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[[Category:Parallel]]
 
[[Category:Parallel]]
   
== Concurrent Programming in GHC ==
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This page contains notes and information about how to write concurrent programs in Haskell. If you're more interested in performance than non-determinism, learn about [[parallelism]] first.
   
This page contains notes and information about how to write concurrent programs in GHC. If you're more interested in performance than non-determinism, learn about writing [[GHC/Parallelism|parallel]] programs instead.
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For practicality, the content is GHC-centric at the moment, although this may change as Haskell evolves.
   
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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== Overview ==
   
* See the [[Parallel/Reading reading list for parallelism in Haskell]].
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GHC provides multi-scale support for parallel and concurrent 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.
 
The concurrent programming models in GHC can be divided into the following forms:
 
   
* Lightweight Haskell threads, explicit synchronization with STM or MVars. See the paper "Tackling the Awkward Squad" below.
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== Getting started ==
* [http://www.cs.kent.ac.uk/projects/ofa/chp/ CHP]: CSP-style concurrency for Haskell.
 
   
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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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.
   
=== Starting points ===
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See "[http://research.microsoft.com/en-us/um/people/simonpj/papers/marktoberdorf/ Tackling the Awkward Squad]" to get started.
   
* '''Basic concurrency: forkIO and MVars'''.
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From there, try the [[Parallel/Reading|reading list for parallelism in Haskell]].
* '''Software Transactional Memory''' (STM) is a new way to coordinate concurrent threads. There's a separate [[Software transactional memory|Wiki page devoted to STM]].
 
: 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.
 
   
* '''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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== Digging deeper ==
   
=== Using concurrency in GHC ===
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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]].
  +
: 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/en-us/um/people/simonpj/papers/stm/#lock-free Lock-free data structures using Software Transactional Memory in Haskell] gives further examples of concurrent programming using STM.
   
* 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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* '''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.
   
* 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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== GHC concurrency specifics ==
   
=== Multicore GHC ===
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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].
   
 
{{GHC/Multicore}}
 
{{GHC/Multicore}}
   
=== Related work ===
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== Alternative approaches ==
  +
  +
* [http://www.cs.kent.ac.uk/projects/ofa/chp/ CHP]: CSP-style concurrency for Haskell.
  +
  +
== See also ==
   
* The Sun project to improve http://ghcsparc.blogspot.com/ GHC performance on Sparc]
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* [[Parallel]] portal
* A [http://www.well-typed.com/blog/38 Microsoft project to improve industrial applications of GHC parallelism].
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* Parallelism and concurrency [[Parallel/Research|research]]
* [http://www.haskell.org/~simonmar/bib/bib.html Simon Marlow's publications on parallelism and GHC]
 
* [http://www.macs.hw.ac.uk/~dsg/gdh/ Glasgow Distributed Haskell]
 
* http://www-i2.informatik.rwth-aachen.de/~stolz/dhs/
 
* http://www.informatik.uni-kiel.de/~fhu/PUBLICATIONS/1999/ifl.html
 
* [http://www.mathematik.uni-marburg.de/~eden Eden]
 

Revision as of 12:48, 17 April 2012


This page contains notes and information about how to write concurrent programs in Haskell. If you're more interested in performance than non-determinism, learn about parallelism first.

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

Contents

1 Overview

GHC provides multi-scale support for parallel and concurrent 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.

2 Getting started

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.

See "Tackling the Awkward Squad" to get started.

From there, try the reading list for parallelism in Haskell.

3 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 Software Transactional Memory in Haskell gives further examples of concurrent programming using STM.

4 GHC concurrency specifics

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

Since 2004, GHC supports running programs in parallel on an SMP or multi-core machine. How to do it:

  • Compile your program using the -threaded switch.
  • Run the program with +RTS -N2 to use 2 threads, for example (RTS stands for runtime system; see the GHC users' guide). You should use a -N value equal to the number of CPU cores on your machine (not including Hyper-threading cores). As of GHC v6.12, you can leave off the number of cores and all available cores will be used (you still need to pass -N however, like so: +RTS -N).
  • Concurrent threads (forkIO) will run in parallel, and you can also use the par combinator and Strategies from the Control.Parallel.Strategies module to create parallelism.
  • Use +RTS -sstderr for timing stats.
  • To debug parallel program performance, use ThreadScope.

5 Alternative approaches

  • CHP: CSP-style concurrency for Haskell.

6 See also