A global variable rarely used may deserve a long name, maxphysaddr say. An array index used on every line of a loop needn't be named any more elaborately than i . When the variable names are huge, it's harder to see what's going on. The problem gets worse fast with real examples. Indices are just notation, so treat them as such.
Pointers also require sensible notation.
More on this in the next essay. As in all other aspects of readable programming, consistency is important in naming. If you call one variable maxphysaddr , don't call its cousin lowestaddress . Globals, for instance, typically have little context when they are used, so their names need to be relatively evocative. This is largely a matter of taste, but taste is relevant to clarity.