First of all, jiffies are supposed to be the counter of ticks happened from system boot. In kernel space, jiffies are designed as a global variable. The “64” bit version of “jiffies” can be called as jiffies_64. jiffies are designed to be a 32 bit value where jiffies_64 is a 64 bit variant of the same. we could say low order 32 bits of jiffies_64 as jiffies.
jiffies(,_64} supposed to be incremented with each tick and it has been counted as a time reference point of various kernel operations. “profiling”, “process accounting”..etc are based on this variable.