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<sect1 id="lang-parallel">
- <title>Parallel Haskell</title>
+ <title>Concurrent and Parallel Haskell</title>
<indexterm><primary>parallelism</primary>
</indexterm>
- <para>There are two implementations of Parallel Haskell: SMP paralellism
- <indexterm><primary>SMP</primary></indexterm>
- which is built-in to GHC (see <xref linkend="sec-using-smp" />) and
- supports running Parallel Haskell programs on a single multiprocessor
- machine, and
- Glasgow Parallel Haskell<indexterm><primary>Glasgow Parallel Haskell</primary></indexterm>
- (GPH) which supports running Parallel Haskell
- programs on both clusters of machines or single multiprocessors. GPH is
- developed and distributed
- separately from GHC (see <ulink url="http://www.cee.hw.ac.uk/~dsg/gph/">The
- GPH Page</ulink>).</para>
-
- <para>Ordinary single-threaded Haskell programs will not benefit from
- enabling SMP parallelism alone. You must expose parallelism to the
- compiler in one of the following two ways.</para>
-
- <sect2>
- <title>Running Concurrent Haskell programs in parallel</title>
+ <para>GHC implements some major extensions to Haskell to support
+ concurrent and parallel programming. Let us first etablish terminology:
+ <itemizedlist>
+ <listitem><para><emphasis>Parallelism</emphasis> means running
+ a Haskell program on multiple processors, with the goal of improving
+ performance. Ideally, this should be done invisibly, and with no
+ semantic changes.
+ </para></listitem>
+ <listitem><para><emphasis>Concurrency</emphasis> means implementing
+ a program by using multiple I/O-performing threads. While a
+ concurrent Haskell program <emphasis>can</emphasis> run on a
+ parallel machine, the primary goal of using concurrency is not to gain
+ performance, but rather because that is the simplest and most
+ direct way to write the program. Since the threads perform I/O,
+ the semantics of the program is necessarily non-deterministic.
+ </para></listitem>
+ </itemizedlist>
+ GHC supports both concurrency and parallelism.
+ </para>
+
+ <sect2 id="concurrent-haskell">
+ <title>Concurrent Haskell</title>
+
+ <para>Concurrent Haskell is the name given to GHC's concurrency extension.
+ It is enabled by default, so no special flags are required.
+ The <ulink
+ url="http://research.microsoft.com/copyright/accept.asp?path=/users/simonpj/papers/concurrent-haskell.ps.gz">
+ Concurrent Haskell paper</ulink> is still an excellent
+ resource, as is <ulink
+ url="http://research.microsoft.com/%7Esimonpj/papers/marktoberdorf">Tackling
+ the awkward squad</ulink>.
+ </para><para>
+ To the programmer, Concurrent Haskell introduces no new language constructs;
+ rather, it appears simply as a library, <ulink
+ url="http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurrent.html">
+ Control.Concurrent</ulink>. The functions exported by this
+ library include:
+ <itemizedlist>
+<listitem><para>Forking and killing threads.</para></listitem>
+<listitem><para>Sleeping.</para></listitem>
+<listitem><para>Synchronised mutable variables, called <literal>MVars</literal></para></listitem>
+<listitem><para>Support for bound threads; see the paper <ulink
+url="http://research.microsoft.com/%7Esimonpj/Papers/conc-ffi/index.htm">Extending
+the FFI with concurrency</ulink>.</para></listitem>
+</itemizedlist>
+</para>
+</sect2>
+
+ <sect2><title>Software Transactional Memory</title>
+
+ <para>GHC now supports a new way to coordinate the activities of Concurrent
+ Haskell threads, called Software Transactional Memory (STM). The
+ <ulink
+ url="http://research.microsoft.com/%7Esimonpj/papers/stm/index.htm">STM
+ papers</ulink> are an excellent introduction to what STM is, and how to use
+ it.</para>
+
+ <para>The main library you need to use STM is <ulink
+ url="http://www.haskell.org/ghc/docs/latest/html/libraries/stm/Control-Concurrent-STM.html">
+ Control.Concurrent.STM</ulink>. The main features supported are these:
+<itemizedlist>
+<listitem><para>Atomic blocks.</para></listitem>
+<listitem><para>Transactional variables.</para></listitem>
+<listitem><para>Operations for composing transactions:
+<literal>retry</literal>, and <literal>orElse</literal>.</para></listitem>
+<listitem><para>Data invariants.</para></listitem>
+</itemizedlist>
+All these features are described in the papers mentioned earlier.
+</para>
+</sect2>
- <para>The first possibility is to use concurrent threads to structure your
- program, and make sure
- that you spread computation amongst the threads. The runtime will
+<sect2><title>Parallel Haskell</title>
+
+ <para>GHC includes support for running Haskell programs in parallel
+ on symmetric, shared-memory multi-processor
+ (SMP)<indexterm><primary>SMP</primary></indexterm>.
+ By default GHC runs your program on one processor; if you
+ want it to run in parallel you must link your program
+ with the <option>-threaded</option>, and run it with the RTS
+ <option>-N</option> option; see <xref linkend="sec-using-smp" />).
+ The runtime will
schedule the running Haskell threads among the available OS
threads, running as many in parallel as you specified with the
<option>-N</option> RTS option.</para>
- </sect2>
+ <para>GHC only supports parallelism on a shared-memory multiprocessor.
+ Glasgow Parallel Haskell<indexterm><primary>Glasgow Parallel Haskell</primary></indexterm>
+ (GPH) supports running Parallel Haskell
+ programs on both clusters of machines, and single multiprocessors. GPH is
+ developed and distributed
+ separately from GHC (see <ulink url="http://www.cee.hw.ac.uk/~dsg/gph/">The
+ GPH Page</ulink>). However, the current version of GPH is based on a much older
+ version of GHC (4.06).</para>
+
+ </sect2>
<sect2>
<title>Annotating pure code for parallelism</title>
- <para>The simplest mechanism for extracting parallelism from pure code is
+ <para>Ordinary single-threaded Haskell programs will not benefit from
+ enabling SMP parallelism alone: you must expose parallelism to the
+ compiler.
+
+ One way to do so is forking threads using Concurrent Haskell (<xref
+ linkend="concurrent-haskell"/>), but the simplest mechanism for extracting parallelism from pure code is
to use the <literal>par</literal> combinator, which is closely related to (and often used
with) <literal>seq</literal>. Both of these are available from <ulink
url="../libraries/base/Control-Parallel.html"><literal>Control.Parallel</literal></ulink>:</para>
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