Jiffies in Linux Kernel

Jiffies in Linux kernel-general Linux technology-Linux programming and kernel information. For details, refer to the following section. The hardware provides a system timer for the kernel to calculate and manage time. The kernel program the preset system timer frequency, that is, the tick rate. Each cycle is called a tick ). The Linux kernel starts to increase the frequency from 2.5 to 100 (of course, it brings many advantages and some disadvantages ).

Jiffies is a global variable in the kernel. It is used to record the number of beats generated from system startup. For example, if the computing system runs for a long time, you can use jiffies/tick rate for calculation. Jiffies is defined in the file Medium:

Extern unsigned long volatile jiffies;

Jiffies can be used to set timeout, for example:
Unsigned long timeout = jiffies + tick_rate * 2; // timeout after 2 seconds


If (time_before (jiffies, timeout ){
// No timeout

}
Else {
// Timeout

}

The kernel provides four macros to compare the cycle count, which are defined in the file Medium:

Time_before (unknown, known)
Time_after (unknown, known)
Time_before_eq (unknown, known)
Time_after_eq (unknown, known)

When comparing these macros, you can avoid the redirection problem caused by the large size of jiffies.

In addition to the system timer, there is also a time-related clock: Real-time clock (RTC), which is a hardware clock used to persistently store the system time, the system shuts down and depends on the micro-battery on the motherboard to maintain timing. When the system starts, the kernel initializes the Wall Time by reading RTC and stores it in the xtime variable. This is the main function of RTC.