Previously, the Linux kernel had by default a periodic interrupt that made the kernel scheduler to balance and schedule threads running among the CPU's, this interrupt is known as the timer tick and it was generated at a fix rate 1000 HZ. This mode enabled by option CONFIG_HZ_PERIODIC. The kernel serviced this interrupt regardless of the power state of the CPU.

NO_HZ_IDLE allows the CPU not to be disturbed when idle and can save power because it just wakes up the CPU when needed to service the timer.

Full dynticks system (tickless), or NO_HZ_FULL, is a configuration option in the kernel that allows it to run without a periodic timer tick at all.

• CPU Isolation – Full dynticks internals