"Boot Camp Part I explained how Linux boots, using the old "SysV
init" system of scripts. But some modern Linux distros have been
gradually migrating to a newer model, called Upstart.
"Upstart has been around since 2006, but it's only in the last
year or so that it's taken a major role in booting distributions
like Ubuntu and Fedora. Debian and OpenSuSE are reportedly joining
in soon, while it's available as an optional component on most
other distros. No distro uses it as the sole boot method yet: even
Fedora 12 and the upcoming Ubuntu 10.04 keep a lot of functionality
in SysV scripts.
"An event-based model
"The normal SysV boot process is synchronous -- meaning things
happen one at a time, one after the other. First you run
S10sysklogd, and only when that's finished you can start running
S11klogd. If anything in the boot process takes a long time,
everything else has to wait.
"Upstart, in contrast, is event based. An "event" can be
something like "booting" ... or it can be a lot more specific, like
"the network is ready to use now". You can specify which scripts
depend on which events. Anything that isn't waiting for an event
can run whenever there's CPU available.