关于Linux nanosleep函数时间精度的测试

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%12s\n", "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   %8d\n", 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 support\n", 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   %8d\n", 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   %8d\n", 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   %8d\n", 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，控制高精度定时器的是CONFIG_HIGH_RES_TIMERS.

结束！