Chapter 4 - DSpace at Waseda University

Chapter 5
Figure 5.4 shows the interrupt process in the non-preemptive kernel. Unlike the preemptive
kernel, a low priority task keeps running even though high priority task occurs during the
working of low priority task. After the low priority task ends, the kernel runs the high priority
task.
5.3 High Resolution Timer
We already have a timer subsystem (kernel/timers.c), why do we need two timer
subsystems? Normally, the most fine-grained time supported by the timer in Linux kernel is
1ms. However, embedded Linux needs much more fine grained time. Therefore, any system
engineers are trying to integrate high-resolution and high-precision features into the existing
timer framework. However, general Linux timer cannot support accuracy of microseconds.
HRTimer provides microsecond resolution with lower overhead and controls time more el
aborately than other timer. It is not possible to use HRTimer in every system. To use
HRTimer supported from hardware. The HRTimer system allows a user space program to be
wake up from a timer event with better accuracy, when using the POSIX timer APIs. Without
this system, the best accuracy that can be obtained for timer events is 1 jiffy. This depends on
the setting of HZ in the kernel. In the 2.4 kernel, HZ was set to 100, which means that the
best accuracy you could get on a timer wakeup in user space was 10 milliseconds.
In other to use HRTimer needs as follows:
� Need to verify that the kernel has support for this feature for your target
processor (and board).
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