</indexterm>
<para>GHC implements some major extensions to Haskell to support
- concurrent and parallel programming. Let us first etablish terminology:
+ concurrent and parallel programming. Let us first establish terminology:
<itemizedlist>
<listitem><para><emphasis>Parallelism</emphasis> means running
a Haskell program on multiple processors, with the goal of improving
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
+ 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">
+ url="../libraries/base/Control-Concurrent.html">
Control.Concurrent</ulink>. The functions exported by this
library include:
<itemizedlist>
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">
+ url="../libraries/stm/Control-Concurrent-STM.html">
Control.Concurrent.STM</ulink>. The main features supported are these:
<itemizedlist>
<listitem><para>Atomic blocks.</para></listitem>
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" />).
+ <option>-N</option> option; see <xref linkend="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
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>
+ url="../libraries/parallel/Control-Parallel.html"><literal>Control.Parallel</literal></ulink>:</para>
<programlisting>
infixr 0 `par`
-infixr 1 `seq`
+infixr 1 `pseq`
-par :: a -> b -> b
-seq :: a -> b -> b</programlisting>
+par :: a -> b -> b
+pseq :: a -> b -> b</programlisting>
<para>The expression <literal>(x `par` y)</literal>
<emphasis>sparks</emphasis> the evaluation of <literal>x</literal>
nfib :: Int -> Int
nfib n | n <= 1 = 1
- | otherwise = par n1 (seq n2 (n1 + n2 + 1))
+ | otherwise = par n1 (pseq n2 (n1 + n2 + 1))
where n1 = nfib (n-1)
n2 = nfib (n-2)</programlisting>
<para>For values of <varname>n</varname> greater than 1, we use
<function>par</function> to spark a thread to evaluate <literal>nfib (n-1)</literal>,
- and then we use <function>seq</function> to force the
+ and then we use <function>pseq</function> to force the
parent thread to evaluate <literal>nfib (n-2)</literal> before going on
to add together these two subexpressions. In this divide-and-conquer
approach, we only spark a new thread for one branch of the computation
(leaving the parent to evaluate the other branch). Also, we must use
- <function>seq</function> to ensure that the parent will evaluate
+ <function>pseq</function> to ensure that the parent will evaluate
<varname>n2</varname> <emphasis>before</emphasis> <varname>n1</varname>
in the expression <literal>(n1 + n2 + 1)</literal>. It is not sufficient
to reorder the expression as <literal>(n2 + n1 + 1)</literal>, because
the compiler may not generate code to evaluate the addends from left to
right.</para>
+ <para>
+ Note that we use <literal>pseq</literal> rather
+ than <literal>seq</literal>. The two are almost equivalent, but
+ differ in their runtime behaviour in a subtle
+ way: <literal>seq</literal> can evaluate its arguments in either
+ order, but <literal>pseq</literal> is required to evaluate its
+ first argument before its second, which makes it more suitable
+ for controlling the evaluation order in conjunction
+ with <literal>par</literal>.
+ </para>
+
<para>When using <literal>par</literal>, the general rule of thumb is that
the sparked computation should be required at a later time, but not too
soon. Also, the sparked computation should not be too small, otherwise
amount of parallelism gained. Getting these factors right is tricky in
practice.</para>
+ <para>It is possible to glean a little information about how
+ well <literal>par</literal> is working from the runtime
+ statistics; see <xref linkend="rts-options-gc" />.</para>
+
<para>More sophisticated combinators for expressing parallelism are
available from the <ulink
- url="../libraries/base/Control-Parallel-Strategies.html"><literal>Control.Parallel.Strategies</literal></ulink> module.
+ url="../libraries/parallel/Control-Parallel-Strategies.html"><literal>Control.Parallel.Strategies</literal></ulink> module.
This module builds functionality around <literal>par</literal>,
expressing more elaborate patterns of parallel computation, such as
parallel <literal>map</literal>.</para>
</sect2>
+<sect2><title>Data Parallel Haskell</title>
+ <para>GHC includes experimental support for Data Parallel Haskell (DPH). This code
+ is highly unstable and is only provided as a technology preview. More
+ information can be found on the corresponding <ulink
+ url="http://www.haskell.org/haskellwiki/GHC/Data_Parallel_Haskell">DPH
+ wiki page</ulink>.</para>
+</sect2>
+
</sect1>
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