Revision as of 13:58, 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 ghc from CVS or use darcs: darcs get --partial http://darcs.haskell.org/ghc. There are also nightly snapshot distributions available.

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.

@@ Line 28: / Line 28: @@
 * 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.
-* 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>
+* 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>).
-<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>
 * 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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 * Use <tt>+RTS -sstderr</tt> for timing stats.
 == Links to related work on parallel and distributed Haskell (many based on GHC) ==