Pre Linking

Description
Pre-Linking is a mechanism for linking programs to shared libraries ahead of time. In general, every time an application is run it must have its external symbols resolved - looked up in the shared library symbol table, and fixed up in the program binary to refer to the correct offsets in the library. To use prelinking, a special utility is run which does this resolution and fixup once for the program. This saves the cost of linking at runtime.

There is an existing package from RedHat which provides this feature.

A drawback of this is that if the shared library is changed, the fixups are no longer correct, and the program must be fixed-up again. This is much less of an issue in an embedded situation, where the programs and libraries are less likely to change than in a desktop or server Linux system.

Overview of linking
There is an excellent paper with an overview of dynamic linking issues at: Pre Linking Overview This paper describes not only pre-linking, but lazy linking and more exotic systems, like compile-on-load.

Expected Improvement
[This is not measured yet.]

We expect that with use of prelinking, there will be a slight reduction in boot time for Linux system, in the area of initial application loading.

We need to use this system and measure the effect of prelinking for a determined set of applications.

RedHat prelinking system

 * The prelink package is at: http://people.redhat.com/jakub/prelink/
 * A white paper is at: prelink

prelink currently supports the following architectures: alpha, arm, cris, i386, ia64, ppc32, ppc64, s390, sh, sparc32, sparc64, x86_64. At present the glibc dynamic linker is required to prelink executables and load prelinked code, uClibc does not support it.

Instructions for using prelinking with Gentoo
The following page has information on how to use prelinking with a Gentoo system:

http://www.gentoo.org/doc/en/prelink-howto.xml

Related Projects
http://mail-index.netbsd.org/tech-userlevel/2002/12/01/0000.html http://www.shiningsilence.com/dbsdlog/2004/01/20/215.html
 * Prebinding (RelCache) - RelCache (aka ELF prebinding) news    http://mail-index.netbsd.org/tech-userlevel/2002/12/04/0017.html
 * RelCache vs. Red Hat prelink
 * Resident - Resident Good (comparisons with prebind)

Specifications
None so far.

Patches
No kernel patches required for kernels 2.4.10 and later.

Case 1 - Panasonic mobile phone prelink
Panasonic used pre-linking on their Linux-based mobile phones. These used a 2.4.x Linux kernel, for an ARM processor. Measuring the time to load a single multimedia application with regular dynamic linking and pre-linking, showed that pre-linking could save a lot of time.
 * Hardware : ARM9 (unspecified CPU frequency)
 * Kernel Version : 2.4.20 (based on Monta Vista Linux CEE 3.1), glibc 2.3
 * Time without change : 2479 ms
 * Time with change : 125 ms
 * Source : page 19 of Making Mobile Phone with CE Linux

Future Work
This item is a work-in-progress, and we are just getting started.

ARM Prelink

 * Japan Jamboree #3
 * http://tree.celinuxforum.org/CelfPubWiki/JapanTechnicalJamboree3#head-1515fb2d64cd91370e9cb2f6ad4847483e729cf3 In the presentation of "Making Mobile Phone with CE Linux", the evaluation of Prelink on ARM architecture was mentioned.
 * by Mr. Mizuyama (Panasonic Mobile)

MIPS Prelink

 * Japan Jamboree #13
 * http://tree.celinuxforum.org/CelfPubWiki/JapanTechnicalJamboree13#head-ab59e6354d343ec0a804b5f440d35b5dcc27304c
 * Evaluation report by Mr. Yagi (Mitsubishi)