I have an Ubuntu 11.04 installation. The /home directory is on a partition
which uses the ext4 filesystem. I haven't changed the default configuration so
the filesystem is mounted at boot time with the defaults mount option. man mount explains which mount options the option defaults implies:
rw, suid, dev, exec, auto, nouser, and async.

I noticed that the access time (atime) of a file isn't updated every time I
access a file (for example with an editor or cat).

I looked around in the man page of mount and found the following option:

• relatime: Update inode access times relative to modify or change time. Access time is only updated if the previous access time was earlier than the current modify or change time.

This is exactly the behaviour which I observe on my system. But the relatime
mount option is not enabled.

Is the behaviour described in the description of the relatime mount option the
default behaviour of atime? Does this apply to all filesystems which support
atime?