October 2017
Intermediate to advanced
586 pages
14h 8m
English
Content preview from Linux Device Drivers Development
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Jiffies and HZ
A jiffy is a kernel unit of time declared in <linux/jiffies.h>. To understand jiffies, we need to introduce a new constant, HZ, which is the number of times jiffies is incremented in one second. Each increment is called a tick. In other words, HZ represents the size of a jiffy. HZ depends on the hardware and on the kernel version, and also determines how frequently the clock interrupt fires. This is configurable on some architectures, fixed on other ones.
What it means is that jiffies is incremented HZ times every second. If HZ = 1,000, then it is incremented 1,000 times (that is, one tick every 1/1,000 seconds). Once defined, the programmable interrupt timer (PIT), which is a hardware component, is programmed with that value ...
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