Added a note to address parallel make procedures

git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@9099 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689

chapter01/changelog.xml

@@ -40,6 +40,11 @@
     <listitem>
       <para>2009-11-12</para>
       <itemizedlist>
+        <listitem>
+          <para>[bdubbs] Added a note to 'About SBUs' to address
+          parallel make procedures and how SBU values will be
+          affected.</para>
+        </listitem>
         <listitem>
           <para>[bdubbs] Minor changes to GRUB-1.9.1 instructions.</para>
         </listitem>

chapter04/aboutsbus.xml

@@ -32,14 +32,33 @@
   Fortunately, most build times are shorter than the one for Binutils.</para>
 
   <para>In general, SBUs are not entirely accurate because they depend on many
-  factors, including the host system's version of GCC. Note that on Symmetric
-  Multi-Processor (SMP)-based machines, SBUs are even less accurate. They are
-  provided here to give an estimate of how long it might take to install a
-  package, but the numbers can vary by as much as dozens of minutes in some
-  cases.</para>
+  factors, including the host system's version of GCC.  They are provided here
+  to give an estimate of how long it might take to install a package, but the
+  numbers can vary by as much as dozens of minutes in some cases.</para>
 
   <para>To view actual timings for a number of specific machines, we recommend
   The LinuxFromScratch SBU Home Page at <ulink url="&lfs-root;~sbu/"/>.</para>
 
+  <note><para>For many modern systems with multiple processors (or cores) the
+  compilation time for a package can be reduced by performing a "parallel make"
+  by either setting an environment variable or telling the
+  <command>make</command> program how many processors are available.  For
+  instance, a Core2Duo can support two simultaneous processes with:</para>
+
+  <screen><userinput>set MAKEFLAGS='-j 2'</userinput></screen>
+
+  <para>or just building with:</para>
+
+  <screen><userinput>make -j2</userinput></screen>
+
+  <para>When multiple processors are used in this way, the SBU units in the
+  book will vary even more than they normally would.  Analyzing the output of
+  the build process will also be more difficult because the lines of different
+  processes will be interleaved.  If you run into a problem with a build step,
+  revert back to a single processor build to properly analyze the error
+  messages.</para>
+
+  </note>
+
 </sect1>