Linux Kernel: Give me a Jiffy

The jiffies variable is a counter that stores the number of elapsed ticks since the system was started

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.

About Anand

Anand is a veteran of Silicon Valley with development experience and patents that span Processors, Operating systems, Networking and Systems development. Anand has been working for the past few years with Service Providers and large Enterprises developing e and Training systems.

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