An example of usage in the Linux Kernel would be in the drivers for XD, where, jiffies + 8*HZ later (as initialized in the member element expires of static struct timer_list) a timer interrupt handler is programmed to execute.

The “watchdog handler” xd_watchdog, in this particular instance, wakes up a sleeping process.

Needless to say, all appropriate caveats emptor apply. Appropriate Processes need to have been sleeping, Timers must have been declared and initialized etc etc

I will blog on this at length later. Please subscribe to our mail list for automated updates on new blog entries.

I explain this specific Linux Kernel concept and more in my classes ( Advanced Linux Kernel Programming @UCSC-Extension, and also in other classes that I teach independently). As always, Feedback, Questions and Comments are appreciated and will be responded to.

Thanks

It is increased by one when a timer interrupt occurs—that is, on every processor tick
i.e. at HZ rate. HZ itself is configured globally.

The Kernel makes generous use of this variable. An example would be to “time” timeouts, “budget” Interrupt Back-Half handlers’ usage of Processors etc to prevent “Hogging”.

The xtime variable derives its information from the jiffies variable and stores the current time and date; it is a structure of type timespec having two fields:

tv_sec: Stores the number of seconds that have elapsed since midnight of January 1, 1970 (UTC)
tv_nsec: Stores the number of nanoseconds that have elapsed within the last second (its value ranges between 0 and 999,999,999)

We also need to blog on back-half handlers. Used in a variety of locations where the non-critical component of Interrupt handling is “deferred”, IO, Packet reception, Transmission etc.