Now that I've looked at readahead too I can say that that and preload
have different and orthogonal purposes.
readahead loads files into linux's file cache during a boot.
preload is a daemon that examines programs when they are run and
determines what libraries they link dynamically and files they open,
typically, and loads those into the file cache when the program is run
before they are needed.
In fact, upstream's benchmarks have determined that using preload
during the boot actually slows it down. There are speedups in
starting heavy applications like openoffice or firefox.