#Unix itimer update#

The global variable jiffies is used to record the total number of beats generated since the system was started. The higher the beat rate, the higher the interrupt frequency of the system clock, which means that the heavier the system burden, that is, the more processor time occupied by the interrupt processing program, which reduces the time for processing other work.Improve the accuracy of process preemption.There will be fine resolution for measurements such as resource consumption and system running time.System calls that rely on timing values, such as select and poll, can run with higher accuracy.The kernel timer can run with higher frequency and accuracy.#define HZ 1000 / * core clock frequency*/Īdvantages and disadvantages of using high frequency for system timer For example, in i386 architecture, the Hz value is 1000 (Hz), representing 1000 beats per second #include The Hz value depends on the architecture. The system timer (beat rate) is defined by static preprocessing, and the hardware is set according to Hz value when the system is started.

#Unix itimer update#

Update statistics on resource consumption and processor time.Check whether the current process has exhausted its own time slice.If the load of running queues is unbalanced, try to balance them. On the smp system, the running queues on each processor in the scheduler are balanced.The system timer interrupts the periodically executed tasks: Wall time = blocking time + ready time + running time, running time = user CPU actual + system CPU time. System running time: the time elapsed since the start of the system, which is very useful for user space and kernel.The number of ticks per second (beat rate) can be obtained through sysconf(_SC_CLK_TCK). Represents the elapsed time (hours) of the system clock from the beginning to the end of the process, including the time of process blocking. Wall clock time: actual time, which is very important for user space applications.It is often used to calculate wall time and system running time. Tick: the pre programmed tick rate is known to the kernel, so the kernel knows the interval between two consecutive clock interrupts.This frequency can be set by programming. The system timer automatically triggers the clock interrupt at a certain frequency (also known as hitting or popping). tick rate: the frequency of the system timer.The kernel can dynamically create and destroy dynamic timers. Dynamic timer: a tool used to delay the execution of a program.Timer interrupt: the interrupt generated by the fixed time cycle of the system timer, in which the interrupt handler is responsible for updating the system time and executing periodic tasks.System timer: a programmable hardware chip that can generate interrupts at a fixed frequency.