Revision as of 14:00, 9 March 2006

Concurrent programming in GHC

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

Please feel free to add stuff here (Edit page link at the bottom).

Basic concurrency: forkIO and MVars. Read Tackling the awkward squad: monadic input/output, concurrency, exceptions, and foreign-language calls in Haskell.
The original paper about Concurrent Haskell contains quite a few examples about how to write concurrent programs. A larger example is Writing High-Performance Server Applications in Haskell, Case Study: A Haskell Web Server

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 Composable memory transactions. The paper 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, Extending the Haskell Foreign Function Interface with Concurrency.

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

The GHC manual gives a few useful flags that control scheduling (not usually necessary) RTS options.

As of version 6.5, GHC supports running programs in parallel on an SMP or multi-core machine. How to do it:

You'll need to get a version of GHC that supports SMP. Either download a nightly snapshot distribution, or get the sources from darcs and build it yourself.

You need to link your program using the -threaded switch. (NOTE: previously it was necessary to compile all code, including libraries, with the -smp switch, this is no longer the case. The -smp flag is now a synonym for -threaded).

Run the program with +RTS -N2 to use 2 threads, for example. You should use a -N value equal to the number of CPU cores on your machine (not including Hyper-threading cores).

Concurrent threads (forkIO and forkOS) will run in parallel, and you can also use the par combinator and Strategies from the Control.Parallel.Strategies module to create parallelism.

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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:
-* 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 distributions], or [http://hackage.haskell.org/trac/ghc/wiki/GhcDarcs get the sources] from darcs and build it yourself.
+* 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.
 * 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>).