时钟机制是驱动Linux内核运转的核心组件，他的工作方式有两种，periodic(周期性的）和NO_HZ_FULL(IDLE).在不同的模式下，时钟周期的精度是不同的，下面做实验验证一下．
测试用例:
#include<stdio.h>#include<stdlib.h>#include<time.h>#include<sys/time.h>#include<errno.h>#include<string.h>#include<unistd.h>#include<sys/types.h>#include<sys/select.h> int main(int argc, char **argv){ unsigned int nTimeTestSec = 0; unsigned int nTimeTest = 0; struct timeval tvBegin; struct timeval tvNow; int ret = 0; unsigned int nDelay = 0; struct timeval tv; int fd = 1; int i = 0; struct timespec req; unsigned int delay[20] = {500000, 100000, 50000, 10000, 1000, 900, 500, 100, 10, 1, 0}; int nReduce = 0; //误差 fprintf(stderr, "%19s%12s%12s%12sn", "fuction", "time(usec)", "realtime", "reduce"); fprintf(stderr, "----------------------------------------------------n"); for (i = 0; i < 20; i++) { if (delay[i] <= 0) break; nDelay = delay[i]; //test sleep gettimeofday(&tvBegin, NULL); ret = usleep(nDelay); if(ret == -1) { fprintf(stderr, "usleep error, errno=%d [%s]n", errno, strerror(errno)); } gettimeofday(&tvNow, NULL); nTimeTest = (tvNow.tv_sec - tvBegin.tv_sec) * 1000000 + tvNow.tv_usec - tvBegin.tv_usec; nReduce = nTimeTest - nDelay; fprintf (stderr, "t usleep %8u %8u %8dn", nDelay, nTimeTest,nReduce); //test nanosleep req.tv_sec = nDelay/1000000; req.tv_nsec = (nDelay%1000000) * 1000; gettimeofday(&tvBegin, NULL); ret = nanosleep(&req, NULL); if (-1 == ret) { fprintf (stderr, "t nanousleep %8u not supportn", nDelay); } gettimeofday(&tvNow, NULL); nTimeTest = (tvNow.tv_sec - tvBegin.tv_sec) * 1000000 + tvNow.tv_usec - tvBegin.tv_usec; nReduce = nTimeTest - nDelay; fprintf (stderr, "t nanosleep %8u %8u %8dn", nDelay, nTimeTest,nReduce); //test select tv.tv_sec = 0; tv.tv_usec = nDelay; gettimeofday(&tvBegin, NULL); ret = select(0, NULL, NULL, NULL, &tv); if (-1 == ret) { fprintf(stderr, "select error. errno = %d [%s]n", errno, strerror(errno)); } gettimeofday(&tvNow, NULL); nTimeTest = (tvNow.tv_sec - tvBegin.tv_sec) * 1000000 + tvNow.tv_usec - tvBegin.tv_usec; nReduce = nTimeTest - nDelay; fprintf (stderr, "t select %8u %8u %8dn", nDelay, nTimeTest,nReduce); //pselcet req.tv_sec = nDelay/1000000; req.tv_nsec = (nDelay%1000000) * 1000; gettimeofday(&tvBegin, NULL); ret = pselect(0, NULL, NULL, NULL, &req, NULL); if (-1 == ret) { fprintf(stderr, "select error. errno = %d [%s]n", errno, strerror(errno)); } gettimeofday(&tvNow, NULL); nTimeTest = (tvNow.tv_sec - tvBegin.tv_sec) * 1000000 + tvNow.tv_usec - tvBegin.tv_usec; nReduce = nTimeTest - nDelay; fprintf (stderr, "t pselect %8u %8u %8dn", nDelay, nTimeTest,nReduce); fprintf (stderr, "--------------------------------n"); } return 0;}在使用高精度定时器的情况下测试开关PREEMPT的情况：
关闭CONFIG_PREEMPT的情况下
打开CONFIG_PREEMPT
重新编译和安装内核：
貌似改善不多，可以得出CONFIG_PREEMPT和精度关系不大的结论，我们继续．
打开周期定时器：
关闭高精度定时器:
运行用例验证，可以明显发现，由于时钟模式变为periodic的了，时钟粒度瞬间缩减为4ms＝4000us,所以例子中即便睡眠１个us,也需要等到4ms后才会被唤醒．和没有打开periodic的模式有显著区别．
CONFIG_HZ修改为100，看有没有变化：
重新编译内核，发现时钟粒度再次变大，CONFIG_HZ从250变为100. 时钟粒度则从4ms变为10ms成反比关系．
打开CONFIG_HIGH_RES_TIMERS
重新编译，运行用例:
可以看到，打开高精度定时器，时间精度恢复了原始的比较精确的误差范围．
关闭CONFIG_NO_HZ
发现时间精度仍然是高精度的范围
所以看起来，控制告警定定时器的是CONFIG_HIGH_RES_TIMERS.
结束！