Add "+RTS -N" to determine the -N value automatically (see #1741)

[ghc-hetmet.git] / docs / users_guide / using.xml

diff --git a/docs/users_guide/using.xml b/docs/users_guide/using.xml
index 20bb687..f769d28 100644 (file)
--- a/docs/users_guide/using.xml
+++ b/docs/users_guide/using.xml
@@ -1834,7 +1834,7 @@ f "2"    = 2
 
       <variablelist>
        <varlistentry>
-         <term><option>-N<replaceable>x</replaceable></option></term>
+         <term><option>-N<optional><replaceable>x</replaceable></optional></option></term>
          <listitem>
            <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
              Use <replaceable>x</replaceable> simultaneous threads when
@@ -1846,9 +1846,19 @@ f "2"    = 2
              on a dual-core machine we would probably use
              <literal>+RTS -N2 -RTS</literal>.</para>
            
+            <para>Omitting <replaceable>x</replaceable>,
+              i.e. <literal>+RTS -N -RTS</literal>, lets the runtime
+              choose the value of <replaceable>x</replaceable> itself
+              based on how many processors are in your machine.</para>
+
+            <para>Be careful when using all the processors in your
+              machine: if some of your processors are in use by other
+              programs, this can actually harm performance rather than
+              improve it.</para>
+
             <para>Setting <option>-N</option> also has the effect of
-             setting <option>-g</option> (the number of OS threads to
-             use for garbage collection) to the same value.</para>
+              enabling the parallel garbage collector (see
+              <xref linkend="rts-options-gc" />).</para>
 
             <para>There is no means (currently) by which this value
              may vary after the program has started.</para>