Herbert started his talk by pointing at all the bloat in our current OSes. Windows went from 6 MLOC in NT3.5 (that was in 1993) to 50 MLOC in XP (released in 2001). That means every year, the codebase grows by about 30%. Free OSes also saw dramatic growth: the Linux kernel was 10 kLOC in 1991, fattened to 13.5 MLOC in 2010; that's an average growth of +60% a year. Similar stats were shown for FreeBSD.

In addition, all software contains bugs. I seem to remember Herbert quoted a single-digit number of bugs per 10 kLOC as a baseline. In this respect, free systems, the BSDs in particular, did better than average (cheers in the audience). FreeBSD did particularly well (louder cheers from part of the audience).

Therefore, current OSes are too complex to be understood. A single kernel hacker can't pretend he understands all the aspects of his kernel. There are bugs lurking around, and the most we can do is limit their impact when we hit them.