Faster booting with Upstart
Loading the Linux kernel takes up just a fraction of the time spent waiting for the login prompt during booting. The system spends most of its time sitting waiting for init (which has its origins in Unix System V) to cycle through the various runlevels, during which it runs innumerable sequences of init scripts. In Ubuntu and Fedora, Upstart has long replaced the traditional SysV init.
The current boot sequence, with services starting consecutively in a fixed sequence, remains unaltered simply because no-one has sat down and adapted the init scripts for these various services to the capabilities of Upstart. Upstart simply emulates the SysV init runlevels (which actually no longer exist on systems running Upstart) and continues to call the old init scripts. For Ubuntu 9.10, the development team have finally started to convert some services to Upstart.
Both Upstart and SysV init are the first processes to be launched by the kernel (with ID 1) as soon as the latter has booted and any boot scripts from the initial ramdisk (initrd) have been run. For SysV init, the lynchpin of system initialisation is the /etc/inittab file. This is where SysV init finds the default runlevel, the name of the first initialisation script and the commands for initialising each runlevel. During runlevel initialisation, the linked init scripts in the relevant runlevel directory (e.g. /etc/rc5.d) are run sequentially.
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