/concurrenthaskell @concurrenthaskell] on Twitter [[image:Twitter-mini.png]]
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* StackOverflow on Haskell: [http://stackoverflow.com/questions/tagged/haskell%2bconcurrency concurrency]
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== Tools ==
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* [[ThreadScope]] - concurrent programs not working as expected? Use the ThreadScope debugger and watch the fireworks.
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* Various [[Applications and libraries/Concurrency and parallelism|libraries]], including those for concurrency.
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== Documentation ==
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* [[Parallel/Glossary|Glossary]]
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* [[Parallel/Reading|Learning to use concurrency in Haskell]]
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* [[Parallel/Research|Current research]]
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* [http://chimera.labs.oreilly.com/books/1230000000929/index.html [...] Concurrent Programming in Haskell] (online book)
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== Alternative approaches ==
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* [http://www.cs.kent.ac.uk/projects/ofa/chp/ CHP]: CSP-style concurrency for Haskell.
   
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=== Multicore GHC ===
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== See also ==
   
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* [[:Category:Concurrency|Concurrency]] category
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{{GHC/Multicore}}
  
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* Concurrency [[Parallel/Research|research]]
   
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=== Related work ===
  
   
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[[Category:GHC]]
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* The Sun project to improve http://ghcsparc.blogspot.com/ GHC performance on Sparc]
  
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[[Category:Concurrency]]
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* A [http://www.well-typed.com/blog/38 Microsoft project to improve industrial applications of GHC parallelism].
  
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* [http://www.haskell.org/~simonmar/bib/bib.html Simon Marlow's publications on parallelism and GHC]
  
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* [http://www.macs.hw.ac.uk/~dsg/gdh/ Glasgow Distributed Haskell]
  
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* http://www-i2.informatik.rwth-aachen.de/~stolz/dhs/
  
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* http://www.informatik.uni-kiel.de/~fhu/PUBLICATIONS/1999/ifl.html
  
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* [http://www.mathematik.uni-marburg.de/~eden Eden]
  

Revision as of 12:02, 9 May 2024

Note: you may want to read Parallelism vs. Concurrency, as the terms have historically been conflated.

This page contains notes and information about how to write concurrent programs in Haskell.

For practicality, the content is GHC-centric at the moment, although this may change as Haskell evolves.

Overview

GHC provides multi-scale support for concurrent programming, from very fine-grained, small tasks to coarse-grained explicit threads and locks, along with other models of concurrent programming including actors, CSP-style concurrency, and Intel Concurrent Collections. Synchronization between tasks is possible via messages, regular Haskell variables, MVar-based shared state or transactional memory.

Getting started

  1. Manage simultaneous I/O actions (eg. multiple connections on a web server)
    Start with Concurrent Haskell (forkIO, MVar)
    Learn more about concurrency, then try using network protocol libraries like HTTP or zeromq.
  2. Work with clusters or do distributed programming
    Look out for ongoing research into distributed Haskell.

Digging deeper

  • Software Transactional Memory (STM) is a newer way to coordinate concurrent threads. There's a separate Wiki page devoted to STM.
STM was added to GHC 6.4, and is described in the paper Composable memory transactions. The paper Lock-free data structures using STM in Haskell gives further examples of concurrent programming using STM.

GHC concurrency specifics

You get access to concurrency operations by importing the library Control.Concurrent.

Community

Tools

  • ThreadScope - concurrent programs not working as expected? Use the ThreadScope debugger and watch the fireworks.
  • Various libraries, including those for concurrency.

Documentation

Alternative approaches

  • CHP: CSP-style concurrency for Haskell.

See also

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