source: chapter04/aboutsbus.xml@ b2f01e8

xry111/clfs-ng
Last change on this file since b2f01e8 was b2f01e8, checked in by Xi Ruoyao <xry111@…>, 9 months ago

cross-ng: explain the unchanged SBU values from trunk

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1<?xml version="1.0" encoding="ISO-8859-1"?>
2<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
3 "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
4 <!ENTITY % general-entities SYSTEM "../general.ent">
5 %general-entities;
6]>
7
8<sect1 id="ch-preps-aboutsbus">
9 <?dbhtml filename="aboutsbus.html"?>
10
11 <title>About SBUs</title>
12
13 <para>Many people would like to know beforehand approximately how long
14 it takes to compile and install each package. Because Linux From
15 Scratch can be built on many different systems, it is impossible to
16 provide accurate time estimates. The biggest package (Glibc) will
17 take approximately 20 minutes on the fastest systems, but could take
18 up to three days on slower systems! Instead of providing actual times,
19 the Standard Build Unit (SBU) measure will be
20 used instead.</para>
21
22 <para>The SBU measure works as follows. The first package to be compiled
23 from this book is binutils in <xref linkend="chapter-cross-tools"/>. The
24 time it takes to compile this package is what will be referred to as the
25 Standard Build Unit or SBU. All other compile times will be expressed relative
26 to this time.</para>
27
28 <para>For example, consider a package whose compilation time is 4.5
29 SBUs. This means that if a system took 10 minutes to compile and
30 install the first pass of binutils, it will take
31 <emphasis>approximately</emphasis> 45 minutes to build this example package.
32 Fortunately, most build times are shorter than the one for binutils.</para>
33
34 <para>In general, SBUs are not entirely accurate because they depend on many
35 factors, including the host system's version of GCC. They are provided here
36 to give an estimate of how long it might take to install a package, but the
37 numbers can vary by as much as dozens of minutes in some cases.</para>
38
39 <note>
40 <para>In the cross edition, the SBUs are kept same as the original LFS
41 book. They should only be considered as a reference. It obviously does
42 not make sense to predict the building time in Chapter 7 or 8 with
43 <quote>1 SBU</quote> measured in Chapter 5, as the target machine and
44 the machine running the host distro may be completely different.</para>
45 </note>
46
47 <note>
48 <para>For many modern systems with multiple processors (or cores) the
49 compilation time for a package can be reduced by performing a "parallel
50 make" by either setting an environment variable or telling the
51 <command>make</command> program how many processors are available. For
52 instance, an Intel i5-6500 CPU can support four simultaneous processes with:</para>
53
54 <screen role="nodump"><userinput>export MAKEFLAGS='-j4'</userinput></screen>
55
56 <para>or just building with:</para>
57
58 <screen role="nodump"><userinput>make -j4</userinput></screen>
59
60 <para>When multiple processors are used in this way, the SBU units in the
61 book will vary even more than they normally would. In some cases, the make
62 step will simply fail. Analyzing the output of the build process will also
63 be more difficult because the lines of different processes will be
64 interleaved. If you run into a problem with a build step, revert back to a
65 single processor build to properly analyze the error messages.</para>
66 </note>
67
68</sect1>
69
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