lfs-book-zh_CN/chapter04/aboutsbus.xml

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<sect1 id="prepare-aboutsbus">
  <?dbhtml filename="aboutsbus.html"?>

  <title>About SBUs</title>

  <para>Many people would like to know beforehand approximately how long
  it takes to compile and install each package. Because Linux From
  Scratch can be built on many different systems, it is impossible to
  provide accurate time estimates. The biggest package (Glibc) will
  take approximately 20 minutes on the fastest systems, but could take
  up to three days on slower systems! Instead of providing actual times,
  the Standard Build Unit (SBU) measure will be
  used instead.</para>

  <para>The SBU measure works as follows. The first package to be compiled
  from this book is Binutils in <xref linkend="chapter-temporary-tools"/>. The
  time it takes to compile this package is what will be referred to as the
  Standard Build Unit or SBU. All other compile times will be expressed relative
  to this time.</para>

  <para>For example, consider a package whose compilation time is 4.5
  SBUs. This means that if a system took 10 minutes to compile and
  install the first pass of Binutils, it will take
  <emphasis>approximately</emphasis> 45 minutes to build this example package.
  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.  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>export 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>