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4.5. About SBUs

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 Static Build Unit (SBU) measure will be used instead.

The SBU measure works as follows. The first package to be compiled from this book is the statically-linked Binutils in Chapter 5. The time it takes to compile this package is what will be referred to as the Static Build Unit or SBU. All other compile times will be expressed relative to this time.

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 static Binutils, it will take approximately 45 minutes to build this example package. Fortunately, most build times are shorter than the one for Binutils.

Please note that if the system compiler on the host is GCC-2.x based, the SBUs listed may be somewhat understated. This is because the SBU is based on the very first package, compiled with the old GCC, while the rest of the system is compiled with the newer GCC-3.4.1 (which is known to be approximately 30 percent slower). SBUs are also not highly accurate for Symmetric Multi-Processor (SMP)-based machines.

To view actual timings for a number of specific machines, we recommend http://www.linuxfromscratch.org/~bdubbs/.

In general, SBUs are not very accurate because they depend on many factors, not just the GCC version. 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.