documentation: BB_NUMBER_THREADS and PARALLEL_MAKE suggestions

Text that suggests setting the BB_NUMBER_THREADS and
PARALLEL_MAKE variables to speed build time has been updated to
suggest a setting of twice the number of host cores supported.

The 2x number came to light through a discussion on the project
mailing list initiated by Robert P. J. Day.

Reported-by: Robert P. J. Day
(From yocto-docs rev: 1846b41a595b7bd905ca828eaa04112d1533d7a1)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/dev-manual/dev-manual-bsp-appendix.xml

@@ -562,9 +562,10 @@
                 You should also be sure any other variables in which you are interested are set.  
                 Some variables to consider are <filename>BB_NUMBER_THREADS</filename>
                 and <filename>PARALLEL_MAKE</filename>, both of which can greatly reduce your build time 
-                if you are using a multi-threaded development system (e.g. values of 
-                <filename>8</filename> and <filename>j 6</filename>, respectively are optimal 
-                for a development machine that has four available cores).</para></listitem>
+                if your development system supports multiple cores.
+                For development systems that support multiple cores, a good rule of thumb is to set 
+                both the <filename>BB_NUMBER_THREADS</filename> and <filename>PARALLEL_MAKE</filename> 
+                variables to twice the number of cores your system supports.</para></listitem>
             <listitem><para>Update the <filename>bblayers.conf</filename> file so that it includes 
                 the path to your new BSP layer.  
                 In this example you need to include the pathname to <filename>meta-mymachine</filename>.

documentation/dev-manual/dev-manual-kernel-appendix.xml

@@ -221,9 +221,9 @@
                     If your host development system supports multi-core and multi-thread capabilities,
                     you can uncomment these statements and set the variables to significantly shorten
                     the full build time.
-                    As a guideline, set <filename>BB_NUMBER_THREADS</filename> to twice the number 
-                    of cores your machine supports and set <filename>PARALLEL_MAKE</filename> to one and 
-                    a half times the number of cores your machine supports.
+                    As a guideline, set both <filename>BB_NUMBER_THREADS</filename> and 
+                    <filename>PARALLEL_MAKE</filename> to twice the number 
+                    of cores your machine supports.
                 </note>
                 </para>
                 <para>
@@ -390,9 +390,8 @@
                        build time if your host supports multi-core and multi-thread capabilities:
                        <filename>BB_NUMBER_THREADS</filename> and <filename>PARALLEL_MAKE</filename>.
                        If the host system has multiple cores then you can optimize build time 
-                       by setting <filename>BB_NUMBER_THREADS</filename> to twice the number of 
-                       cores and setting <filename>PARALLEL_MAKE</filename> to one and a half times the 
-                       number of cores.</para></listitem>
+                       by setting both these variables to twice the number of 
+                       cores.</para></listitem>
                    <listitem><para><emphasis>Identify Your <filename>meta-kernel-dev</filename>
                        Layer:</emphasis> The <filename>BBLAYERS</filename> variable in the 
                        <filename>bblayers.conf</filename> file found in the 

documentation/poky-ref-manual/ref-variables.xml

@@ -927,7 +927,7 @@
                     This variable is usually in the form <filename>-j 4</filename>, where the number
                     represents the maximum number of parallel threads make can run.
                     If you development host supports multiple cores a good rule of thumb is to set 
-                    this variable to one and a half times the number of cores on the host.</para>
+                    this variable to twice the number of cores on the host.</para>
             </glossdef>
         </glossentry>
 

documentation/yocto-project-qs/yocto-project-qs.xml

@@ -410,11 +410,8 @@
              <ulink url='http://www.yoctoproject.org/docs/latest/poky-ref-manual/poky-ref-manual.html#var-PARALLEL_MAKE'><filename>PARALLEL_MAKE</filename></ulink> variables.
              By default, these variables are commented out. 
              However, if you have a multi-core CPU you might want to uncomment
-             the lines and set the variable
-             <filename>BB_NUMBER_THREADS</filename> equal to twice the number of your 
+             the lines and set both variables equal to twice the number of your 
              host's processor cores.
-             Also, you could set the variable <filename>PARALLEL_MAKE</filename> equal to 
-             1.5 times the number of processor cores.
              Setting these variables can significantly shorten your build time.
          </para>