- <para>Link the program with the "threaded" runtime system.
- This version of the runtime is designed to be used in
- programs that use multiple operating-system threads. It
- supports calls to foreign-exported functions from multiple
- OS threads. Calls to foreign functions are made using the
- same OS thread that created the Haskell thread (if it was
- created by a call-in), or an arbitrary OS thread otherwise
- (if the Haskell thread was created by
+ <para>Link the program with the "threaded" version of the
+ runtime system. The threaded runtime system is so-called
+ because it manages multiple OS threads, as opposed to the
+ default runtime system which is purely
+ single-threaded.</para>
+
+ <para>Note that you do <emphasis>not</emphasis> need
+ <option>-threaded</option> in order to use concurrency; the
+ single-threaded runtime supports concurrency between Haskell
+ threads just fine.</para>
+
+ <para>The threaded runtime system provides the following
+ benefits:</para>
+
+ <itemizedlist>
+ <listitem>
+ <para>Parallelism<indexterm><primary>parallelism</primary></indexterm> on a multiprocessor<indexterm><primary>multiprocessor</primary></indexterm><indexterm><primary>SMP</primary></indexterm> or multicore<indexterm><primary>multicore</primary></indexterm>
+ machine. See <xref linkend="using-smp" />.</para>
+
+ <para>The ability to make a foreign call that does not
+ block all other Haskell threads.</para>
+
+ <para>The ability to invoke foreign exported Haskell
+ functions from multiple OS threads.</para>
+ </listitem>
+ </itemizedlist>
+
+ <para>With <option>-threaded</option>, calls to foreign
+ functions are made using the same OS thread that created the
+ Haskell thread (if it was created by a call to a foreign
+ exported Haskell function), or an arbitrary OS thread
+ otherwise (if the Haskell thread was created by