A conversation with Linus at LinuxCon Japan

If people would prefer some concrete metric as the deciding point for the switch for 3.x -> 4.0 ... how about "When either a third of the code present in 3.0 has been replaced, or the total lines of code doubles - whichever comes first".

That way you have a clear reason why you want to start the 4.x series - either because there is not much 3.0 left, or because the additions dwarf the original.

Here's an out-of-date graph that shows how total code size grew from 2.6.12 to 2.6.37 ... while code from each release was gradually nibbled away by later releases:
http://www.kernel.org/pub/linux/kernel/people/aegl/codede...