Difference between revisions of "Concurrency"

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m (clear some confusion, using '+RTS' wasn't working, '+RTS -N' is needed (ghc 6.12.1 Linux x86_64))
(Relevant content from "Parallel" transferred to here; various other changes)
 
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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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== Parallel and Concurrent Programming in GHC ==
  
   
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This page contains notes and information about how to write concurrent and/or parallel 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.
⚫
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 ==
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* See "Real World Haskell" [http://book.realworldhaskell.org/read/concurrent-and-multicore-programming.html chapter 24], for an introduction to the most common forms of concurrent and parallel programming in GHC.
  
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* A [http://donsbot.wordpress.com/2009/09/03/parallel-programming-in-haskell-a-reading-list/ reading list for parallelism in Haskell].
  
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* The [http://stackoverflow.com/questions/3063652/whats-the-status-of-multicore-programming-in-haskell status of parallel and concurrent programming] in Haskell.
  
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The concurrent and parallel programming models in GHC can be divided into the following forms:
  
   
  ⚫
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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* Very fine grained: parallel sparks and futures, as described in the paper "[http://www.haskell.org/~simonmar/bib/multicore-ghc-09_abstract.html Runtime Support for Multicore Haskell]"
  
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* Fine grained: lightweight Haskell threads, explicit synchronization with STM or MVars. See the paper "Tackling the Awkward Squad" below.
  
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* Nested data parallelism: a parallel programming model based on bulk data parallelism, in the form of the [http://www.haskell.org/haskellwiki/GHC/Data_Parallel_Haskell DPH] and [http://hackage.haskell.org/package/repa Repa] libraries for transparently parallel arrays.
  
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* Intel [http://software.intel.com/en-us/blogs/2010/05/27/announcing-intel-concurrent-collections-for-haskell-01/ Concurrent Collections for Haskell]: a graph-oriented parallel programming model.
  
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* [http://www.cs.kent.ac.uk/projects/ofa/chp/ CHP]: CSP-style concurrency for Haskell.
  
   
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== Getting started ==
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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, implicit parallelism via sparks and, if you're interested in scientific programming specifically, nested data parallelism in Haskell.
  
   
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# Manage simultaneous I/O actions (eg. multiple connections on a web server)
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=== Starting points ===
  
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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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* '''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.
  
   
  ⚫
* '''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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* '''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.
  
  ⚫
: 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.
   
  ⚫
* '''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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* '''Nested Data Parallelism'''. For an approach to exploiting the implicit parallelism in array programs for multiprocessors, see [[GHC/Data Parallel Haskell|Data Parallel Haskell]] (work in progress).
  
   
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=== Using concurrency in GHC ===
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== GHC concurrency specifics ==
   
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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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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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== Community ==
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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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* Ask questions on [[Mailing lists|Haskell Cafe]]
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=== Multicore GHC ===
  
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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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Since 2004, GHC supports running programs in parallel on an SMP or multi-core machine. How to do it:
  
   
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* [[ThreadScope]] - concurrent programs not working as expected? Use the ThreadScope debugger and watch the fireworks.
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* [http://haskell.org/platform Download a recent GHC].
  
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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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* Compile your program using the <tt>-threaded</tt> switch.
  
   
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* [[Parallel/Glossary|Glossary]]
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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). 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 <tt>-N</tt> however, like so: <tt>+RTS -N</tt>).
  
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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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* Concurrent threads (<tt>forkIO</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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* [http://www.cs.kent.ac.uk/projects/ofa/chp/ CHP]: CSP-style concurrency for Haskell.
   
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== See also ==
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* Use <tt>+RTS -sstderr</tt> for timing stats.
  
   
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* [[:Category:Concurrency|Concurrency]] category
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* To debug parallel program performance, use [http://research.microsoft.com/en-us/projects/threadscope/ ThreadScope].
  
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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/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]
  

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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