"Ten years ago, most computer users were young people or professionals with lots of technical expertise. When things went wrong – which they often did – they knew how to fix things. Nowadays, the average user is far less sophisticated, perhaps a 12-year-old girl or a grandfather. Most of them know about as much about fixing computer problems as the average computer nerd knows about repairing his car. What they want more than anything else is a computer that works all the time, with no glitches and no failures. Many users automatically compare their computer to their television set. Both are full of magical electronics and have big screens. Most users have an implicit model of a television set: (1) you buy the set; (2) you plug it in; (3) it works perfectly without any failures of any kind for the next 10 years. They expect that from the computer, and when they do not get it, they get frustrated. When computer experts tell them: "If God had wanted computers to work all the time, He wouldn't have invented ‘Reset’ buttons" they are not impressed.

For lack of a better definition of dependability, let us adopt this one: A device is said to be dependable if 99% of the users never experience any failures during the entire period they own the device. By this definition, virtually no computers are dependable, whereas most TVs, iPods, digital cameras, camcorders, etc. are. Techies are willing to forgive a computer that crashes once or twice a year; ordinary users are not. Home users aren't the only ones annoyed by the poor dependability of computers. Even in highly technical settings, the low dependability of computers is a problem. Companies like Google and Amazon, with hundreds of thousands of servers, experience many failures every day. They have learned to live with this, but they would really prefer systems that just worked all the time. Unfortunately, current software fails them.

The basic problem is that software contains bugs, and the more software there is, the more bugs there are. Various studies have shown that the number of bugs per thousand lines of code (KLoC) varies from 1 to 10 in large production systems. A really well-written piece of software might have 2 bugs per KLoC over time, but not fewer. An operating system with, say, 4 million lines of code is thus likely to have at least 8000 bugs. Not all are fatal, but some will be. A study at Stanford University showed that device drivers – which make up 70% of the code base of a typical operating system – have bug rates 3x to 7x higher than the rest of the system. Device drivers have higher bug rates because (1) they are more complicated and (2) they are inspected less. While many people study the scheduler, few look at printer drivers.

The Solution: Smaller Kernels

The solution to this problem is to move code out of the kernel, where it can do maximal damage, and put it into user-space processes, where bugs cannot cause system crashes. This is how Minix 3 is designed."

From http://www.linux-magazine.com/Issues/2009/99/Minix-3