Understanding /dev

If you wish to get every possible millisecond out of your system, there are a few things you should know about directories in UNIX file systems. (Though technically I'm talking about the implementation of the EXT2 file system, the points apply to almost all UNIX file systems.)

First note that though the command ls always displays the directory in a sorted order, directories are not stored with their entries sorted. Instead, each new entry is placed as close to the front of the directory as possible.

To find a file in a directory, the file name is compared with the first file in the directory. If it doesn't match, the second name is checked, and so on until either a match is found, or the end of the directory is reached. Consequently, if you have a large number of files in a directory and are frequently opening the last file in the directory, your CPU is doing a lot of comparisons. If, however, you could control the order of the entries, so as to place the most-frequently-used entries at the beginning of the directory, this would not be such an issue.

If you want to redo the internal ordering of the entries in /dev, boot from a floppy and then mount your primary file system. If your regular file system is mounted as /mnt, your regular device directory is /mnt/dev. Create a new directory called /mnt/dev2. Now you can move device files from /mnt/dev to /mnt/dev2. You will probably want to start with /mnt/dev/zero and /mnt/dev/null, as these two are opened far more frequently than any other devices.

If that sounds like a hassle, then don't bother with it. You probably won't notice any difference unless you are running on an old 386. Furthermore, the new directory cache under development in the 2.1.x kernel series will most likely make this a non-issue.

Serial Port Devices