jiffies64

Jiffies64 is a 64-bit counter used by the Linux kernel to track the number of timer ticks since system boot. It serves as a long-running time base for the kernel’s timekeeping and scheduling subsystems, providing a monotonic measure of elapsed time that can be used for timeouts, wakeups, and delayed work.

The conventional jiffies counter is a 32-bit value on many architectures, which can wrap around after a relatively short uptime depending on the timer frequency (HZ). Jiffies64 extends the same concept to 64 bits, greatly reducing wrap concerns and allowing longer uptimes to be measured directly. In kernels that support it, a safe 64-bit read interface, such as get_jiffies_64(), is provided to obtain a stable value even on platforms where the underlying representation may be split across multiple registers.

In 64-bit kernels, jiffies64 represents the same timing source as jiffies, while on 32-bit architectures it is implemented to assemble a coherent 64-bit value from separate parts to avoid torn reads. This mechanism is transparent to most kernel code and enables precise time calculations, timers, and delayed work without frequent wrap-arounds.

Jiffies64 is primarily an internal kernel construct and is not directly exposed to userspace. Applications and libraries typically interact with time through higher-level interfaces such as CLOCK_MONOTONIC, gettimeofday, or high-resolution timers, which rely on the kernel’s timekeeping subsystem, including the underlying jiffies64 counter.

See also: jiffies, timekeeping, get_jiffies_64, ktime.