Mastering Linux Kernel Development

Jiffies

The jiffies variable holds the number of ticks elapsed since system bootup. Every time a tick occurs, jiffies is incremented by one. It's a 32-bit variable, meaning for a tick rate of 100 Hz, overflow will occur in approximately 497 days (and in 49 days, 17 hours for a 1000 Hz tick rate).

To overcome this issue, a 64-bit variable jiffies_64 is used instead, which allows for thousands of millions of years before the overflow occurs. The jiffies variable is equated to the 32 least significant bits of jiffies_64. The reason for having both jiffies and jiffies_64 variables is that in 32-bit machines, a 64-bit variable can not be accessed atomically; some synchronization is required in order to avoid any counter update while these two ...