readahead changes

[Lean Fuglsang]

> 
> http://people.ubuntu.com/~scott/bootcharts/edgy-20060920-21.png
> 
> 
> Scott

Looks really nice!

What we need now is a way to reorder blocks on the disk for services, and
let a service start at the moment the have all the blocks and all
dependencies satisfied.
If the blocks are reordered we should get full troughput in 7s-20s zone -
which should cut that down to 1-3 seconds (if disk is able to sustain
31MB/s all the time, and the throughput graph is right). There is no
dependency graph in the picture, but there should be a couple of those
services from 20s-23s that can be started earlier, and maybe also some of
them from 25-45. So maybe 2 seconds all in all.
Then the only thing left is that weird modprobe, that blocks everything.
It must be possible to start a lot of services in parallel? Or maybe it
can split out it services, like graphic card, sound, net, disk etc. So
alsa can start as soon as the soundcard is ready.
So all in all it should be possible to get your boot time down to under 30
seconds, _without_ optimising any services (except a better modprobe).

So start by getting those blocks juggled - there is a huge perfomance
boost by getting 31MB/S sustained.