Concurrency: Difference between revisions

From HaskellWiki
No edit summary
mNo edit summary
 
(37 intermediate revisions by 17 users not shown)
Line 1: Line 1:
= Concurrent programming in GHC =
''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 GHC.
This page contains notes and information about how to write concurrent programs in Haskell.


Please feel free to add stuff here (Edit page link at the bottom).
For practicality, the content is GHC-centric at the moment, although this may change as Haskell evolves.


== Starting points ==
== Overview ==


* '''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]
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.
</p>
* '''Software Transactional Memory''' (STM) is a new way to coordinate concurrent threads. STM will be in GHC 6.6, 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].
== Getting started ==


# Manage simultaneous I/O actions (eg. multiple connections on a web server)
#: Start with Concurrent Haskell (<code>forkIO</code>, <code>MVar</code>)
#: [[Parallel/Reading|Learn more about concurrency]], then try using [[Applications_and_libraries/Network#Libraries|network protocol libraries]] like HTTP or zeromq.
# Work with clusters or do distributed programming
#: Look out for [[Parallel/Research|ongoing research]] into distributed Haskell.


== Digging deeper ==


== Using concurrency in GHC ==
* '''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 [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.


* 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].
* '''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.


* 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].
== GHC concurrency specifics ==


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].


== Multiprocessor GHC ==
== Community ==


As of version 6.5, GHC supports running programs in parallel on an SMP or multi-core machine. How to do it:
* Ask questions on [[Mailing lists|Haskell Cafe]]
* See what [https://groups.google.com/group/concurrent-haskell concurrent-haskell] researchers and developers are working on
* Follow [http://twitter.com/#!/concurrenthaskell @concurrenthaskell] on Twitter [[image:Twitter-mini.png]]
* StackOverflow on Haskell: [http://stackoverflow.com/questions/tagged/haskell%2bconcurrency concurrency]


* You'll need to get a version of GHC that supports SMP.  Either download ghc from [http://www.haskell.org/ghc/docs/latest/html/building/sec-cvs.html CVS] or use darcs: <tt>darcs get --partial http://darcs.haskell.org/ghc</tt>.  There are also [http://www.haskell.org/ghc/dist/current/dist nightly snapshot distributions] available.
== Tools ==
* All code currently has to be built using the <tt>-smp</tt> switch, including the libraries.  If you downloaded a binary snapshot, then you already have the required libraries.  If you build GHC from source, you need to add<p>
<pre>
GhcLibWays += s
</pre>
to the file <tt>mk/build.mk</tt> in the build tree before building.</p>


* Compile your program with <tt>-smp</tt>
* [[ThreadScope]] - concurrent programs not working as expected? Use the ThreadScope debugger and watch the fireworks.
* Various [[Applications and libraries/Concurrency and parallelism|libraries]], including those for concurrency.


* 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).
== Documentation ==


* 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.
* [[Parallel/Glossary|Glossary]]
* [[Parallel/Reading|Learning to use concurrency in Haskell]]
* [[Parallel/Research|Current research]]
* [http://chimera.labs.oreilly.com/books/1230000000929/index.html ... Concurrent Programming in Haskell] (online book)


* Use <tt>+RTS -sstderr</tt> for timing stats.
== Alternative approaches ==


* [http://www.cs.kent.ac.uk/projects/ofa/chp/ CHP]: CSP-style concurrency for Haskell.


== Links to related work on parallel and distributed Haskell (many based on GHC) ==
== See also ==


* [http://www.macs.hw.ac.uk/~dsg/gph/ Glasgow Parallel Haskell]
* [[:Category:Concurrency|Concurrency]] category
* [http://www.macs.hw.ac.uk/~dsg/gdh/ Glasgow Distributed Haskell]
* Concurrency [[Parallel/Research|research]]
* http://www-i2.informatik.rwth-aachen.de/~stolz/dhs/
* http://www.informatik.uni-kiel.de/~fhu/PUBLICATIONS/1999/ifl.html


----
 
[[Category:GHC]]
[[Category:Concurrency]]

Latest revision as of 09:56, 15 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

Retrieved from "https://wiki.haskell.org/index.php?title=Concurrency&oldid=66658"