Jiffies

Jiffies is a term used in many Unix-like kernels to denote the global timer tick counter that measures system time in discrete units. The kernel maintains a variable, commonly called jiffies, which increments on each timer interrupt. The rate at which it increments is determined by the system’s timer frequency, denoted by HZ. Jiffies serve as the basic time unit for short durations and form the foundation for scheduling decisions, timeouts, and other time-sensitive kernel logic.

In Linux and similar kernels, jiffies is typically an unsigned long. Because it is a counter that advances with every tick, it eventually overflows and wraps around. The wrap time is approximately 2^32 / HZ seconds; for example, with HZ = 1000, jiffies wraps roughly every 49.7 days, while with HZ = 100 it would wrap roughly every 497 days. Some kernels provide a 64-bit counter variant (jiffies_64) to minimize or eliminate wrap concerns on longer-running systems.

Jiffies are used to calculate elapsed time by taking the difference between two jiffy values and converting the result using HZ, or via helper macros such as jiffies_to_msecs() and similar utilities. They underpin short-term scheduling decisions, timers, and delay loops within the kernel. However, direct reading of jiffies is a kernel-internal operation; user-space applications typically rely on high-level time APIs or system clocks rather than reading jiffies directly.

Related concepts include the HZ value (the timer interrupt frequency), tickless kernel configurations, and higher-resolution timekeeping mechanisms such as ktime and CLOCK_MONOTONIC in user space.