Minor documentation bug, odd defn of fibn.
On this page: http://haskell.org/ghc/docs/6.6/html/users_guide/lang-parallel.html the function fibn is defined. I assume fibn is meant to give the fibonacci sequence, but it doesn't. The problem is the term 'n1 + n2 + 1' in the function definition. It should be just 'n1 + n2'. This change needs to be made in two places on this page, and additionally, an occurence of 'n2 + n1 + 1' needs to be changed to 'n2 + n1'.
In case it is helpful, here is the corrected html.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>7.15. Parallel Haskell</title><link rel="stylesheet" href="lang-parallel_files/fptools.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.68.1"><link rel="start" href="http://haskell.org/ghc/docs/6.6/html/users_guide/index.html" title="The Glorious Glasgow Haskell Compilation System User's Guide, Version 6.6"><link rel="up" href="http://haskell.org/ghc/docs/6.6/html/users_guide/ghc-language-features.html" title="Chapter 7. GHC Language Features"><link rel="prev" href="http://haskell.org/ghc/docs/6.6/html/users_guide/monomorphism.html" title="7.14. Control over monomorphism"><link rel="next" href="http://haskell.org/ghc/docs/6.6/html/users_guide/ffi.html" title="Chapter 8. Foreign function interface (FFI) "></head> <body alink="#0ff" bgcolor="white" link="#0ff" text="black" vlink="#840084"><div class="navheader"><table summary="Navigation header" width="100%"><tbody><tr><th colspan="3" align="center">7.15. Parallel Haskell</th></tr><tr><td align="left" width="20%"><a accesskey="p" href="http://haskell.org/ghc/docs/6.6/html/users_guide/monomorphism.html">Prev</a> </td><th align="center" width="60%">Chapter 7. GHC Language Features</th><td align="right" width="20%"> <a accesskey="n" href="http://haskell.org/ghc/docs/6.6/html/users_guide/ffi.html">Next</a></td></tr></tbody></table><hr></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both;"><a name="lang-parallel"></a>7.15. Parallel Haskell</h2></div></div></div><a class="indexterm" name="id3181622"></a><p>There are two implementations of Parallel Haskell: SMP paralellism
<a class="indexterm" name="id3181636"></a>
which is built-in to GHC (see <a href="http://haskell.org/ghc/docs/6.6/html/users_guide/sec-using-smp.html" title="4.12. Using SMP parallelism">Section 4.12, “Using SMP parallelism”</a>) and
supports running Parallel Haskell programs on a single multiprocessor
machine, and
Glasgow Parallel Haskell<a class="indexterm" name="id3181653"></a>
(GPH) which supports running Parallel Haskell
programs on both clusters of machines or single multiprocessors. GPH is
developed and distributed
separately from GHC (see <a href="http://www.cee.hw.ac.uk/%7Edsg/gph/" target="_top">The
GPH Page</a>).</p><p>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.</p><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id3181681"></a>7.15.1. Running Concurrent Haskell programs in parallel</h3></div></div></div><p>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
schedule the running Haskell threads among the available OS
threads, running as many in parallel as you specified with the
<code class="option">-N</code> RTS option.</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id3181704"></a>7.15.2. Annotating pure code for parallelism</h3></div></div></div><p>The simplest mechanism for extracting parallelism from pure code is
to use the <code class="literal">par</code> combinator, which is closely related to (and often used
with) <code class="literal">seq</code>. Both of these are available from <a href="http://haskell.org/ghc/docs/6.6/html/libraries/base/Control-Parallel.html" target="_top"><code class="literal">Control.Parallel</code></a>:</p><pre class="programlisting">infixr 0
par
infixr 1 seq
par :: a -> b -> b
seq :: a -> b -> b</pre><p>The expression <code class="literal">(x par y)</code>
<span class="emphasis"><em>sparks</em></span> the evaluation of <code class="literal">x</code>
(to weak head normal form) and returns <code class="literal">y</code>. Sparks are
queued for execution in FIFO order, but are not executed immediately. If
the runtime detects that there is an idle CPU, then it may convert a
spark into a real thread, and run the new thread on the idle CPU. In
this way the available parallelism is spread amongst the real
CPUs.</p><p>For example, consider the following parallel version of our old
nemesis, <code class="function">nfib</code>:</p><pre class="programlisting">import Control.Parallel
nfib :: Int -> Int nfib n | n <= 1 = 1
| otherwise = par n1 (seq n2 (n1 + n2))
where n1 = nfib (n-1)
n2 = nfib (n-2)</pre><p>For values of <code class="varname">n</code> greater than 1, we use
<code class="function">par</code> to spark a thread to evaluate <code class="literal">nfib (n-1)</code>,
and then we use <code class="function">seq</code> to force the
parent thread to evaluate <code class="literal">nfib (n-2)</code> 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
<code class="function">seq</code> to ensure that the parent will evaluate
<code class="varname">n2</code> <span class="emphasis"><em>before</em></span> <code class="varname">n1</code>
in the expression <code class="literal">(n1 + n2)</code>. It is not sufficient
to reorder the expression as <code class="literal">(n2 + n1)</code>, because
the compiler may not generate code to evaluate the addends from left to
right.</p><p>When using <code class="literal">par</code>, 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
the cost of forking it in parallel will be too large relative to the
amount of parallelism gained. Getting these factors right is tricky in
practice.</p><p>More sophisticated combinators for expressing parallelism are
available from the <a href="http://haskell.org/ghc/docs/6.6/html/libraries/base/Control-Parallel-Strategies.html" target="_top"><code class="literal">Control.Parallel.Strategies</code></a> module.
This module builds functionality around <code class="literal">par</code>,
expressing more elaborate patterns of parallel computation, such as
parallel <code class="literal">map</code>.</p></div></div><div class="navfooter"><hr><table summary="Navigation footer" width="100%"><tbody><tr><td align="left" width="40%"><a accesskey="p" href="http://haskell.org/ghc/docs/6.6/html/users_guide/monomorphism.html">Prev</a> </td><td align="center" width="20%"><a accesskey="u" href="http://haskell.org/ghc/docs/6.6/html/users_guide/ghc-language-features.html">Up</a></td><td align="right" width="40%"> <a accesskey="n" href="http://haskell.org/ghc/docs/6.6/html/users_guide/ffi.html">Next</a></td></tr><tr><td align="left" valign="top" width="40%">7.14. Control over monomorphism </td><td align="center" width="20%"><a accesskey="h" href="http://haskell.org/ghc/docs/6.6/html/users_guide/index.html">Home</a></td><td align="right" valign="top" width="40%"> Chapter 8.
Foreign function interface (FFI) </td></tr></tbody></table></div></body></html>