Linux下的微秒级定时器: usleep, nanosleep, select, pselect

    今天在公司代码中看到了使用select函数的超时功能作定时器的用法，便整理了如下几个Linux下的微秒级别的定时器。在我的Ubutu10.10 双核环境中，编译通过。

/*
 * @FileName: test_sleep.c
 * @Author: wzj
 * @Brief: 
 *   
 *  
 * @History: 
 * 
 * @Date: 2012年02月07日星期二22:20:00
 * 
 */ 
#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   %8unot 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;
}

    老大建议我们在对精度要求较高的情况下使用select()作为定时器，最大的好处就是不会影响信号处理，线程安全，而且精度能得到保证。在这个实验中，当时间延时时间较长时，select和pselect表现较差，当时间小于1毫秒时，他们的精确度便提高了，表现与usleep、nanosleep不相上下，有时精度甚至超过后者。