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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= Concurrent programming in GHC =
  
   
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This page contains notes and information about how to write concurrent programs in GHC.
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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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Please feel free to add stuff here (Edit page link at the bottom).
  
   
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== Starting points ==
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== Overview ==
   
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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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* '''Basic concurrency: forkIO and MVars'''. Read [http://research.microsoft.com/Users/simonpj/papers/marktoberdorf/marktoberdorf.ps.gz Tackling the awkward squad: monadic input/output, concurrency, exceptions, and foreign-language calls in Haskell].<p>The [http://www.haskell.org/ghc/docs/papers/concurrent-haskell.ps.gz original paper about Concurrent Haskell] contains quite a few examples about how to write concurrent programs. A larger example is [http://www.haskell.org/~simonmar/papers/web-server.ps.gz Writing High-Performance Server Applications in Haskell, Case Study: A Haskell Web Server]
  
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</p>
  
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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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== Getting started ==
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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://www.cse.unsw.edu.au/~chak/haskell/ghc/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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# Manage simultaneous I/O actions (eg. multiple connections on a web server)
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#: Start with Concurrent Haskell (<code>forkIO</code>, <code>MVar</code>)
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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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* '''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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== Using concurrency in GHC ==
  
  ⚫
: 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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* 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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== GHC concurrency specifics ==
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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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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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== Multiprocessor GHC ==
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== Community ==
   
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* Ask questions on [[Mailing lists|Haskell Cafe]]
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As of version 6.5, GHC supports running programs in parallel on an SMP or multi-core machine. How to do it:
  
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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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* You'll need to get a version of GHC that supports SMP. Either download a [http://www.haskell.org/ghc/dist/current/dist nightly snapshot distribution], or [http://hackage.haskell.org/trac/ghc/wiki/GhcDarcs get the sources] from darcs and build it yourself.
  
   
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* [[ThreadScope]] - concurrent programs not working as expected? Use the ThreadScope debugger and watch the fireworks.
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* You need to link your program using the <tt>-threaded</tt> switch. (NOTE: previously it was necessary to compile all code, including libraries, with the <tt>-smp</tt> switch, this is no longer the case. The <tt>-smp</tt> flag is now a synonym for <tt>-threaded</tt>).
  
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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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* Run the program with <tt>+RTS -N2</tt> to use 2 threads, for example. You should use a <tt>-N</tt> value equal to the number of CPU cores on your machine (not including Hyper-threading cores).
  
   
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* [[Parallel/Glossary|Glossary]]
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* Concurrent threads (<tt>forkIO</tt> and <tt>forkOS</tt>) will run in parallel, and you can also use the <tt>par</tt> combinator and Strategies from the [http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Parallel-Strategies.html Control.Parallel.Strategies] module to create parallelism.
  
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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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* Use <tt>+RTS -sstderr</tt> for timing stats.
  
   
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* [http://www.cs.kent.ac.uk/projects/ofa/chp/ CHP]: CSP-style concurrency for Haskell.
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== Links to related work on parallel and distributed Haskell (many based on GHC) ==
  
   
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== See also ==
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* [http://www.macs.hw.ac.uk/~dsg/gph/ Glasgow Parallel Haskell]
  
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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]
  
   
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* [[:Category:Concurrency|Concurrency]] category
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----
  
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* Concurrency [[Parallel/Research|research]]
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[[Category:GHC]]
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[[Category:Concurrency]]

Latest 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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