I would assume that most large fleets have background periodic tasks that perform basic self-checks (the fleets I know about certainly do). Using "idle" cycles

I would assume that most large fleets have background periodic tasks that perform basic self-checks (the fleets I know about certainly do). Using "idle" cycles is not a great idea though: - They may seem "free", but in fact you would end up using more power: CPU turns itself off during idle time, and you'd replace that with an intensive process. Power (and, as a consequence, cooling) is one of the main costs of a data center. - Machines that are properly utilized (close to 100% CPU utilization) would get less coverage, and those are the ones that need it the most. So it is better to allocate a certain percentage of your CPU budget to self checks, based on risk and sensitivity of the tests. And have some easy way to put a machine under stress testing if it is suspected of having rare memory or CPU errors.