
Mastering Linux Kernel Development
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We will now broaden our awareness by looking at some key timekeeping structures, macros, and helper routines that can assist programmers in extracting specific time-related data.
Thejiffies
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 32-bit halves are...
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