Dependency based boot sequencing is the default in Debian.  The
insserv program is used by sysv-rc to order init.d scripts based on
their declared dependencies.

An experimental feature to run init.d scripts in parallel is also
available.  Use the CONCURRENCY setting in /etc/default/rcS to
enable it.

In short:

  # Enable parallel booting
  echo CONCURRENCY=makefile >> /etc/default/rcS

  # Ready to reboot
  shutdown -r now

The next boot should then start services in parallel, as early as
possible during the boot process based on the dependency information
provided.

The 'makefile' style concurrency depend on complete and correct
dependencies in all init.d scripts.  There is a simpler and less
strict style 'startpar', which only require all scripts with the same
sequence number to be able to run in parallel, and thus accept
incomplete dependencies as long as the boot sequence number is OK.

To monitor the boot sequence, the bootchart project is a good choice.
Debian packages are available in etch and sid.  The project itself is
available from <URL:http://www.bootchart.org/>.

Background info on alternative boot systems in Debian is available
from
<URL:http://alioth.debian.org/docman/view.php/30730/38/debconf2-initscripts-bkg.pdf>.

Information on the LSB initscript comment convention is available from
<URL:http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/iniscrptact.html>,
<URL:http://wiki.debian.org/LSBInitScripts> and
<URL:http://wiki.debian.org/LSBInitScripts/DependencyBasedBoot>.

Information on an alternative approach to parallelism during boot is
available from
<URL:http://www-128.ibm.com/developerworks/linux/library/l-boot.html>.

A tool to detect cycles is available from Adam Jensen using the git
repository <URL:http://linuxguy.org/git/initcycle.git>.