clock_gettime和CLOCK_PROCESS_CPUTIME_ID错误的数字

Question

我在64位Ubuntu 12.04系统上并尝试以下代码:

#include <unistd.h>
#include <time.h>
#include <stdio.h>


int
main(void)
{
  struct timespec user1,user2;
  struct timespec sys1,sys2;
  double user_elapsed;
  double sys_elapsed;

  clock_gettime(CLOCK_REALTIME, &user1);
  clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &sys1);
  sleep(10);
  clock_gettime(CLOCK_REALTIME, &user2);
  clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &sys2);
  user_elapsed = user2.tv_sec + user2.tv_nsec/1E9;
  user_elapsed -= user1.tv_sec + user1.tv_nsec/1E9;
  printf("CLOCK_REALTIME: %f\n", user_elapsed);
  sys_elapsed = sys2.tv_sec + sys2.tv_nsec/1E9;
  sys_elapsed -= sys1.tv_sec + sys1.tv_nsec/1E9;
  printf("CLOCK_PROCESS_CPUTIME_ID: %f\n", sys_elapsed);
}

据我了解,这应该打印出类似的东西

CLOCK_REALTIME: 10.000117
CLOCK_PROCESS_CPUTIME_ID: 10.001

但就我而言,我得到的是

CLOCK_REALTIME: 10.000117
CLOCK_PROCESS_CPUTIME_ID: 0.000032

这是正确的行为吗？如果是这样,我如何确定sys1和sys2的实际秒数？

当我将CLOCK_PROCESS_CPUTIME_ID更改为CLOCK_REALTIME然后我得到了预期的结果,但这不是我想要的,因为我们需要精度.

[编辑]显然CLOCK_PROCESS_CPUTIME_ID返回cpu在prcessing上花费的实际时间.CLOCK_MONOTONIC似乎返回正确的值.但是精确度如何呢？

Answer 1

基本上我们所需要的是精确地获取应用程序的当前运行时间(以微秒为单位).

如果我没有误解,这里的运行时间意味着经过的时间.通常,这CLOCK_REALTIME对此有好处,但如果在应用程序运行期间设置时间,则CLOCK_REALTIME经过时间的概念也会发生变化.为了防止这种情况 - 不太可能 - 我建议使用CLOCK_MONOTONIC或,如果存在,CLOCK_MONOTONIC_RAW.从手册页中的描述

   CLOCK_REALTIME
          System-wide real-time clock.  Setting this clock requires appro-
          priate privileges.

   CLOCK_MONOTONIC
          Clock that cannot be set and  represents  monotonic  time  since
          some unspecified starting point.

   CLOCK_MONOTONIC_RAW (since Linux 2.6.28; Linux-specific)
          Similar  to  CLOCK_MONOTONIC, but provides access to a raw hard-
          ware-based time that is not subject to NTP adjustments.

CLOCK_MONOTONIC可能受到NTP调整的影响,而CLOCK_MONOTONIC_RAW不是.所有这些时钟通常具有1纳秒的分辨率(检查一下clock_getres()),但为了您的目的,低于1微秒的分辨率就足够了.

计算经过的时间(以微秒为单位)

#define USED_CLOCK CLOCK_MONOTONIC // CLOCK_MONOTONIC_RAW if available
#define NANOS 1000000000LL

int main(int argc, char *argv[]) {
    /* Whatever */
    struct timespec begin, current;
    long long start, elapsed, microseconds;
    /* set up start time data */
    if (clock_gettime(USED_CLOCK, &begin)) {
        /* Oops, getting clock time failed */
        exit(EXIT_FAILURE);
    }
    /* Start time in nanoseconds */
    start = begin.tv_sec*NANOS + begin.tv_nsec;

    /* Do something interesting */

    /* get elapsed time */
    if (clock_gettime(USED_CLOCK, &current)) {
        /* getting clock time failed, what now? */
        exit(EXIT_FAILURE);
    }
    /* Elapsed time in nanoseconds */
    elapsed = current.tv_sec*NANOS + current.tv_nsec - start;
    microseconds = elapsed / 1000 + (elapsed % 1000 >= 500); // round up halves

    /* Display time in microseconds or something */

    return EXIT_SUCCESS;
}